{"id":66720,"date":"2026-10-13T08:43:55","date_gmt":"2026-10-13T05:43:55","guid":{"rendered":"https:\/\/wpcalc.com\/en\/?p=66720"},"modified":"2026-10-13T08:43:55","modified_gmt":"2026-10-13T05:43:55","slug":"duct-pressure-drop","status":"publish","type":"post","link":"https:\/\/wpcalc.com\/en\/engineering\/duct-pressure-drop\/","title":{"rendered":"Duct Pressure Drop"},"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\">Estimate Pressure Loss in a Duct<\/h2><\/div><div class=\"calcdash-favorite group\">\n  <button type=\"button\" class=\"favorite\" id=\"favorite\" data-favorite-id=\"66720\" data-favorite-title=\"Duct Pressure Drop\" data-favorite-url=\"https:\/\/wpcalc.com\/en\/engineering\/duct-pressure-drop\/\" data-favorite-excerpt=\"Estimate the pressure drop in a rectangular or circular ventilation duct from its size, length, airflow and fittings. Enter duct dimensions, airflow rate and friction factor to get friction loss, fitting loss and total pressure drop in Pa, mm H2O and in. w.g.\" aria-label=\"Add to Favorites\" data-favorite-icon=\"icon icon-ventilation\">\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\/engineering\/duct-pressure-drop\/\" method=\"POST\" class=\"calculator\" id=\"calculator-66720\" data-post=\"66720\"><fieldset class=\"fieldset-input is-2\"><legend class=\"sr-only\">Input Fields<\/legend><div class=\"field is-single\" id=\"input-1\"><label for=\"field-1\">Duct shape<\/label><div class=\"grid grid-cols-1\"><select id=\"field-1\" name=\"duct_shape\"><option value=\"rect\" selected>Rectangular<\/option><option value=\"circ\">Circular<\/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 class=\"field has-term\" id=\"input-2\"><label for=\"field-2\">Duct width (W)<\/label><div class=\"term\">W<\/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=\"duct_width_w\" id=\"field-2\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" placeholder=\"Width\" min=\"10\" value=\"400\"\/><div class=\"grid grid-cols-1\"><select id=\"field-3\" name=\"unit\" aria-label=\"Unit\"><option value=\"mm\" selected>mm<\/option><option value=\"in\">in<\/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-4\">Duct height (H)<\/label><div class=\"term\">H<\/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=\"duct_height_h\" id=\"field-4\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" placeholder=\"Height\" min=\"10\" value=\"200\"\/><div class=\"grid grid-cols-1\"><select id=\"field-5\" name=\"unit\" aria-label=\"Unit\"><option value=\"mm\" selected>mm<\/option><option value=\"in\">in<\/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-4\" hidden><label for=\"field-6\">Duct diameter (D)<\/label><div class=\"term\">D<\/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=\"duct_diameter_d\" id=\"field-6\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" placeholder=\"Diameter\" min=\"10\" value=\"250\"\/><div class=\"grid grid-cols-1\"><select id=\"field-7\" name=\"unit\" aria-label=\"Unit\"><option value=\"mm\" selected>mm<\/option><option value=\"in\">in<\/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-5\"><label for=\"field-8\">Duct length (L)<\/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=\"duct_length_l\" id=\"field-8\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" placeholder=\"Length\" min=\"0.1\" value=\"10\"\/><div class=\"grid grid-cols-1\"><select id=\"field-9\" name=\"unit\" aria-label=\"Unit\"><option value=\"m\" selected>m<\/option><option value=\"ft\">ft<\/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-6\"><label for=\"field-10\">Airflow (Q)<\/label><div class=\"term\">Q<\/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=\"airflow_q\" id=\"field-10\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" placeholder=\"Airflow\" min=\"1\" value=\"1000\"\/><div class=\"grid