{"id":66512,"date":"2026-10-06T17:39:14","date_gmt":"2026-10-06T14:39:14","guid":{"rendered":"https:\/\/wpcalc.com\/en\/?p=66512"},"modified":"2026-10-06T17:39:14","modified_gmt":"2026-10-06T14:39:14","slug":"isoelectric-point-calculator","status":"publish","type":"post","link":"https:\/\/wpcalc.com\/en\/chemistry\/isoelectric-point-calculator\/","title":{"rendered":"Isoelectric Point 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\">Find the pH of Zero Net Charge<\/h2><\/div><div class=\"calcdash-favorite group\">\n  <button type=\"button\" class=\"favorite\" id=\"favorite\" data-favorite-id=\"66512\" data-favorite-title=\"Isoelectric Point\" data-favorite-url=\"https:\/\/wpcalc.com\/en\/chemistry\/isoelectric-point-calculator\/\" data-favorite-excerpt=\"Enter the pKa values of a molecule&#039;s acidic and basic groups to find its isoelectric point (pI) \u2014 the pH at which the net charge is zero. Based on Henderson-Hasselbalch charge fractions, with a per-group charge breakdown.\" aria-label=\"Add to Favorites\" data-favorite-icon=\"icon icon-biology\">\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\/chemistry\/isoelectric-point-calculator\/\" method=\"POST\" class=\"calculator\" id=\"calculator-66512\" data-post=\"66512\"><fieldset class=\"fieldset-input is-1\"><legend class=\"sr-only\">Input Fields<\/legend><div class=\"is-single field\" id=\"input-1\"><span class=\"font-semibold\">Acidic Groups (negative when deprotonated)<\/span><\/div><div class=\"field\" id=\"input-2\"><label for=\"field-2\">Acidic pKa Values<\/label><textarea id=\"field-2\" name=\"acidic_pka_values\">2.10; 3.86<\/textarea><\/div><div class=\"is-single field\" id=\"input-3\"><span class=\"font-semibold\">Basic Groups (positive when protonated)<\/span><\/div><div class=\"field\" id=\"input-4\"><label for=\"field-4\">Basic pKa Values<\/label><textarea id=\"field-4\" name=\"basic_pka_values\">9.82<\/textarea><\/div><\/fieldset><div class=\"buttons\"><button type=\"submit\" data-text=\"Re-Calculate\" id=\"calculate-button\" data-post=\"66512\">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> Isoelectric Point Results<\/h3><div class=\"result-box\"><div class=\"field-result\" id=\"output-1\"><span class=\"field-title\"><span>Estimated Isoelectric Point<\/span><span class=\"result-term\">pI<\/span><\/span><span class=\"field-value\" id=\"result-1\" data-unit=\"pH\"><\/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>Net Charge at pI<\/span><span class=\"result-term\">Z(pI)<\/span><\/span><span class=\"field-value\" id=\"result-2\"><\/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-3\"><span class=\"field-title\"><span>Calculation Details<\/span><\/span><span class=\"field-value\" id=\"result-3\"><\/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=\"66512\" 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>Isoelectric Point 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(\\mathrm{pH}) \\;=\\; \\sum_{\\text{basic}} \\frac{1}{1+10^{\\,\\mathrm{pH}-pK_a}} \\;-\\; \\sum_{\\text{acidic}} \\frac{1}{1+10^{\\,pK_a-\\mathrm{pH}}} \\;=\\; 0 \\]\n<\/div>\r\n\t<\/figure>\r\n\t<\/p>\n<p><strong>Where:<\/strong><\/p>\n<ul>\n<li>\\(Z(\\mathrm{pH})\\) \u2014 net charge of the molecule at a given pH<\/li>\n<li>\\(pK_a\\) \u2014 dissociation constant of each ionizable group (acidic or basic)<\/li>\n<li>\\(\\mathrm{pI}\\) \u2014 the pH at which the sum of all partial charges equals zero<\/li>\n<\/ul>\n<p>Each fraction is the Henderson\u2013Hasselbalch charge contribution of one group: an acidic group contributes between 0 and \\(-1\\), a basic group between 0 and \\(+1\\), depending on how far the pH sits from that group's pKa. The pI is found by solving the equation numerically.<\/p>\n<p>For a molecule with only one group of each type (a simple amino acid without ionizable side chains), the equation reduces to the well-known textbook rule:<\/p>\n<p>\\[ \\mathrm{pI} \\;=\\; \\frac{pK_a(\\text{lower}) + pK_a(\\text{higher})}{2} \\]<\/p>\n<p>where the two pKa values are those flanking the neutral (zwitterionic) form.