{"id":64443,"date":"2025-05-12T09:29:55","date_gmt":"2025-05-12T06:29:55","guid":{"rendered":"https:\/\/wpcalc.com\/en\/?p=64443"},"modified":"2026-09-30T18:26:54","modified_gmt":"2026-09-30T15:26:54","slug":"microstrip-differential-impedance","status":"publish","type":"post","link":"https:\/\/wpcalc.com\/en\/engineering\/microstrip-differential-impedance\/","title":{"rendered":"Microstrip PCB Differential Impedance 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\">Differential Impedance Estimator for Microstrip Traces<\/h2><\/div><div class=\"calcdash-favorite group\">\n  <button type=\"button\" class=\"favorite\" id=\"favorite\" data-favorite-id=\"64443\" data-favorite-title=\"Microstrip Differential Impedance\" data-favorite-url=\"https:\/\/wpcalc.com\/en\/engineering\/microstrip-differential-impedance\/\" data-favorite-excerpt=\"Estimate the differential impedance of an edge-coupled microstrip pair on a PCB by entering the single-ended impedance, trace spacing, and substrate height \u2014 essential for USB, HDMI, LVDS, and PCIe routing.\" aria-label=\"Add to Favorites\" data-favorite-icon=\"icon icon-engineering\">\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\/microstrip-differential-impedance\/\" method=\"POST\" class=\"calculator\" id=\"calculator-64443\" data-post=\"64443\"><fieldset class=\"fieldset-input\"><legend class=\"sr-only\">Input Fields<\/legend><div class=\"field has-term\" id=\"input-1\"><label for=\"field-1\">Single-ended impedance<\/label><div class=\"term\">Zo<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"singleended_impedance\" id=\"field-1\" aria-describedby=\"calc-unit-1 calc-info-1\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"1\" value=\"50\"\/><div class=\"unit\" id=\"calc-unit-1\">\u03a9<\/div><small id=\"calc-info-1\">Impedance of a single microstrip trace<\/small><\/div><div class=\"field has-term\" id=\"input-2\"><label for=\"field-2\">Trace spacing<\/label><div class=\"term\">s<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"trace_spacing\" id=\"field-2\" aria-describedby=\"calc-unit-2 calc-info-2\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"0.01\" value=\"0.15\"\/><div class=\"unit\" id=\"calc-unit-2\">mm<\/div><small id=\"calc-info-2\">Spacing between the two differential traces<\/small><\/div><div class=\"field has-term\" id=\"input-3\"><label for=\"field-3\">Substrate height<\/label><div class=\"term\">h<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"substrate_height\" id=\"field-3\" aria-describedby=\"calc-unit-3 calc-info-3\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"0.01\" value=\"1.6\"\/><div class=\"unit\" id=\"calc-unit-3\">mm<\/div><small id=\"calc-info-3\">Height from trace to reference ground plane<\/small><\/div><\/fieldset><div class=\"buttons\"><button type=\"submit\" data-text=\"Re-Calculate\" id=\"calculate-button\" data-post=\"64443\">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> Your Results<\/h3><div class=\"result-box\"><div class=\"field-result\" id=\"output-1\"><span class=\"field-title\"><span>Differential Impedance<\/span><\/span><span class=\"field-value\" id=\"result-1\" data-unit=\"\u03a9\"><\/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=\"64443\" 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>Microstrip Differential Impedance 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\\[ Z_{\\text{diff}} = 2Z_0\\left(1 - 0.48\\,e^{-0.96\\,\\frac{s}{h}}\\right) \\]\n<\/div>\r\n\t<\/figure>\r\n\t<\/p>\n<p><strong>Where:<\/strong><\/p>\n<ul>\n<li>\\(Z_{\\text{diff}}\\) \u2014 differential impedance of the pair (\u03a9)<\/li>\n<li>\\(Z_0\\) \u2014 single-ended impedance of one isolated microstrip trace (\u03a9)<\/li>\n<li>\\(s\\) \u2014 spacing between the two traces of the pair (mm)<\/li>\n<li>\\(h\\) \u2014 height of the dielectric substrate between the traces and the ground plane (mm)<\/li>\n<\/ul>\n<p>This empirical expression describes a symmetrical, edge-coupled microstrip pair over a reference plane. As the traces move closer together (smaller \\(s\/h\\)), the coupling between them increases and the differential impedance falls below twice the single-ended value.<\/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>Microstrip Differential Impedance \u2013 Calculation Example<\/h2><\/div>\r\n\r\n<\/p>\n<p><strong>Example:<\/strong> a pair of 50 \u03a9 microstrip traces on a 1.6 mm thick substrate, spaced 0.15 mm apart.<\/p>\n<p>\\[ Z_{\\text{diff}} = 2 \\cdot 50 \\cdot \\left(1 - 0.48\\,e^{-0.96 \\cdot \\frac{0.15}{1.6}}\\right) \\]<\/p>\n<p>\\[ = 100 \\cdot \\left(1 - 0.48\\,e^{-0.09}\\right) = 100 \\cdot \\left(1 - 0.48 \\cdot 0.9139\\right) \\]<\/p>\n<p>\\[ = 100 \\cdot 0.5613 \\approx 56.13~\\Omega \\]<\/p>\n<p><strong>Result:<\/strong> the differential impedance of the pair is approximately 56.13 \u03a9.<\/p>\n<p>\n\r\n<\/section>\n<h2>How to use the calculator<\/h2>\n<ol>\n<li>Enter the <strong>Single-ended impedance<\/strong> of one trace in ohms. If you do not know it yet, compute it first with the trace geometry using the <a href=\"https:\/\/wpcalc.com\/en\/engineering\/microstrip-pcb-impedance\/\">Microstrip PCB Impedance<\/a> calculator.<\/li>\n<li>Enter the <strong>Trace spacing<\/strong> \\(s\\) in mm \u2014 the edge-to-edge gap between the two traces of the pair.<\/li>\n<li>Enter the <strong>Substrate height<\/strong> \\(h\\) in mm \u2014 the distance from the trace layer to the reference ground plane.