{"id":391,"date":"2026-04-06T10:21:00","date_gmt":"2026-04-06T14:21:00","guid":{"rendered":"\/blog\/?p=391"},"modified":"2026-05-10T00:48:28","modified_gmt":"2026-05-10T04:48:28","slug":"pump-sizing-imperial-metric-flow-rate-conversion","status":"publish","type":"post","link":"\/blog\/pump-sizing-imperial-metric-flow-rate-conversion","title":{"rendered":"Pump Sizing Guide: Convert Manufacturer Flow Rates Between Imperial and Metric"},"content":{"rendered":"\n<p>When the pump on your spec sheet was designed in Denmark and the system you\u2019re replacing was built in Texas, the units don\u2019t line up. European catalogs lead with <strong>m\u00b3\/h<\/strong> and meters of head; US ones lead with <strong>GPM<\/strong> and feet of head. Get the conversion wrong (or confuse US and Imperial gallons \u2014 the Imperial gallon is ~20% larger) and you\u2019ll under- or oversize the pump. This guide walks you through doing it right, with real model numbers from Grundfos, Pentair, Goulds, and Wilo.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Jump to a section<\/h2>\n\n\n\n<ul class=\"wp-block-list\"><li><a href=\"#conversions\">The four conversions you need<\/a><\/li><li><a href=\"#us-curve\">Reading a US pump curve<\/a><\/li><li><a href=\"#metric-curve\">Reading a metric pump curve<\/a><\/li><li><a href=\"#example-bg-grundfos\">Worked example: replacing a Bell &amp; Gossett with a Grundfos<\/a><\/li><li><a href=\"#example-wilo\">Worked example: spec\u2019ing a metric Wilo for a US system<\/a><\/li><li><a href=\"#mistakes\">Common mistakes that cost you 10\u201320%<\/a><\/li><li><a href=\"#faq\">FAQ<\/a><\/li><\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"conversions\">The four conversions you need<\/h2>\n\n\n\n<p>Pump catalogs publish four numbers that matter. To compare specs across regions, you need to convert all four:<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"table table-hover\"><thead><tr><th>Quantity<\/th><th>US units \u2192 metric<\/th><th>Metric \u2192 US units<\/th><\/tr><\/thead><tbody><tr><td>Flow rate<\/td><td>GPM \u00d7 0.227125 = m\u00b3\/h<\/td><td>m\u00b3\/h \u00d7 4.40287 = GPM<\/td><\/tr><tr><td>Head<\/td><td>ft \u00d7 0.3048 = m<\/td><td>m \u00d7 3.2808 = ft<\/td><\/tr><tr><td>Power (motor)<\/td><td>HP \u00d7 0.7457 = kW<\/td><td>kW \u00d7 1.34102 = HP<\/td><\/tr><tr><td>Pressure (system)<\/td><td>psi \u00d7 6.89476 = kPa<\/td><td>kPa \u00d7 0.145038 = psi<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p>Use <a href=\"https:\/\/www.xconvert.com\/unit-converter\/volumeFlowRate\">xconvert flow-rate calculators<\/a> for the flow conversions and <a href=\"https:\/\/www.xconvert.com\/unit-converter\/length\">Length<\/a> \/ <a href=\"https:\/\/www.xconvert.com\/unit-converter\/power\">Power<\/a> \/ <a href=\"https:\/\/www.xconvert.com\/unit-converter\/pressure\">Pressure<\/a> for the rest.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1600\" height=\"1000\" src=\"\/blog\/wp-content\/uploads\/2026\/05\/step-01-converter-12.png\" alt=\"The xconvert GPM to m\u00b3\/h converter showing the conversion factor\" class=\"wp-image-530\" srcset=\"\/blog\/wp-content\/uploads\/2026\/05\/step-01-converter-12.png 1600w, \/blog\/wp-content\/uploads\/2026\/05\/step-01-converter-12-300x188.png 300w, \/blog\/wp-content\/uploads\/2026\/05\/step-01-converter-12-1024x640.png 1024w, \/blog\/wp-content\/uploads\/2026\/05\/step-01-converter-12-768x480.png 768w, \/blog\/wp-content\/uploads\/2026\/05\/step-01-converter-12-1536x960.png 1536w\" sizes=\"auto, (max-width: 1600px) 100vw, 1600px\" \/><\/figure>\n\n\n\n<p>A practical rule: never compare just one operating point. A pump curve is a curve \u2014 two pumps that match at one flow can diverge significantly at another. Always convert and compare at the actual system operating point, not just the rated point.