{"id":66746,"date":"2026-10-13T08:52:58","date_gmt":"2026-10-13T05:52:58","guid":{"rendered":"https:\/\/wpcalc.com\/en\/?p=66746"},"modified":"2026-10-13T08:52:58","modified_gmt":"2026-10-13T05:52:58","slug":"heat-index-calculator","status":"publish","type":"post","link":"https:\/\/wpcalc.com\/en\/converters\/heat-index-calculator\/","title":{"rendered":"Heat Index 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\">How Hot Does It Feel? Enter Temperature and Humidity<\/h2><\/div><div class=\"calcdash-favorite group\">\n  <button type=\"button\" class=\"favorite\" id=\"favorite\" data-favorite-id=\"66746\" data-favorite-title=\"Heat Index\" data-favorite-url=\"https:\/\/wpcalc.com\/en\/converters\/heat-index-calculator\/\" data-favorite-excerpt=\"Enter air temperature (\u00b0F or \u00b0C) and relative humidity to find the heat index \u2014 how hot it actually feels \u2014 with the matching NWS heat risk level.\" aria-label=\"Add to Favorites\" data-favorite-icon=\"icon icon-physic\">\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\/converters\/heat-index-calculator\/\" method=\"POST\" class=\"calculator\" id=\"calculator-66746\" data-post=\"66746\"><fieldset class=\"fieldset-input is-1\"><legend class=\"sr-only\">Input Fields<\/legend><div class=\"field has-term\" id=\"input-1\"><label for=\"field-1\">Air temperature (T)<\/label><div class=\"term\">T<\/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=\"air_temperature_t\" id=\"field-1\" aria-describedby=\"calc-info-1\" placeholder=\"e.g. 90\" min=\"-80\" step=\"any\" value=\"90\"\/><div class=\"grid grid-cols-1\"><select id=\"field-2\" name=\"unit\" aria-label=\"Unit\"><option value=\"f\" selected>\u00b0F<\/option><option value=\"c\">\u00b0C<\/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\">Shaded air temperature as measured by a standard thermometer, not in direct sun.<\/small><\/div><div class=\"field has-term\" id=\"input-2\"><label for=\"field-3\">Relative humidity (RH)<\/label><div class=\"term\">RH<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"relative_humidity_rh\" id=\"field-3\" aria-describedby=\"calc-unit-3 calc-info-3\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" placeholder=\"e.g. 60\" min=\"0\" max=\"100\" value=\"60\"\/><div class=\"unit\" id=\"calc-unit-3\">%<\/div><small id=\"calc-info-3\">Percentage of moisture in the air relative to saturation (0\u2013100%).<\/small><\/div><\/fieldset><div class=\"buttons\"><button type=\"submit\" data-text=\"Re-Calculate\" id=\"calculate-button\" data-post=\"66746\">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> Heat Index Results<\/h3><div class=\"result-box\"><div class=\"field-result is-column\" id=\"output-1\"><span class=\"field-title\"><span>Heat index (how hot it feels)<\/span><span class=\"result-term\">HI<\/span><\/span><span class=\"field-value\" id=\"result-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-2\"><span class=\"field-title\"><span>Heat risk level and calculation details<\/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><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=\"66746\" 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>Heat Index 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\\[ \\mathrm{HI} = -42.379 + 2.04901523\\,T + 10.14333127\\,\\mathrm{RH} - 0.22475541\\,T\\,\\mathrm{RH} - 0.00683783\\,T^2 - 0.05481717\\,\\mathrm{RH}^2 \\\\ + 0.00122874\\,T^2\\mathrm{RH} + 0.00085282\\,T\\,\\mathrm{RH}^2 - 0.00000199\\,T^2\\mathrm{RH}^2 \\]\n<\/div>\r\n\t<\/figure>\r\n\t<\/p>\n<p><strong>Where:<\/strong><\/p>\n<ul>\n<li>\\(T\\) \u2014 air temperature in degrees Fahrenheit (\u00b0F)<\/li>\n<li>\\(\\mathrm{RH}\\) \u2014 relative humidity in percent (%)<\/li>\n<li>\\(\\mathrm{HI}\\) \u2014 heat index in degrees Fahrenheit (\u00b0F)<\/li>\n<\/ul>\n<p>This is the Rothfusz regression used by the US National Weather Service (NWS). It is applied only when conditions are hot enough to matter; for cooler conditions the NWS first evaluates a simpler approximation,<\/p>\n<p>\\[ \\mathrm{HI} = 0.5\\left(T + 61.0 + (T - 68.0) \\cdot 1.2 + \\mathrm{RH} \\cdot 0.094\\right) \\]<\/p>\n<p>and switches to the full regression only if the average of this value and \\(T\\) is 80\u00b0F or higher. Two small corrections are added at very low humidity (below 13%) or very high humidity (above 85%) in the 80\u2013112\u00b0F range.<\/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>Heat Index \u2013 Calculation Example<\/h2><\/div>\r\n\r\n<\/p>\n<p><strong>Example:<\/strong> the air temperature is 90\u00b0F and the relative humidity is 60% \u2014 a typical humid summer afternoon (these are the calculator's default values).<\/p>\n<p>The simple formula first gives \\(\\mathrm{HI} \\approx 91.5^\\circ\\mathrm{F}\\). Since \\((91.5 + 90)\/2 \\approx 90.8 \\ge 80\\), the full Rothfusz regression is used:<\/p>\n<p>\\[ \\mathrm{HI} = -42.379 + 2.04901523(90) + 10.14333127(60) - 0.22475541(90)(60) \\\\ - 0.00683783(90)^2 - 0.05481717(60)^2 + 0.00122874(90)^2(60) \\\\ + 0.00085282(90)(60)^2 - 0.00000199(90)^2(60)^2 \\approx 99.7^\\circ\\mathrm{F} \\]<\/p>\n<p><strong>Result:<\/strong> the heat index is about <strong>99.7\u00b0F (37.6\u00b0C)<\/strong> \u2014 an \"Extreme caution\" level, where heat cramps and heat exhaustion are possible with prolonged exposure and physical activity.<\/p>\n<p>\n\r\n<\/section>\n<h2>How to Use the Heat Index Calculator<\/h2>\n<ol>\n<li>Enter the <strong>Air temperature (T)<\/strong> in the field. This should be the shaded air temperature measured by a standard thermometer, not in direct sun.