{"id":67387,"date":"2026-10-21T10:11:05","date_gmt":"2026-10-21T07:11:05","guid":{"rendered":"https:\/\/wpcalc.com\/en\/?p=67387"},"modified":"2026-10-21T10:11:05","modified_gmt":"2026-10-21T07:11:05","slug":"engine-compression-ratio-calculator","status":"publish","type":"post","link":"https:\/\/wpcalc.com\/en\/life\/engine-compression-ratio-calculator\/","title":{"rendered":"Engine Compression Ratio Calculator"},"content":{"rendered":"<section class=\"not-prose calculator-box\" aria-labelledby=\"calculator-title\"><div class=\"flex items-baseline sm:items-center justify-between gap-2 sm:gap-3\"><div class=\"flex flex-col sm:flex-row sm:items-center gap-2\"><span class=\"icon icon-calculator text-primary-700! text-base! dark:text-primary-300!\" aria-hidden=\"true\"><\/span><h2 id=\"calculator-title\" class=\"text-lg font-display font-bold\">Calculate Static Compression Ratio, Displacement and DCR<\/h2><\/div><div class=\"calcdash-favorite group\">\n  <button type=\"button\" class=\"favorite\" id=\"favorite\" data-favorite-id=\"67387\" data-favorite-title=\"Compression Ratio\" data-favorite-url=\"https:\/\/wpcalc.com\/en\/life\/engine-compression-ratio-calculator\/\" data-favorite-excerpt=\"Enter bore, stroke, cylinder count, chamber volume, gasket, piston and deck specs to get the static compression ratio, the dynamic compression ratio, and total engine displacement in cc, liters and cubic inches.\" aria-label=\"Add to Favorites\" data-favorite-icon=\"icon icon-roadways\">\n    <span class=\"icon icon-shape-star-empty\" aria-hidden=\"true\"><\/span>\n  <\/button>\n  <div class=\"calcdash-favorite__tip\" aria-hidden=\"true\">\n   <span class=\"favorite-tooltip\" id=\"favorite-tooltip\"><\/span>\n  <\/div>\n  <span class=\"sr-only\" role=\"status\" id=\"favorite-status\"><\/span>\n<\/div><\/div><form action=\"https:\/\/wpcalc.com\/en\/life\/engine-compression-ratio-calculator\/\" method=\"POST\" class=\"calculator\" id=\"calculator-67387\" data-post=\"67387\"><fieldset class=\"fieldset-input is-2\"><legend class=\"sr-only\">Input Fields<\/legend><div class=\"field has-term\" id=\"input-1\"><label for=\"field-1\">Bore<\/label><div class=\"term\">B<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><div class=\"field-group\"><input type=\"text\" name=\"bore\" id=\"field-1\" aria-describedby=\"calc-info-1\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"1\" max=\"400\" value=\"4\"\/><div class=\"grid grid-cols-1\"><select id=\"field-2\" name=\"unit\" aria-label=\"Unit\"><option value=\"in\" selected>in<\/option><option value=\"mm\">mm<\/option><\/select><svg class=\"pointer-events-none col-start-1 row-start-1 mr-2 size-5 self-center justify-self-end text-gray-500 sm:size-4\" viewBox=\"0 0 16 16\" fill=\"currentColor\" aria-hidden=\"true\" data-slot=\"icon\">\n              <path fill-rule=\"evenodd\" d=\"M4.22 6.22a.75.75 0 0 1 1.06 0L8 8.94l2.72-2.72a.75.75 0 1 1 1.06 1.06l-3.25 3.25a.75.75 0 0 1-1.06 0L4.22 7.28a.75.75 0 0 1 0-1.06Z\" clip-rule=\"evenodd\" \/>\n            <\/svg><\/div><\/div><small id=\"calc-info-1\">Cylinder diameter as measured at the piston.<\/small><\/div><div class=\"field has-term\" id=\"input-2\"><label for=\"field-3\">Stroke<\/label><div class=\"term\">S<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><div class=\"field-group\"><input type=\"text\" name=\"stroke\" id=\"field-3\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"1\" max=\"400\" value=\"3.48\"\/><div class=\"grid grid-cols-1\"><select id=\"field-4\" name=\"unit\" aria-label=\"Unit\"><option value=\"in\" selected>in<\/option><option value=\"mm\">mm<\/option><\/select><svg class=\"pointer-events-none col-start-1 row-start-1 mr-2 size-5 self-center justify-self-end text-gray-500 sm:size-4\" viewBox=\"0 0 16 