grid-cols-1\"><select id=\"field-11\" name=\"unit\" aria-label=\"Unit\"><option value=\"m3h\" selected>m\u00b3\/h<\/option><option value=\"cfm\">CFM<\/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-7\"><label for=\"field-12\">Air density (\u03c1)<\/label><div class=\"term\">\u03c1<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"air_density_\" id=\"field-12\" aria-describedby=\"calc-unit-12 calc-info-12\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" placeholder=\"Density\" min=\"0.1\" value=\"1.2\"\/><div class=\"unit\" id=\"calc-unit-12\">kg\/m\u00b3<\/div><small id=\"calc-info-12\">1.2 kg\/m\u00b3 \u2248 air at 20 \u00b0C<\/small><\/div><div class=\"field has-term\" id=\"input-8\"><label for=\"field-13\">Friction factor (f)<\/label><div class=\"term\">f<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"friction_factor_f\" id=\"field-13\" aria-describedby=\"calc-info-13\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" placeholder=\"Factor\" min=\"0.001\" max=\"0.2\" value=\"0.02\"\/><small id=\"calc-info-13\">Darcy friction factor; 0.015\u20130.025 is typical for galvanized steel<\/small><\/div><div class=\"field has-term\" id=\"input-9\"><label for=\"field-14\">Fitting loss coefficient (\u03a3K)<\/label><div class=\"term\">\u03a3K<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"fitting_loss_coefficient_k\" id=\"field-14\" aria-describedby=\"calc-info-14\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" placeholder=\"\u03a3K\" min=\"0\" value=\"1\"\/><small id=\"calc-info-14\">Sum of K-values for bends, tees, grilles etc. Enter 0 for a straight duct only<\/small><\/div><\/fieldset><div class=\"buttons\"><button type=\"submit\" data-text=\"Re-Calculate\" id=\"calculate-button\" data-post=\"66720\">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> Estimated Duct Pressure Loss<\/h3><div class=\"result-box\"><div class=\"field-result\" id=\"output-1\"><span class=\"field-title\"><span>Air velocity<\/span><span class=\"result-term\">v<\/span><\/span><span class=\"field-value\" id=\"result-1\" data-unit=\"m\/s\"><\/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>Friction loss (straight duct)<\/span><span class=\"result-term\">\u0394p<\/span><\/span><span class=\"field-value\" id=\"result-2\" data-unit=\"Pa\"><\/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>Fitting loss<\/span><span class=\"result-term\">\u0394p<\/span><\/span><span class=\"field-value\" id=\"result-3\" data-unit=\"Pa\"><\/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 pressure drop<\/span><span class=\"result-term\">\u0394p<\/span><\/span><span class=\"field-value\" id=\"result-4\" data-unit=\"Pa\"><\/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 pressure drop<\/span><span class=\"result-term\">\u0394p<\/span><\/span><span class=\"field-value\" id=\"result-5\" data-unit=\"mm H\u2082O\"><\/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>Total pressure drop<\/span><span class=\"result-term\">\u0394p<\/span><\/span><span class=\"field-value\" id=\"result-6\" data-unit=\"in. w.g.\"><\/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=\"66720\" 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>Duct Pressure Drop 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\\[ \\Delta p = f \\cdot \\frac{L}{D_h} \\cdot \\frac{\\rho v^2}{2} + \\Sigma K \\cdot \\frac{\\rho v^2}{2}, \\qquad v = \\frac{Q}{A}, \\qquad D_h = \\frac{2WH}{W+H} \\]\n<\/div>\r\n\t<\/figure>\r\n\t<\/p>\n<p><strong>Where:<\/strong><\/p>\n<ul>\n<li>\\(\\Delta p\\) \u2014 total pressure drop (Pa)<\/li>\n<li>\\(f\\) \u2014 Darcy friction factor (dimensionless)<\/li>\n<li>\\(L\\) \u2014 duct length (m)<\/li>\n<li>\\(D_h\\) \u2014 hydraulic diameter (m); for a circular duct it equals the diameter, for a rectangular duct it is \\(2WH\/(W+H)\\)<\/li>\n<li>\\(\\rho\\) \u2014 air