<\/li>\n<li>Read the <strong>Differential impedance<\/strong> result in ohms, shown to two decimal places.<\/li>\n<\/ol>\n<h2>What is microstrip differential impedance?<\/h2>\n<p>A differential pair carries one signal on two traces with equal and opposite voltages. The receiver responds only to the <em>difference<\/em> between the two lines, so the impedance that matters for termination and reflections is the <strong>differential impedance<\/strong> \\(Z_{\\text{diff}}\\) \u2014 the impedance seen between the two traces of the pair.<\/p>\n<p>For microstrips (traces on an outer layer, with air above and dielectric below the copper), the two traces of a pair couple to each other as well as to the ground plane. Because of this mutual coupling, the differential impedance is always <em>less<\/em> than \\(2Z_0\\). The tighter the spacing relative to the substrate height, the stronger the coupling and the lower the differential impedance. The formula used here captures that effect: at large spacing (\\(s \\gg h\\)) the coupling term vanishes and \\(Z_{\\text{diff}} \\to 2Z_0\\), while tightly coupled pairs can land well below that value.<\/p>\n<p>Getting this number right matters because most high-speed serial standards specify a differential impedance target \u2014 typically 90 \u03a9 for USB 2.0\/3.0, 100 \u03a9 for Ethernet pairs, LVDS, PCIe, and many HDMI lanes. If the actual board impedance deviates from the target, part of the signal reflects at the receiver and driver, causing overshoot, ringing, jitter, and increased EMI. Poor impedance control is one of the most common causes of failed eye-diagram and return-loss tests.<\/p>\n<p>Note that this calculator takes the <em>single-ended<\/em> impedance as an input rather than the trace width and dielectric constant. This makes it flexible: you can obtain \\(Z_0\\) from your stackup calculator, a field solver, or a measurement, then use this tool to see how a chosen spacing changes the pair impedance.<\/p>\n<h2>Assumptions, limitations, and practical tips<\/h2>\n<ul>\n<li><strong>Symmetrical pair.<\/strong> The formula assumes two identical traces of equal width, routed side by side at the same height above an unbroken ground plane. Asymmetric pairs behave differently and need a field solver.<\/li>\n<li><strong>Edge coupling only.<\/strong> Only the lateral (edge-to-edge) coupling between the traces is modeled. Broadside-coupled pairs on adjacent layers are not covered.<\/li>\n<li><strong>Empirical accuracy.<\/strong> This is a well-known closed-form approximation, accurate to a few percent for typical geometries. For final sign-off on gigabit-class designs, verify with a 2D field solver or an impedance test coupon.<\/li>\n<li><strong>Choose spacing deliberately.<\/strong> Tight spacing lowers impedance and improves noise immunity, but it also increases insertion loss and crosstalk susceptibility to neighboring aggressors. To evaluate coupling to nearby traces, see the <a href=\"https:\/\/wpcalc.com\/en\/engineering\/pcb-microstrip-crosstalk\/\">PCB Microstrip Crosstalk<\/a> calculator.<\/li>\n<li><strong>Common design workflow:<\/strong> pick a trace width for the target \\(Z_0\\), then tune the pair spacing until \\(Z_{\\text{diff}}\\) matches the specification. Keep the spacing constant along the entire route and avoid vias and layer changes inside a pair whenever possible.<\/li>\n<li><strong>Inner layers differ.<\/strong> If your differential pair runs on an inner layer between two reference planes, use the <a href=\"https:\/\/wpcalc.com\/en\/engineering\/stripline-pcb-differential-impedance\/\">Stripline PCB Differential Impedance<\/a> calculator instead, and the <a href=\"https:\/\/wpcalc.com\/en\/engineering\/stripline-impedance\/\">Stripline Impedance<\/a> calculator for the single-ended value.<\/li>\n<li><strong>Watch manufacturing tolerances.<\/strong> Etch-back, copper roughness, and dielectric thickness tolerance (\u00b110% on prepreg is common) all shift the impedance. Design with margin, and confirm with a coupon if the spec is tight.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Estimate the differential impedance of an edge-coupled microstrip pair on a PCB by entering the single-ended impedance, trace spacing, and substrate height \u2014 essential for USB, HDMI, LVDS, and PCIe routing.<\/p>\n","protected":false},"author":70,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[4],"tags":[92],"class_list":["post-64443","post","type-post","status-publish","format-standard","hentry","category-engineering","tag-rf-microwave"],"acf":[],"_links":{"self":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/64443","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\/70"}],"replies":[{"embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/comments?post=64443"}],"version-history":[{"count":3,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/64443\/revisions"}],"predecessor-version":[{"id":65990,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/64443\/revisions\/65990"}],"wp:attachment":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/media?parent=64443"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/categories?post=64443"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/tags?post=64443"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}