16\" fill=\"currentColor\" aria-hidden=\"true\" data-slot=\"icon\">\n              <path fill-rule=\"evenodd\" d=\"M4.22 6.22a.75.75 0 0 1 1.06 0L8 8.94l2.72-2.72a.75.75 0 1 1 1.06 1.06l-3.25 3.25a.75.75 0 0 1-1.06 0L4.22 7.28a.75.75 0 0 1 0-1.06Z\" clip-rule=\"evenodd\" \/>\n            <\/svg><\/div><\/div><\/div><div class=\"field has-term\" id=\"input-3\"><label for=\"field-5\">Cylinders<\/label><div class=\"term\">n<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"cylinders\" id=\"field-5\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"1\" max=\"16\" value=\"8\"\/><\/div><div class=\"field has-term\" id=\"input-4\"><label for=\"field-6\">Chamber volume<\/label><div class=\"term\">Vch<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"chamber_volume\" id=\"field-6\" aria-describedby=\"calc-unit-6 calc-info-6\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"0.1\" max=\"100000\" value=\"64\"\/><div class=\"unit\" id=\"calc-unit-6\">cc<\/div><small id=\"calc-info-6\">Combustion chamber volume in the head, per cylinder.<\/small><\/div><div class=\"field\" id=\"input-5\"><label for=\"field-7\">Gasket bore<\/label><div class=\"field-group\"><input type=\"text\" name=\"gasket_bore\" id=\"field-7\" aria-describedby=\"calc-info-7\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"1\" max=\"400\" value=\"4.1\"\/><div class=\"grid grid-cols-1\"><select id=\"field-8\" name=\"unit\" aria-label=\"Unit\"><option value=\"in\" selected>in<\/option><option value=\"mm\">mm<\/option><\/select><svg class=\"pointer-events-none col-start-1 row-start-1 mr-2 size-5 self-center justify-self-end text-gray-500 sm:size-4\" viewBox=\"0 0 16 16\" fill=\"currentColor\" aria-hidden=\"true\" data-slot=\"icon\">\n              <path fill-rule=\"evenodd\" d=\"M4.22 6.22a.75.75 0 0 1 1.06 0L8 8.94l2.72-2.72a.75.75 0 1 1 1.06 1.06l-3.25 3.25a.75.75 0 0 1-1.06 0L4.22 7.28a.75.75 0 0 1 0-1.06Z\" clip-rule=\"evenodd\" \/>\n            <\/svg><\/div><\/div><small id=\"calc-info-7\">Fire-ring or gasket opening diameter.<\/small><\/div><div class=\"field\" id=\"input-6\"><label for=\"field-9\">Gasket thickness<\/label><div class=\"field-group\"><input type=\"text\" name=\"gasket_thickness\" id=\"field-9\" aria-describedby=\"calc-info-9\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"0\" max=\"100\" value=\"0.04\"\/><div class=\"grid grid-cols-1\"><select id=\"field-10\" name=\"unit\" aria-label=\"Unit\"><option value=\"in\" selected>in<\/option><option value=\"mm\">mm<\/option><\/select><svg class=\"pointer-events-none col-start-1 row-start-1 mr-2 size-5 self-center justify-self-end text-gray-500 sm:size-4\" viewBox=\"0 0 16 16\" fill=\"currentColor\" aria-hidden=\"true\" data-slot=\"icon\">\n              <path fill-rule=\"evenodd\" d=\"M4.22 6.22a.75.75 0 0 1 1.06 0L8 8.94l2.72-2.72a.75.75 0 1 1 1.06 1.06l-3.25 3.25a.75.75 0 0 1-1.06 0L4.22 7.28a.75.75 0 0 1 0-1.06Z\" clip-rule=\"evenodd\" \/>\n            <\/svg><\/div><\/div><small id=\"calc-info-9\">Compressed thickness. Use 0 to ignore the gasket.<\/small><\/div><div class=\"field has-term\" id=\"input-7\"><label for=\"field-11\">Piston volume<\/label><div class=\"term\">Vp<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"number\" name=\"piston_volume\" id=\"field-11\" aria-describedby=\"calc-unit-11 calc-info-11\" min=\"-2000\" max=\"2000\" step=\"0.1\" value=\"-7\"\/><div class=\"unit\" id=\"calc-unit-11\">cc<\/div><small id=\"calc-info-11\">Dome: positive. Dish or valve reliefs: negative. A dome reduces, a dish adds clearance volume. 0 for a flat top.