<\/li>\n<li>Choose the unit \u2014 <strong>\u00b0F<\/strong> or <strong>\u00b0C<\/strong> \u2014 from the dropdown next to the temperature field. Celsius values are converted internally, since the NWS equations are defined in Fahrenheit.<\/li>\n<li>Enter the <strong>Relative humidity (RH)<\/strong> as a percentage between 0 and 100. If you only know the dew point, many weather apps and reports list relative humidity directly.<\/li>\n<li>Read the results: the heat index in both Fahrenheit and Celsius, the heat risk level, and details about which formula and validity range were used.<\/li>\n<\/ol>\n<h2>What Is the Heat Index?<\/h2>\n<p>The heat index is the \"feels-like\" temperature: what the combination of heat and humidity feels like to the human body, compared with an equivalent dry-air temperature. On a 90\u00b0F day with 60% humidity, the air does not feel like 90\u00b0F \u2014 it feels closer to 100\u00b0F, because your body cannot cool itself as effectively as it could in drier air.<\/p>\n<p>The reason is evaporative cooling. Your body's main defense against heat is sweating: as sweat evaporates from your skin, it carries heat away. When the air is already nearly saturated with moisture, sweat evaporates slowly or not at all, so this cooling mechanism fails and body temperature rises. That is why a dry 100\u00b0F day in the desert can be more tolerable than a humid 90\u00b0F day in the Southeast.<\/p>\n<p>Because it reflects real physiological stress rather than just thermometer readings, the heat index is the number public health agencies use for heat warnings. The NWS classifies heat index values into risk bands, each with recommended precautions:<\/p>\n<table>\n<thead>\n<tr>\n<th>Heat index<\/th>\n<th>Category<\/th>\n<th>Main effect<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>80\u201390\u00b0F (27\u201332\u00b0C)<\/td>\n<td>Caution<\/td>\n<td>Fatigue possible with prolonged exposure and activity<\/td>\n<\/tr>\n<tr>\n<td>90\u2013103\u00b0F (32\u201339\u00b0C)<\/td>\n<td>Extreme caution<\/td>\n<td>Heat cramps and heat exhaustion possible<\/td>\n<\/tr>\n<tr>\n<td>103\u2013125\u00b0F (39\u201352\u00b0C)<\/td>\n<td>Danger<\/td>\n<td>Heat exhaustion likely; heat stroke possible<\/td>\n<\/tr>\n<tr>\n<td>\u2265 125\u00b0F (\u2265 52\u00b0C)<\/td>\n<td>Extreme danger<\/td>\n<td>Heat stroke imminent with continued exposure<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>A heat index below 80\u00b0F is generally comfortable for most people, which is why the full regression is only applied above that threshold.<\/p>\n<h2>Assumptions, Limitations and Practical Tips<\/h2>\n<ul>\n<li><strong>Shade and light wind.<\/strong> The heat index assumes you are in the shade with a light breeze. Full sunshine can make it feel up to 15\u00b0F (about 8\u00b0C) hotter than the calculated value.<\/li>\n<li><strong>Valid range.<\/strong> The Rothfusz regression is empirically valid for temperatures of 80\u2013112\u00b0F and humidities of 20\u201399%. Outside these ranges the result is an estimate, and below 80\u00b0F the simplified formula simply returns a value close to the air temperature.<\/li>\n<li><strong>It describes typical adults<\/strong> in light clothing. Sensitivity to heat varies with age, fitness, hydration, medication, and acclimatization \u2014 children, older adults, and people with chronic conditions can experience heat stress at lower heat index values.<\/li>\n<li><strong>Not a wind-chill equivalent.<\/strong> The heat index applies to hot weather. In cold and windy conditions, a different model (wind chill) is used.<\/li>\n<li><strong>Watch the trend, not just the number.<\/strong> A jump in humidity overnight can push an unchanged air temperature into a higher risk category the next day, so re-check the heat index when planning outdoor work or exercise.<\/li>\n<\/ul>\n<p>For other unit conversions you may need alongside weather and science work, try our <a href=\"https:\/\/wpcalc.com\/en\/converters\/ppm-to-percent-mg-l-converter\/\">PPM to percent converter<\/a> (useful for moisture and concentration figures) or the <a href=\"https:\/\/wpcalc.com\/en\/converters\/time-units-converter\/\">time units converter<\/a> when tracking exposure durations. If you work with heat transfer calculations, the <a href=\"https:\/\/wpcalc.com\/en\/converters\/thermal-conductivity\/\">thermal conductivity converter<\/a> may also come in handy.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Enter air temperature (\u00b0F or \u00b0C) and relative humidity to find the heat index \u2014 how hot it actually feels \u2014 with the matching NWS heat risk level.<\/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":[3],"tags":[34],"class_list":["post-66746","post","type-post","status-publish","format-standard","hentry","category-converters","tag-physics-units"],"acf":[],"_links":{"self":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66746","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=66746"}],"version-history":[{"count":1,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66746\/revisions"}],"predecessor-version":[{"id":66747,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66746\/revisions\/66747"}],"wp:attachment":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/media?parent=66746"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/categories?post=66746"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/tags?post=66746"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}