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"us-curve\">Reading a US pump curve<\/h2>\n\n\n\n<p>A typical US-format pump curve plots:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>X-axis:<\/strong> GPM (flow)<\/li><li><strong>Y-axis (left):<\/strong> Head, in feet of water column<\/li><li><strong>Y-axis (right):<\/strong> Efficiency, percent<\/li><li><strong>Curves:<\/strong> One per impeller diameter, plus a \u201csystem curve\u201d overlaid<\/li><\/ul>\n\n\n\n<p>Example: A <strong>Bell &amp; Gossett 1510-3BC<\/strong> with a 7.5&quot; impeller curve passes through 80 GPM at 30 ft of head and 75% efficiency. The motor is rated 1 HP. To compare to a European candidate, convert all four:<\/p>\n\n\n\n<p>So this US point is equivalent to <strong>18.17 m\u00b3\/h at 9.14 m head, with a 0.75 kW motor<\/strong>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"metric-curve\">Reading a metric pump curve<\/h2>\n\n\n\n<p>A European catalog plots:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>X-axis:<\/strong> m\u00b3\/h (or sometimes L\/s for smaller pumps)<\/li><li><strong>Y-axis (left):<\/strong> Head, in meters<\/li><li><strong>Curves:<\/strong> One per impeller trim<\/li><li><strong>Power table:<\/strong> Motor rating in kW, often with a \u201cP2\u201d (shaft) and \u201cP1\u201d (electrical) value<\/li><\/ul>\n\n\n\n<p>Example: A <strong>Grundfos UPS 32-80 N<\/strong> circulator at 18 m\u00b3\/h passes 9 m of head with the larger impeller; rated 0.18 kW shaft. To express in US terms:<\/p>\n\n\n\n<p>Now you can compare the two pumps at the same operating point: ~80 GPM at ~30 ft of head, with the Grundfos using significantly less motor (0.24 HP vs 1 HP). That isn\u2019t apples-to-apples, though \u2014 the B&amp;G 1510 is a base-mounted end-suction pump, the UPS is an inline circulator. Topology matters at least as much as the head\/flow numbers.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"example-bg-grundfos\">Worked example: replacing a Bell &amp; Gossett with a Grundfos<\/h2>\n\n\n\n<p>You\u2019re replacing an aging <strong>B&amp;G 1510-3BC<\/strong> in a closed-loop hydronic chiller plant. The system curve hits the pump at:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>80 GPM at 30 ft of head<\/strong><\/li><li>460 V, 3-phase, 1 HP motor<\/li><\/ul>\n\n\n\n<p>You need a Grundfos equivalent to specify. Walk through it:<\/p>\n\n\n\n<p><strong>Step 1 \u2014 Convert the operating point.<\/strong><\/p>\n\n\n\n<p><strong>Step 2 \u2014 Find a Grundfos with that point on its curve.<\/strong><\/p>\n\n\n\n<p>Looking at the Grundfos catalog: the <strong>TPE2 32-100\/2<\/strong> family covers 5\u201325 m\u00b3\/h at heads from 4\u201313 m. The 32-100\/2 with the largest impeller hits ~18 m\u00b3\/h at ~9.5 m head \u2014 slightly above the operating point, which is what you want (system curve will balance just below the pump curve).<\/p>\n\n\n\n<p><strong>Step 3 \u2014 Match the motor.<\/strong><\/p>\n\n\n\n<p>The TPE2 32-100\/2 ships with a 0.55 kW motor. In HP: 0.55 \u00d7 1.34102 \u2248 <strong>0.74 HP<\/strong>. The B&amp;G uses 1 HP. The difference is partly efficiency (Grundfos\u2019s IE5-rated PM motor is more efficient than a typical 1990s-vintage induction motor) and partly conservative oversizing in the original spec.