<\/small><\/div><div class=\"field has-term\" id=\"input-8\"><label for=\"field-12\">Deck clearance<\/label><div class=\"term\">deck<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><div class=\"field-group\"><input type=\"number\" name=\"deck_clearance\" id=\"field-12\" aria-describedby=\"calc-info-12\" min=\"-20\" max=\"400\" step=\"0.001\" value=\"0.015\"\/><div class=\"grid grid-cols-1\"><select id=\"field-13\" name=\"unit\" aria-label=\"Unit\"><option value=\"in\" selected>in<\/option><option value=\"mm\">mm<\/option><\/select><svg class=\"pointer-events-none col-start-1 row-start-1 mr-2 size-5 self-center justify-self-end text-gray-500 sm:size-4\" viewBox=\"0 0 16 16\" fill=\"currentColor\" aria-hidden=\"true\" data-slot=\"icon\">\n              <path fill-rule=\"evenodd\" d=\"M4.22 6.22a.75.75 0 0 1 1.06 0L8 8.94l2.72-2.72a.75.75 0 1 1 1.06 1.06l-3.25 3.25a.75.75 0 0 1-1.06 0L4.22 7.28a.75.75 0 0 1 0-1.06Z\" clip-rule=\"evenodd\" \/>\n            <\/svg><\/div><\/div><small id=\"calc-info-12\">Piston top below the block deck at TDC. Negative if the piston protrudes above the deck.<\/small><\/div><div class=\"is-single field\" id=\"input-9\"><span class=\"font-semibold\">Dynamic compression ratio (optional)<\/span><\/div><div class=\"field has-term\" id=\"input-10\"><label for=\"field-15\">Intake valve closes<\/label><div class=\"term\">IVC<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"intake_valve_closes\" id=\"field-15\" aria-describedby=\"calc-unit-15 calc-info-15\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"0\" max=\"120\" value=\"60\"\/><div class=\"unit\" id=\"calc-unit-15\">\u00b0 ABDC<\/div><small id=\"calc-info-15\">Effective intake valve closing point, in degrees after bottom dead center.<\/small><\/div><div class=\"field has-term\" id=\"input-11\"><label for=\"field-16\">Rod length<\/label><div class=\"term\">L<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><div class=\"field-group\"><input type=\"text\" name=\"rod_length\" id=\"field-16\" aria-describedby=\"calc-info-16\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"1\" max=\"400\" value=\"5.7\"\/><div class=\"grid grid-cols-1\"><select id=\"field-17\" name=\"unit\" aria-label=\"Unit\"><option value=\"in\" selected>in<\/option><option value=\"mm\">mm<\/option><\/select><svg class=\"pointer-events-none col-start-1 row-start-1 mr-2 size-5 self-center justify-self-end text-gray-500 sm:size-4\" viewBox=\"0 0 16 16\" fill=\"currentColor\" aria-hidden=\"true\" data-slot=\"icon\">\n              <path fill-rule=\"evenodd\" d=\"M4.22 6.22a.75.75 0 0 1 1.06 0L8 8.94l2.72-2.72a.75.75 0 1 1 1.06 1.06l-3.25 3.25a.75.75 0 0 1-1.06 0L4.22 7.28a.75.75 0 0 1 0-1.06Z\" clip-rule=\"evenodd\" \/>\n            <\/svg><\/div><\/div><small id=\"calc-info-16\">Connecting rod length, center to center. Enter together with the IVC angle for the dynamic ratio.<\/small><\/div><\/fieldset><div class=\"buttons\"><button type=\"submit\" data-text=\"Re-Calculate\" id=\"calculate-button\" data-post=\"67387\">Calculate<\/button><button type=\"reset\">Reset<\/button><\/div><div class=\"field is-checkbox hidden!\" id=\"field-auto-calc\"><input type=\"checkbox\" id=\"auto-calc\"><label for=\"auto-calc\">Calculate automatically<\/label><small>If enabled, the result will update automatically when you change any value.<\/small><\/div><div class=\"fieldset-result is-hidden\" aria-labelledby=\"result-title\" aria-live=\"polite\" role=\"region\"> <h3 class=\"result-title bg-gradient-to-r from-primary-50 to-gray-50 dark:from-primary-900 dark:to-gray-800\" id=\"result-title\"><span class=\"icon icon-s-pulse\" aria-hidden=\"true\"><\/span> Compression Ratio and Displacement<\/h3><div class=\"result-box\"><div class=\"field-result\" id=\"output-1\"><span class=\"field-title\"><span>Compression ratio<\/span><\/span><span class=\"field-value\" id=\"result-1\" data-unit=\": 1\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result\" id=\"output-2\"><span class=\"field-title\"><span>Displacement