<\/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>Isoelectric Point \u2013 Calculation Example<\/h2><\/div>\r\n\r\n<\/p>\n<p><strong>Example:<\/strong> aspartic acid has three ionizable groups: the C-terminal carboxyl (pKa 2.10), the acidic side-chain carboxyl (pKa 3.86), and the N-terminal amine (pKa 9.82). This is the default set of values in the calculator.<\/p>\n<p>\\[ Z(\\mathrm{pH}) = \\frac{1}{1+10^{\\,\\mathrm{pH}-9.82}} - \\frac{1}{1+10^{\\,2.10-\\mathrm{pH}}} - \\frac{1}{1+10^{\\,3.86-\\mathrm{pH}}} \\]<\/p>\n<p>Solving \\(Z(\\mathrm{pH}) = 0\\) by bisection over the range pH 0\u201314:<\/p>\n<p>\\[ \\mathrm{pI} \\approx 2.98 \\]<\/p>\n<p>At pH 2.98 the individual charges are: C-terminal carboxyl \\(\\approx -0.883\\), side-chain carboxyl \\(\\approx -0.116\\), N-terminal amine \\(\\approx +1.000\\). They sum to zero, confirming the result.<\/p>\n<p><strong>Result:<\/strong> pI \u2248 2.98 (net charge 0.0000). Note that this equals the average of the two pKa values flanking the neutral form, \\((2.10 + 3.86)\/2 = 2.98\\), since the N-terminal amine stays fully protonated in this pH region.<\/p>\n<p>\n\r\n<\/section>\n<h2>How to Use the Isoelectric Point Calculator<\/h2>\n<ol>\n<li>List the <strong>Acidic pKa Values<\/strong> \u2014 every group that becomes negatively charged when deprotonated: the C-terminal carboxyl plus side chains such as Asp (\u2248 3.9), Glu (\u2248 4.2), Cys (\u2248 8.3) and Tyr (\u2248 10.1). Separate values with spaces, semicolons, commas or new lines.<\/li>\n<li>List the <strong>Basic pKa Values<\/strong> \u2014 every group that is positive when protonated: the N-terminal amine plus side chains such as His (\u2248 6.0), Lys (\u2248 10.5) and Arg (\u2248 12.5).<\/li>\n<li>Read the <strong>Estimated Isoelectric Point<\/strong> (the pH of zero net charge) and the <strong>Net Charge at pI<\/strong>, which should be 0 or negligibly close to it.<\/li>\n<li>Check the <strong>Calculation Details<\/strong> table to see the partial charge of every individual group at the pI \u2014 a quick way to verify that all your pKa values were entered correctly.<\/li>\n<\/ol>\n<h2>What Is the Isoelectric Point?<\/h2>\n<p>The isoelectric point (pI) is the pH at which a molecule \u2014 typically an amino acid, peptide or protein \u2014 carries no net electrical charge. Below the pI the molecule is overall positive; above it, overall negative. At the pI itself the positive and negative charges balance exactly: the species exists mostly as a zwitterion or as a set of charge states whose contributions cancel.<\/p>\n<p>Ionizable groups on a molecule do not switch on and off abruptly. Each one transitions gradually around its own pKa, following the Henderson\u2013Hasselbalch relationship (see the <a href=\"https:\/\/wpcalc.com\/en\/chemistry\/henderson-hasselbalch-calculator\/\">Henderson\u2013Hasselbalch calculator<\/a> for the underlying equation). This calculator adds up the fractional charge of every group at each pH and locates the point where the total crosses zero \u2014 the same way research tools estimate peptide pI values.<\/p>\n<p>The pI matters because charge governs behavior. A molecule at its pI has no electrostatic repulsion, so it is least soluble and most likely to precipitate or aggregate \u2014 a fact exploited in protein crystallization and in isoelectric focusing, where proteins migrate in a pH gradient until they stop at their pI. The pI also determines electrophoretic mobility and helps predict which peptides will bind to ion-exchange resins at a given buffer pH. If you work with buffer systems, the <a href=\"https:\/\/wpcalc.com\/en\/chemistry\/ph-calculator\/\">pH calculator<\/a> is a useful companion for setting up the solutions themselves.<\/p>\n<h2>Interpreting the Results<\/h2>\n<p>The <strong>Estimated Isoelectric Point<\/strong> is the pH of zero net charge. The <strong>Calculation Details<\/strong> table shows each group's partial charge at that pH: values near \u00b11 mean the group is essentially fully ionized, values near 0 mean it is essentially neutral, and intermediate values mark groups that straddle the pI \u2014 these are the ones that actually define it.