<\/p>\n\n\n\n<p><strong>Step 4 \u2014 Verify the system curve still hits in the safe zone.<\/strong><\/p>\n\n\n\n<p>Plot the system curve on the Grundfos curve. The operating point should land in the middle 60% of the flow range, away from both deadhead and run-out. If it lands at 90% of the run-out flow, go up one frame size to keep cavitation margin.<\/p>\n\n\n\n<p>This is where unit conversion matters: if you\u2019d compared \u201c1 HP\u201d to \u201c0.55 kW\u201d without converting, you\u2019d think the Grundfos was undersized. It isn\u2019t \u2014 the conversion shows it\u2019s roughly 0.74 HP equivalent, which is appropriate for an 80 GPM @ 30 ft duty.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"example-wilo\">Worked example: spec\u2019ing a metric Wilo for a US system<\/h2>\n\n\n\n<p>You\u2019re sourcing pumps for a chilled-water plant in a building partly designed in Germany. The system curve passes through <strong>40 m\u00b3\/h at 14 m of head<\/strong>. You want to confirm a <strong>Wilo Stratos PICO<\/strong> is the right size.<\/p>\n\n\n\n<p><strong>Step 1 \u2014 Convert to US units to sanity-check against US pump curves you\u2019re familiar with.<\/strong><\/p>\n\n\n\n<p><strong>Step 2 \u2014 Compare against the Wilo curve.<\/strong><\/p>\n\n\n\n<p>The Stratos PICO 30\/0.5-12 model maxes out around 12 m\u00b3\/h. That\u2019s 53 GPM \u2014 way too small for a 176 GPM duty. You need to step up to the <strong>Wilo IL<\/strong> or <strong>CronoLine<\/strong> in-line pump series, which cover 50+ m\u00b3\/h.<\/p>\n\n\n\n<p>Looking at the Wilo IL 50\/130-2.2\/2: rated for 60 m\u00b3\/h at 13 m head. Convert:<\/p>\n\n\n\n<p>Slightly above the duty (176 GPM at 45.9 ft). With variable-speed control, this pump can ride down its curve to the 40 m\u00b3\/h operating point and run efficiently there. Good match.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"mistakes\">Common mistakes that cost you 10\u201320%<\/h2>\n\n\n\n<p>A few mistakes that recur in pump-sizing work, and how to avoid them:<\/p>\n\n\n\n<p><strong>1. Using UK gallons when the spec is US.<\/strong> The Imperial gallon (UK) is 4.54609 L; the US gallon is 3.78541 L \u2014 the Imperial is <strong>~20% larger<\/strong> than the US gallon (equivalently, the US gallon is 16.7% smaller than the Imperial). UK pump catalogs and older Commonwealth specs sometimes write \u201cGPM\u201d without qualifying it. If you\u2019re matching a UK pump to a US system, double-check the gallon definition.<\/p>\n\n\n\n<p><strong>2. Treating \u201chead\u201d and \u201cpressure\u201d as interchangeable.<\/strong> They aren\u2019t. Head depends only on the pump and is measured in length units (ft or m). Pressure depends on the <em>fluid density<\/em> \u2014 for water at 4 \u00b0C, 1 ft of head = 0.433 psi, but for diesel or oil the same head produces less pressure. If you\u2019re pumping anything other than water, convert head to pressure using the actual fluid density.<\/p>\n\n\n\n<p><strong>3. Comparing rated-flow points instead of operating points.<\/strong> A pump\u2019s \u201crated flow\u201d is the design point at maximum efficiency. The system curve might run at 60% of rated flow \u2014 that\u2019s a different head, different efficiency, and different power draw. Always convert and compare at the actual operating point.<\/p>\n\n\n\n<p><strong>4. Forgetting that NPSH-A is in absolute units.<\/strong> Net Positive Suction Head Available is given in feet absolute (US) or meters absolute (metric). The conversion is straightforward (\u00d7 0.3048 or \u00d7 3.2808), but NPSH-Required from the manufacturer is in the same units as the pump curve\u2019s head axis \u2014 so they must match before subtracting.