per cylinder<\/span><\/span><span class=\"field-value\" id=\"result-2\" data-unit=\"cc\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result\" id=\"output-3\"><span class=\"field-title\"><span>Total displacement<\/span><\/span><span class=\"field-value\" id=\"result-3\" data-unit=\"cc\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result\" id=\"output-4\"><span class=\"field-title\"><span>Total displacement<\/span><\/span><span class=\"field-value\" id=\"result-4\" data-unit=\"liters\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result\" id=\"output-5\"><span class=\"field-title\"><span>Total displacement<\/span><\/span><span class=\"field-value\" id=\"result-5\" data-unit=\"ci\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result\" id=\"output-6\"><span class=\"field-title\"><span>Dynamic compression ratio<\/span><\/span><span class=\"field-value\" id=\"result-6\" data-unit=\": 1\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result is-column\" id=\"output-7\"><span class=\"field-title\"><span>Volume breakdown<\/span><\/span><span class=\"field-value\" id=\"result-7\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><div class=\"field-result is-column\" id=\"output-8\"><span class=\"field-title\"><span>Method and assumptions<\/span><\/span><span class=\"field-value\" id=\"result-8\"><\/span><button type=\"button\" class=\"copy-result\" data-tooltip=\"Copy Result\" aria-label=\"Copy Result\"><span class=\"copy-icon icon icon-document-copy\" aria-hidden=\"true\"><\/span><\/button><\/div><\/div><div class=\"result-actions\"><button type=\"button\" class=\"copy-link\"><svg viewBox=\"0 0 20 20\" fill=\"currentColor\" aria-hidden=\"true\"><path d=\"M12.232 4.232a2.5 2.5 0 0 1 3.536 3.536l-1.225 1.224a.75.75 0 0 0 1.061 1.06l1.224-1.224a4 4 0 0 0-5.656-5.656l-3 3a4 4 0 0 0 .225 5.865.75.75 0 0 0 .977-1.138 2.5 2.5 0 0 1-.142-3.667l3-3Z\"\/><path d=\"M11.603 7.963a.75.75 0 0 0-.977 1.138 2.5 2.5 0 0 1 .142 3.667l-3 3a2.5 2.5 0 0 1-3.536-3.536l1.225-1.224a.75.75 0 0 0-1.061-1.06l-1.224 1.224a4 4 0 1 0 5.656 5.656l3-3a4 4 0 0 0-.225-5.865Z\"\/><\/svg><span class=\"copy-link__text\">Copy link to this calculation<\/span><\/button><span class=\"sr-only\" role=\"status\" id=\"copy-link-status\"><\/span><\/div><\/div><a href=\"#respond\" class=\"hidden transition-opacity duration-300 opacity-0 w-50 text-sm justify-center items-center gap-2 px-4 py-2 rounded bg-gray-200 text-gray-700 hover:bg-gray-300\" id=\"leave-comment\"><span class=\"icon icon-comments\" aria-hidden=\"true\"><\/span>Leave a Comment<\/a><\/form><\/section><section id=\"calc-reactions\" class=\"not-prose hidden my-12 bg-gradient-to-r from-primary-50 to-gray-50 border border-indigo-100 rounded-xl px-6 py-4 shadow-sm\" data-post=\"67387\" aria-live=\"polite\" aria-labelledby=\"calc-reactions-title\"><p id=\"calc-reactions-title\" class=\"text-sm text-gray-500 text-center mb-2\">How did this result make you feel?<\/p><div role=\"group\" aria-labelledby=\"calc-reactions-title\" class=\"grid grid-cols-3 sm:flex sm:flex-row sm:justify-around gap-4 sm:items-center sm:flex-wrap\"><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"like\"><span class=\"reaction-count\" data-reaction-count=\"like\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\ude00 <\/span><span class=\"reaction-description font-medium\">Like <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"helpful\"><span class=\"reaction-count\" data-reaction-count=\"helpful\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\udca1 <\/span><span class=\"reaction-description font-medium\">Helpful <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"confused\"><span class=\"reaction-count\" data-reaction-count=\"confused\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\ude15 <\/span><span class=\"reaction-description font-medium\">Confused <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"disappointed\"><span class=\"reaction-count\" data-reaction-count=\"disappointed\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\ud83d\ude1e <\/span><span class=\"reaction-description font-medium\">Disappointed <\/span><\/button><button type=\"button\" aria-pressed=\"false\" class=\"reaction-btn flex flex-col items-center gap-1 text-sm text-gray-700 hover:text-primary-600 cursor-pointer transition-transform hover:scale-105\" data-reaction=\"inaccurate\"><span class=\"reaction-count\" data-reaction-count=\"inaccurate\">0<\/span><span class=\"reaction text-3xl\" aria-hidden=\"true\">\u274c <\/span><span class=\"reaction-description font-medium\">Inaccurate <\/span><\/button><\/div><div id=\"reaction-message\" class=\"hidden mt-8 rounded-md border border-primary-100 bg-white\/50 backdrop-blur-sm px-4 py-2 text-sm text-center shadow-sm\"><\/div><\/section><div class=\"calcdash-ad not-prose calcdash-ad--calc_result calcdash-ad--in-article\"><ins class=\"adsbygoogle\" style=\"display:block\" data-ad-client=\"ca-pub-1721569815777345\" data-ad-slot=\"6317458308\" data-ad-layout=\"in-article\" data-ad-format=\"fluid\"><\/ins><script>(adsbygoogle = window.adsbygoogle || []).push({});<\/script><\/div>\n<style id=\"wpcoderpro-inline-1\">.formula-box {\r\n\tborder-bottom: none !important;\r\n}<\/style><section class=\"formula-box\" aria-labelledby=\"formula-title\">\r\n\t<div id=\"formula-title\" class=\"not-prose font-display mb-4\"><span class=\"icon icon-formula\" aria-hidden=\"true\"><\/span><h2>Compression Ratio Formula<\/h2><\/div>\r\n\t<figure class=\"not-prose formula\">\r\n\t\t<figcaption class=\"formula-title\">Formula<\/figcaption>\r\n\t\t<div class=\"text-base text-gray-800\" id=\"formula\">\n\\[ CR \\;=\\; \\frac{V_{swept} + V_{clear}}{V_{clear}} \\\\[4pt] V_{swept} \\;=\\; \\frac{\\pi}{4}\\,B^{2}S, \\qquad V_{clear} \\;=\\; V_{ch} + V_{g} + V_{deck} - V_{p} \\]\n<\/div>\r\n\t<\/figure>\r\n\t<\/p>\n<p><strong>Where:<\/strong><\/p>\n<ul>\n<li>\\(B\\) \u2014 cylinder bore, measured at the piston (cm)<\/li>\n<li>\\(S\\) \u2014 piston stroke (cm)<\/li>\n<li>\\(V_{ch}\\) \u2014 combustion chamber volume in the head, per cylinder (cc)<\/li>\n<li>\\(V_{g}\\) \u2014 head gasket volume, \\( \\tfrac{\\pi}{4}B_{g}^{2}t \\), where \\(B_{g}\\) is the gasket (fire-ring) bore and \\(t\\) is the compressed gasket thickness (cc)<\/li>\n<li>\\(V_{deck}\\) \u2014 deck clearance volume \\(= \\tfrac{\\pi}{4}B^{2}d\\), where \\(d\\) is the piston-crown-to-deck distance at TDC, positive when the piston sits below the deck (cc)<\/li>\n<li>\\(V_{p}\\) \u2014 piston volume: positive for a dome, negative for a dish or valve reliefs, 0 for a flat top (cc)<\/li>\n<\/ul>\n<p>Every length is converted to centimeters internally, so all volumes come out in cc. The formula expresses the static compression ratio: the full cylinder volume at bottom dead center (BDC) divided by the clearance volume that remains when the piston is at top dead center (TDC). A piston dome removes clearance volume; a dish or valve reliefs add it.<\/p>\n<p>\r\n<\/section>\n<style id=\"wpcoderpro-inline-459\">.calculation-box {\r\n\tborder-bottom: none !important;\r\n}<\/style><section id=\"calculation\" class=\"calculation-box\" aria-labelledby=\"calculation-title\">\r\n\t<div id=\"calculation-title\" class=\"not-prose font-display\"><span class=\"icon icon-unordered-list\" aria-hidden=\"true\"><\/span><h2>Compression Ratio \u2013 Calculation Example<\/h2><\/div>\r\n\r\n<\/p>\n<p><strong>Example:<\/strong> a 350 ci small-block V8 \u2014 4 in bore, 3.48 in stroke, 8 cylinders, 64 cc chamber volume, 4.1 in gasket bore, 0.04 in compressed gasket thickness, 7 cc dished pistons (entered as \u22127), 0.015 in deck clearance, 5.7 in rod length and an intake valve closing at 60\u00b0 ABDC. These are the calculator's default values.