<\/p>\n<p>For simple amino acids the result usually matches the textbook average of the two pKa values bracketing the neutral form. For peptides and molecules with several overlapping pKas, the numerical solution used here is more accurate than any simple average, because it accounts for all groups simultaneously.<\/p>\n<h2>Assumptions and Limitations<\/h2>\n<ul>\n<li><strong>Independent protonation:<\/strong> each group is assumed to ionize independently, without influencing its neighbors. This holds well for amino acids and short peptides.<\/li>\n<li><strong>Standard pKa values:<\/strong> tabulated pKa values (typically measured at 25 \u00b0C) are used as given. In a real protein, the local environment can shift a side chain's pKa by one or more units, so computed pI values for large proteins are estimates only.<\/li>\n<li><strong>Physiological range:<\/strong> the calculator searches for the zero-charge point between pH 0 and 14 and reports an error if the net charge never crosses zero in that range.<\/li>\n<li><strong>Ionic strength effects:<\/strong> activity coefficients are ignored. At high salt concentrations the effective pKa values \u2014 and hence the pI \u2014 shift; see the <a href=\"https:\/\/wpcalc.com\/en\/chemistry\/ionic-strength-calculator\/\">Ionic Strength calculator<\/a> to gauge how strong your buffer is.<\/li>\n<li><strong>Enter every ionizable group:<\/strong> forgetting a side chain (for example the single His in a peptide) is the most common source of wrong pI values. The details table makes missing groups easy to spot.<\/li>\n<\/ul>\n<h2>Typical pKa Reference Values<\/h2>\n<table>\n<thead>\n<tr>\n<th>Group<\/th>\n<th>pKa (approx.)<\/th>\n<th>Type<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>C-terminal carboxyl<\/td>\n<td>2.0\u20132.2<\/td>\n<td>Acidic<\/td>\n<\/tr>\n<tr>\n<td>Asp side chain<\/td>\n<td>3.9<\/td>\n<td>Acidic<\/td>\n<\/tr>\n<tr>\n<td>Glu side chain<\/td>\n<td>4.2<\/td>\n<td>Acidic<\/td>\n<\/tr>\n<tr>\n<td>His side chain<\/td>\n<td>6.0<\/td>\n<td>Basic<\/td>\n<\/tr>\n<tr>\n<td>Cys side chain<\/td>\n<td>8.3<\/td>\n<td>Acidic<\/td>\n<\/tr>\n<tr>\n<td>N-terminal amine<\/td>\n<td>9.0\u20139.8<\/td>\n<td>Basic<\/td>\n<\/tr>\n<tr>\n<td>Tyr side chain<\/td>\n<td>10.1<\/td>\n<td>Acidic<\/td>\n<\/tr>\n<tr>\n<td>Lys side chain<\/td>\n<td>10.5<\/td>\n<td>Basic<\/td>\n<\/tr>\n<tr>\n<td>Arg side chain<\/td>\n<td>12.5<\/td>\n<td>Basic<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Use these values as a starting point; always substitute measured or literature pKa values for your specific molecule when accuracy matters. For acid\u2013base behavior of the buffer itself, the <a href=\"https:\/\/wpcalc.com\/en\/chemistry\/acid-base-titration-calculator\/\">Titration calculator<\/a> shows how the pKa region appears in practice.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Enter the pKa values of a molecule's acidic and basic groups to find its isoelectric point (pI) \u2014 the pH at which the net charge is zero. Based on Henderson-Hasselbalch charge fractions, with a per-group charge breakdown.<\/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":[54],"tags":[60],"class_list":["post-66512","post","type-post","status-publish","format-standard","hentry","category-chemistry","tag-biochemistry"],"acf":[],"_links":{"self":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66512","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=66512"}],"version-history":[{"count":1,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66512\/revisions"}],"predecessor-version":[{"id":66513,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66512\/revisions\/66513"}],"wp:attachment":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/media?parent=66512"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/categories?post=66512"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/tags?post=66512"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}