{"id":66768,"date":"2026-10-13T08:58:02","date_gmt":"2026-10-13T05:58:02","guid":{"rendered":"https:\/\/wpcalc.com\/en\/?p=66768"},"modified":"2026-10-13T08:58:02","modified_gmt":"2026-10-13T05:58:02","slug":"rainfall-volume-calculator","status":"publish","type":"post","link":"https:\/\/wpcalc.com\/en\/converters\/rainfall-volume-calculator\/","title":{"rendered":"Rainfall Volume 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 Much Water Does That Rainfall Collect?<\/h2><\/div><div class=\"calcdash-favorite group\">\n  <button type=\"button\" class=\"favorite\" id=\"favorite\" data-favorite-id=\"66768\" data-favorite-title=\"Rainfall Volume\" data-favorite-url=\"https:\/\/wpcalc.com\/en\/converters\/rainfall-volume-calculator\/\" data-favorite-excerpt=\"Calculate how much water a rainfall event delivers onto any roof or surface. Enter the rainfall depth, collection area and an optional roof efficiency to get the volume in liters, US gallons and cubic meters.\" aria-label=\"Add to Favorites\" data-favorite-icon=\"icon icon-volume\">\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\/rainfall-volume-calculator\/\" method=\"POST\" class=\"calculator\" id=\"calculator-66768\" data-post=\"66768\"><fieldset class=\"fieldset-input is-3\"><legend class=\"sr-only\">Input Fields<\/legend><div class=\"field has-term\" id=\"input-1\"><label for=\"field-1\">Rainfall Depth (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=\"rainfall_depth_d\" id=\"field-1\" aria-describedby=\"calc-info-1\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"0\" value=\"25\"\/><div class=\"grid grid-cols-1\"><select id=\"field-2\" name=\"unit\" aria-label=\"Unit\"><option value=\"mm\" selected>mm<\/option><option value=\"cm\">cm<\/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><small id=\"calc-info-1\">Depth of rain that fell, e.g. from a rain gauge or forecast<\/small><\/div><div class=\"field has-term\" id=\"input-2\"><label for=\"field-3\">Collection Area (A)<\/label><div class=\"term\">A<\/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=\"collection_area_a\" id=\"field-3\" aria-describedby=\"calc-info-3\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"0\" value=\"100\"\/><div class=\"grid grid-cols-1\"><select id=\"field-4\" name=\"unit\" aria-label=\"Unit\"><option value=\"m2\" selected>m\u00b2<\/option><option value=\"ft2\">ft\u00b2<\/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-3\">Roof, tarp or ground area the rain falls on \u2014 use the footprint (length \u00d7 width), not the sloped roof surface<\/small><\/div><div class=\"field has-term\" id=\"input-3\"><label for=\"field-5\">Roof Efficiency<\/label><div class=\"term\">\u03b7<\/div>  <div class=\"absolute bottom-0 left-16 top-8 w-2 h-0.5 bg-blue-500\"><\/div><input type=\"text\" name=\"roof_efficiency\" id=\"field-5\" aria-describedby=\"calc-info-5\" data-number inputmode=\"decimal\" autocomplete=\"off\" spellcheck=\"false\" min=\"0\" max=\"100\" value=\"80\"\/><small id=\"calc-info-5\">Optional \u2014 share of rain actually collected (runoff losses, evaporation, gutter spills). Typical roofs: 75\u201390%; leave blank to ignore<\/small><\/div><\/fieldset><div class=\"buttons\"><button type=\"submit\" data-text=\"Re-Calculate\" id=\"calculate-button\" data-post=\"66768\">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>Water Volume in Liters<\/span><span class=\"result-term\">V<\/span><\/span><span class=\"field-value\" id=\"result-1\" data-unit=\"L\"><\/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>Water Volume in US Gallons<\/span><span class=\"result-term\">V<\/span><\/span><span class=\"field-value\" id=\"result-2\" data-unit=\"gal (US)\"><\/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>Water Volume in Cubic Meters<\/span><span class=\"result-term\">V<\/span><\/span><span class=\"field-value\" id=\"result-3\" data-unit=\"m\u00b3\"><\/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-4\"><span class=\"field-title\"><span>Rainwater Harvesting Potential<\/span><span class=\"result-term\">V<\/span><\/span><span class=\"field-value\" id=\"result-4\"><\/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=\"66768\" 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\">1<\/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>Rainfall Volume<\/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\\[ V = A \\times d \\]\n\\[ V_{\\text{collected}} = A \\times d \\times \\frac{\\eta}{100} \\]\n<\/div>\r\n\t<\/figure>\r\n\t<\/p>\n<p><strong>Where:<\/strong><\/p>\n<ul>\n<li>\\(V\\) \u2014 volume of water (m\u00b3)<\/li>\n<li>\\(A\\) \u2014 collection area, the flat footprint of the roof or surface (m\u00b2)<\/li>\n<li>\\(d\\) \u2014 rainfall depth (m)<\/li>\n<li>\\(\\eta\\) \u2014 collection efficiency in percent (optional)<\/li>\n<\/ul>\n<p>The first formula gives the gross volume of rain that falls on the area: since 1 mm of rain over 1 m\u00b2 equals exactly 1 liter, the math is a simple multiplication once units match. The second formula subtracts real-world losses when you specify an efficiency below 100%.<\/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>Rainfall Volume \u2013 Calculation Example<\/h2><\/div>\r\n\r\n<\/p>\n<p><strong>Example:<\/strong> a steady storm drops 25 mm of rain, and you want to know how much water lands on a 100 m\u00b2 roof with 80% collection efficiency (gutter spills, evaporation and wetting losses included).