<\/p>\n<p>\\[ V_{swept} = \\tfrac{\\pi}{4}(10.16)^{2}(8.8392) = 716.62\\ \\text{cc} \\]<\/p>\n<p>\\[ V_{clear} = 64 + 8.65 + 3.09 + 7 = 82.74\\ \\text{cc} \\]<\/p>\n<p>\\[ CR = \\frac{716.62 + 82.74}{82.74} = 9.66 \\]<\/p>\n<p><strong>Result:<\/strong> static compression ratio 9.66 : 1, dynamic compression ratio 8.00 : 1, displacement 716.62 cc per cylinder and 5,733 cc (5.73 L, 349.8 ci) in total.<\/p>\n<p>\n\r\n<\/section>\n<h2>How to Use the Compression Ratio Calculator<\/h2>\n<ol>\n<li>Enter the <strong>Bore<\/strong> and <strong>Stroke<\/strong> of one cylinder and choose inches or millimeters from the unit menus.<\/li>\n<li>Set <strong>Cylinders<\/strong> (1\u201316) so the calculator can total the engine displacement.<\/li>\n<li>Enter the <strong>Chamber volume<\/strong> in cc \u2014 the combustion chamber in the head, per cylinder. If the figure is not published, measure it by \"cc'ing\" the head with a burette.<\/li>\n<li>Enter the <strong>Gasket bore<\/strong> (the fire-ring or gasket opening) and the compressed <strong>Gasket thickness<\/strong>; use 0 to ignore the gasket.<\/li>\n<li>Enter the <strong>Piston volume<\/strong>: positive for a dome, negative for a dish or valve reliefs, 0 for a flat top.<\/li>\n<li>Enter the <strong>Deck clearance<\/strong> \u2014 positive when the piston top sits below the block deck at TDC, negative if the piston protrudes above the deck.<\/li>\n<li>For the dynamic ratio, fill in <strong>Intake valve closes<\/strong> (degrees ABDC) and <strong>Rod length<\/strong> (center to center). Leave either field empty to skip the dynamic result.<\/li>\n<li>Read the <strong>compression ratio<\/strong>, the <strong>displacement<\/strong> in cc, liters and cubic inches, and the volume breakdown, which shows exactly where every cc of clearance volume comes from.<\/li>\n<\/ol>\n<h2>What Is Compression Ratio?<\/h2>\n<p>The compression ratio (CR) compares the volume inside a cylinder when the piston is at bottom dead center with the volume that remains when the piston is at top dead center. A ratio of 9.66 : 1 means the air-fuel charge is squeezed into roughly one tenth of its original volume before the spark fires. It is a purely geometric quantity: it is fixed by the bore, stroke, chamber size, gasket, piston crown shape and deck height \u2014 not by tuning.<\/p>\n<p>Compression ratio matters because it drives thermal efficiency and octane demand at the same time. A higher ratio extracts more work from every unit of fuel, improving torque and fuel economy, but it also raises peak cylinder pressure and temperature, which pushes the engine toward detonation (knock). Choosing a ratio is therefore a balance: build as much compression as the fuel you intend to run will tolerate.<\/p>\n<p>The static ratio above is only half the story. In a running engine the cylinder is not sealed until the intake valve closes, so a camshaft with a late intake closing point lets some charge push back into the port before real compression begins. The <em>dynamic compression ratio<\/em> (DCR) captures this: it is computed from the volume actually trapped from the effective intake valve closing point onward, using the rod length and the exact slider-crank piston position. A long-duration cam lowers the DCR well below the static figure, which is why race camshafts and pump gas rarely mix with high static ratios.<\/p>\n<p>As a guideline, most street engines on pump gasoline work best with a dynamic ratio in the 7.5 : 1 to 8.5 : 1 range. The volume breakdown table in the results shows each component separately \u2014 chamber, gasket, deck and piston \u2014 so you can see which change (a thinner gasket, a milled head, a different piston) moves the ratio and by how much.