density (kg\/m\u00b3)<\/li>\n<li>\\(v\\) \u2014 air velocity (m\/s)<\/li>\n<li>\\(Q\\) \u2014 airflow (m\u00b3\/s)<\/li>\n<li>\\(A\\) \u2014 cross-section area (m\u00b2)<\/li>\n<li>\\(\\Sigma K\\) \u2014 sum of fitting loss coefficients<\/li>\n<\/ul>\n<p>The formula is the Darcy\u2013Weisbach equation: the first term is the friction lost along the straight duct run, the second term adds the dynamic losses from fittings such as bends, tees and grilles. Both terms are driven by the velocity pressure \\(\\rho v^2\/2\\), which is why smaller ducts produce disproportionately higher losses.<\/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>Duct Pressure Drop \u2013 Calculation Example<\/h2><\/div>\r\n\r\n<\/p>\n<p><strong>Example:<\/strong> a rectangular galvanized steel duct 400 mm wide \u00d7 200 mm high, 10 m long, carrying 1000 m\u00b3\/h of air (\u03c1 = 1.2 kg\/m\u00b3, f = 0.02), with fittings totalling \u03a3K = 1.<\/p>\n<p>\\[ A = 0.4 \\times 0.2 = 0.08\\ \\text{m}^2, \\qquad Q = \\frac{1000}{3600} = 0.2778\\ \\text{m}^3\/\\s \\]<\/p>\n<p>\\[ v = \\frac{0.2778}{0.08} = 3.47\\ \\text{m\/s}, \\qquad D_h = \\frac{2 \\times 0.4 \\times 0.2}{0.4 + 0.2} = 0.267\\ \\text{m} \\]<\/p>\n<p>\\[ \\frac{\\rho v^2}{2} = \\frac{1.2 \\times 3.47^2}{2} = 7.23\\ \\text{Pa} \\]<\/p>\n<p>\\[ \\Delta p_{\\text{friction}} = 0.02 \\times \\frac{10}{0.267} \\times 7.23 = 5.43\\ \\text{Pa}, \\qquad \\Delta p_{\\text{fittings}} = 1 \\times 7.23 = 7.23\\ \\text{Pa} \\]<\/p>\n<p><strong>Result:<\/strong> total pressure drop = 5.43 + 7.23 = 12.66 Pa \u2248 1.29 mm H\u2082O \u2248 0.051 in. w.g., at an air velocity of 3.47 m\/s.<\/p>\n<p>\n\r\n<\/section>\n<h2>How to Use the Calculator<\/h2>\n<ol>\n<li>Select the <strong>Duct shape<\/strong> \u2014 Rectangular or Circular. The width, height and diameter fields switch automatically.<\/li>\n<li>For a rectangular duct, enter the <strong>Duct width (W)<\/strong> and <strong>Duct height (H)<\/strong> with their units (mm or in). For a circular duct, enter the <strong>Duct diameter (D)<\/strong>.<\/li>\n<li>Enter the <strong>Duct length (L)<\/strong> in metres or feet \u2014 the straight-run length only.<\/li>\n<li>Enter the <strong>Airflow (Q)<\/strong> in m\u00b3\/h or CFM.<\/li>\n<li>Check the <strong>Air density (\u03c1)<\/strong>; the default 1.2 kg\/m\u00b3 corresponds to air at about 20 \u00b0C. Adjust for hot, cold or high-altitude air.<\/li>\n<li>Set the <strong>Friction factor (f)<\/strong>. A value of 0.015\u20130.025 is typical for galvanized steel ductwork.<\/li>\n<li>Enter the <strong>Fitting loss coefficient (\u03a3K)<\/strong> \u2014 the sum of the K-values of all bends, tees, dampers and grilles in the section. Use 0 for a straight duct with no fittings.<\/li>\n<\/ol>\n<p>The results show the air velocity, the friction loss along the straight duct, the fitting loss, and the total pressure drop in Pa, mm H\u2082O and inches of water gauge.<\/p>\n<h2>What Is Duct Pressure Drop?<\/h2>\n<p>As air moves through a duct, it loses energy to friction against the duct walls and to turbulence at every change of direction, junction or restriction. This lost energy appears as a drop in static pressure along the duct, and the fan must make it up to keep the air flowing. Duct pressure drop \u2014 also called duct pressure loss or friction loss \u2014 is therefore one of the key numbers in sizing ventilation systems: too much pressure drop means an oversized, noisy, energy-hungry fan, while underestimating it means poor airflow at the grilles.<\/p>\n<p>The Darcy\u2013Weisbach equation used here splits the loss into two parts. The friction term grows with the length-to-hydraulic-diameter ratio: a long, narrow duct loses far more pressure than a short, wide one carrying the same airflow. The fitting term accounts for local losses, each fitting contributing a multiple of the velocity pressure according to its K-value. Since both terms scale with \\(v^2\\), halving the duct cross-section roughly quadruples the pressure drop at a given airflow \u2014 the single most common cause of underperforming systems.