<\/p>\n<p>\\[ V = 100\\ \\text{m}^2 \\times 0.025\\ \\text{m} = 2.5\\ \\text{m}^3 \\]<\/p>\n<p>\\[ V_{\\text{collected}} = 2.5 \\times \\frac{80}{100} = 2.0\\ \\text{m}^3 \\]<\/p>\n<p><strong>Result:<\/strong> 2,000 L (about 528 US gallons), or roughly 10 standard rain barrels of 200 L each.<\/p>\n<p>\n\r\n<\/section>\n<h2>How to Use the Calculator<\/h2>\n<ol>\n<li>Enter the <strong>Rainfall Depth (d)<\/strong> from your rain gauge or weather forecast, and pick the unit: mm, cm or in.<\/li>\n<li>Enter the <strong>Collection Area (A)<\/strong> \u2014 the flat footprint of your roof, tarp or catchment (length \u00d7 width), not the sloped surface \u2014 and choose m\u00b2 or ft\u00b2.<\/li>\n<li>Optionally enter a <strong>Roof Efficiency<\/strong> between 0 and 100. Typical pitched roofs with gutters collect 75\u201390%. Leave this field blank to see the full theoretical volume.<\/li>\n<li>Read the results: water volume in liters, US gallons and cubic meters, plus a rain-barrel comparison that puts the number in context.<\/li>\n<\/ol>\n<h2>What Is Rainfall Volume and Why It Matters<\/h2>\n<p>Rainfall depth alone tells you how wet a storm was, but not how much water you can actually use. A modest 10 mm drizzle on a small house roof can still deliver a thousand liters of free water. Rainfall volume converts a depth reading into a real, harvestable quantity by multiplying it by the area the rain lands on.<\/p>\n<p>The key insight that makes the calculation so clean is the identity <em>1 mm of rain over 1 m\u00b2 = 1 liter<\/em>. So 25 mm over a 50 m\u00b2 roof is 1,250 L before losses. In US units, 1 inch of rain over 1 ft\u00b2 is about 0.623 gallons \u2014 the calculator handles all of these conversions for you.<\/p>\n<p>This number matters in several practical situations: sizing a rainwater harvesting tank, checking whether a drainage system or French drain can handle a storm, estimating irrigation savings for a garden, or comparing the water yield of different roof sections. For rainwater harvesting, the annual potential is simply this calculation repeated with your yearly rainfall total.<\/p>\n<p>The <strong>Rainwater Harvesting Potential<\/strong> line translates the result into standard 200 L (55 gal) rain barrels, which is the most common storage unit for home systems. If you entered an efficiency, that comparison already accounts for it.<\/p>\n<h2>Assumptions, Limitations and Practical Tips<\/h2>\n<ul>\n<li><strong>Use the footprint, not the roof surface.<\/strong> A pitched roof has more surface area than its flat projection, but rain falls vertically, so only the footprint counts. A 10 m \u00d7 8 m house gives 80 m\u00b2 no matter how steep the roof is.<\/li>\n<li><strong>Efficiency losses are real.<\/strong> The first flush of rain wets the roof and washes off dust, some water overshoots gutters in heavy downpours, and a little evaporates. For planning a harvesting system, 75\u201390% is a realistic range; use 100% only for theoretical maximums.<\/li>\n<li><strong>Heavy rain can exceed gutter capacity.<\/strong> During intense cloudbursts, gutters overflow even when the total volume is modest, so actual collection in extreme events may be lower than the calculated figure.<\/li>\n<li><strong>One storm vs. a season.<\/strong> For tank sizing, repeat the calculation with monthly or annual rainfall totals, then compare against your water demand.<\/li>\n<li><strong>Flat areas collect less than you think.<\/strong> Paved or soil areas absorb water; roofs and tarps deliver nearly everything to the gutter.<\/li>\n<\/ul>\n<p>To plan around the weather itself, you may also find our <a href=\"https:\/\/wpcalc.com\/en\/converters\/heat-index-calculator\/\">Heat Index Calculator<\/a>, <a href=\"https:\/\/wpcalc.com\/en\/converters\/dew-point-calculator\/\">Dew Point Calculator<\/a> and <a href=\"https:\/\/wpcalc.com\/en\/converters\/wind-chill-calculator\/\">Wind Chill Calculator<\/a> useful for other conditions in your area.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Calculate how much water a rainfall event delivers onto any roof or surface. Enter the rainfall depth, collection area and an optional roof efficiency to get the volume in liters, US gallons and cubic meters.<\/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":[40],"class_list":["post-66768","post","type-post","status-publish","format-standard","hentry","category-converters","tag-volume-units"],"acf":[],"_links":{"self":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66768","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=66768"}],"version-history":[{"count":1,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66768\/revisions"}],"predecessor-version":[{"id":66770,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/posts\/66768\/revisions\/66770"}],"wp:attachment":[{"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/media?parent=66768"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/categories?post=66768"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wpcalc.com\/en\/wp-json\/wp\/v2\/tags?post=66768"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}