<\/p>\n<h2>Assumptions, Limitations and Practical Tips<\/h2>\n<ul>\n<li>Use <strong>measured<\/strong> chamber volumes whenever possible. Published \"nominal\" chambers often vary by 1\u20133 cc, which is enough to shift the ratio by several tenths.<\/li>\n<li>Always enter the <strong>compressed<\/strong> gasket thickness, not the thickness as supplied. A gasket at 0.04 in installed may measure 0.06 in in the box.<\/li>\n<li>Watch the sign conventions: a dome is positive and <em>reduces<\/em> clearance volume, a dish is negative and <em>adds<\/em> it. A piston protruding above the deck takes a negative deck clearance.<\/li>\n<li>If the clearance volume reaches zero, the result is invalid \u2014 a oversized dome, a protruding piston or an empty chamber leaves nothing to compress.<\/li>\n<li>The dynamic ratio is a geometric estimate. It ignores valve-event ramp rates, manifold pressure, altitude and charge temperature, so treat it as a cam-selection guideline, not a measurement of cranking pressure.<\/li>\n<li>Quench area, ignition timing, coolant temperature and combustion-chamber design affect knock as much as the ratio itself, so the fuel table below is only a starting point.<\/li>\n<\/ul>\n<table>\n<thead>\n<tr>\n<th>Static CR<\/th>\n<th>Typical fuel<\/th>\n<th>Typical use<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>8.0\u20139.5 : 1<\/td>\n<td>Regular, 87 octane<\/td>\n<td>Stock and vintage engines, forced-induction street builds<\/td>\n<\/tr>\n<tr>\n<td>9.5\u201310.5 : 1<\/td>\n<td>Mid-grade to premium<\/td>\n<td>Typical naturally aspirated street engine; iron heads sit at the low end<\/td>\n<\/tr>\n<tr>\n<td>10.5\u201312.0 : 1<\/td>\n<td>Premium to 100+ octane<\/td>\n<td>Aluminum heads, tight quench, careful tuning<\/td>\n<\/tr>\n<tr>\n<td>Over 12 : 1<\/td>\n<td>Race fuel or E85<\/td>\n<td>Competition engines<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Once the engine is back together and running, our <a href=\"https:\/\/wpcalc.com\/en\/life\/fuel-cost-calculator-trip-gas-cost\/\">fuel cost calculator<\/a>, <a href=\"https:\/\/wpcalc.com\/en\/life\/gas-mileage-calculator\/\">gas mileage calculator<\/a> and <a href=\"https:\/\/wpcalc.com\/en\/life\/car-maintenance-due-calculator\/\">car maintenance due calculator<\/a> help you keep track of the everyday costs of driving it.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Enter bore, stroke, cylinder count, chamber volume, gasket, piston and deck specs to get the static compression ratio, the dynamic compression ratio, and total engine displacement in cc, liters and cubic inches.<\/p>\n","protected":false},"author":3168,"featured_media":0,"comment_status":"open","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[6],"tags":[99],"class_list":["post-67387","post","type-post","status-publish","format-standard","hentry","category-life","tag-auto-travel"],"acf":[],"_links":{"self":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/67387","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/users\/3168"}],"replies":[{"embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/comments?post=67387"}],"version-history":[{"count":1,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/67387\/revisions"}],"predecessor-version":[{"id":67388,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/67387\/revisions\/67388"}],"wp:attachment":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/media?parent=67387"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/categories?post=67387"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/tags?post=67387"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}