<\/p>\n<p>The hydraulic diameter is what makes a rectangular duct comparable to a round one. A flat, wide duct has a smaller hydraulic diameter than its area suggests, so it suffers higher friction per metre than a square or circular duct of the same cross-section. That is why round ducts are usually the most efficient shape when space allows.<\/p>\n<p>To judge whether your result is reasonable, check the air velocity: comfort ventilation typically runs at 3\u20135 m\/s in main ducts and below 2 m\/s near occupied spaces, while industrial systems may reach 10 m\/s or more. Pressure drops of about 1 Pa per metre of duct are a common design target for low-energy systems.<\/p>\n<h2>Assumptions, Limitations and Practical Tips<\/h2>\n<ul>\n<li>The calculation assumes incompressible, steady airflow with uniform properties along the duct \u2014 valid for normal ventilation pressures (a few hundred pascals).<\/li>\n<li>The friction factor is a fixed input, not computed from the surface roughness and Reynolds number. For most galvanized steel ducts, 0.02 is a safe value; flexible ducts can be significantly worse and may need f = 0.03 or higher. To check the flow regime, calculate the <a href=\"https:\/\/wpcalc.com\/en\/engineering\/reynolds-number\/\">Reynolds number<\/a> for your duct.<\/li>\n<li>Each fitting must be counted once in \u03a3K. Typical K-values: 90\u00b0 smooth bend 0.2\u20130.3, sharp 90\u00b0 elbow 0.8\u20131.2, branch tee 0.5\u20131.5, grille or diffuser 1\u20133, sudden expansion 0.5\u20131. Check manufacturer data where accuracy matters.<\/li>\n<li>The tool computes one duct section with constant dimensions and airflow. Real systems have tapered ducts and branches; split the network into sections, calculate each one, and sum the losses along the critical path (usually the longest or most resistive run).<\/li>\n<li>Leakage, crushed flexible sections, dirty filters and fan entry\/exit losses are not included. A design margin of 10\u201320 % on the calculated total is common practice.<\/li>\n<li>The same Darcy\u2013Weisbach method applies to liquids in pipes \u2014 see the <a href=\"https:\/\/wpcalc.com\/en\/engineering\/pipe-pressure-drop\/\">Pipe Pressure Drop calculator<\/a> \u2014 and once you know the total pressure loss and airflow, you can size the fan with the <a href=\"https:\/\/wpcalc.com\/en\/engineering\/pump-power\/\">Pump Power calculator<\/a>.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Estimate the pressure drop in a rectangular or circular ventilation duct from its size, length, airflow and fittings. Enter duct dimensions, airflow rate and friction factor to get friction loss, fitting loss and total pressure drop in Pa, mm H2O and in. w.g.<\/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":[4],"tags":[39],"class_list":["post-66720","post","type-post","status-publish","format-standard","hentry","category-engineering","tag-ventilation"],"acf":[],"_links":{"self":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66720","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=66720"}],"version-history":[{"count":1,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66720\/revisions"}],"predecessor-version":[{"id":66721,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66720\/revisions\/66721"}],"wp:attachment":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/media?parent=66720"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/categories?post=66720"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/tags?post=66720"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}