<\/p>\n\n\n\n<p><strong>5. Ignoring the variable-frequency-drive math.<\/strong> A pump on a VFD scales flow, head, and power with the affinity laws (linear, square, and cube respectively). When converting units across a VFD-controlled system, do all the unit conversions FIRST, then apply the affinity laws \u2014 not the other way around.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"faq\">Frequently Asked Questions<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">What\u2019s the conversion factor between GPM and m\u00b3\/h?<\/h3>\n\n\n\n<p>1 GPM = 0.227125 m\u00b3\/h, and 1 m\u00b3\/h = 4.40287 GPM. xconvert\u2019s <a href=\"https:\/\/www.xconvert.com\/unit-converter\/gallons-per-minute-to-cubic-meters-per-hour\">GPM to m\u00b3\/h converter<\/a> gives precise results. For mental math: m\u00b3\/h \u2248 GPM \u00f7 4.4, or GPM \u2248 m\u00b3\/h \u00d7 4.4.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Why do US pumps use HP and European pumps use kW?<\/h3>\n\n\n\n<p>Conventional inertia. The mechanical horsepower (745.7 W) was defined in 18th-century Britain by James Watt to compare steam engines to draft horses. The metric horsepower (735.5 W) emerged in mainland Europe at roughly the same time but with a slightly different definition. Both have been formally replaced by the SI watt, but motor manufacturers still publish in HP for the US market because that\u2019s what specifiers expect. 1 HP \u2248 0.7457 kW (mechanical) or 0.7355 kW (metric horsepower).<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Are pump curves linear after conversion?<\/h3>\n\n\n\n<p>Yes. Unit conversion is multiplication by a constant, which preserves the shape of any curve. A US pump curve plotted against GPM-and-feet looks identical to the same curve plotted against m\u00b3\/h-and-meters \u2014 just with different axis labels. The operating point lands at the same percent of run-out flow either way.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">How do I convert a centrifugal pump\u2019s affinity-law math when units differ?<\/h3>\n\n\n\n<p>The affinity laws relate flow (Q), head (H), and power (P) to speed (N): Q \u221d N, H \u221d N\u00b2, P \u221d N\u00b3. The proportionality is unit-independent. So if you have a pump curve at 1750 RPM and want to predict the curve at 1450 RPM:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>Q_new = Q_old \u00d7 (1450 \/ 1750)<\/li><li>H_new = H_old \u00d7 (1450 \/ 1750)\u00b2<\/li><li>P_new = P_old \u00d7 (1450 \/ 1750)\u00b3<\/li><\/ul>\n\n\n\n<p>Apply the ratios in whatever units you\u2019re in. The conversion to other units (GPM &#x2194; m\u00b3\/h, etc.) happens separately and doesn\u2019t interact with the affinity laws.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">What\u2019s the difference between \u201cduty point\u201d and \u201crated point\u201d?<\/h3>\n\n\n\n<p>The <strong>rated point<\/strong> is what the manufacturer prints on the pump nameplate \u2014 typically the best-efficiency point. The <strong>duty point<\/strong> is where your system actually runs the pump, defined by the intersection of the pump curve and the system head curve. They should be close (ideally within 80\u2013110% of rated flow), but they\u2019re rarely identical.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Why do European pump catalogs sometimes use bar instead of meters?<\/h3>\n\n\n\n<p>For pressure-rated equipment (boosters, pressure tanks, water-treatment skids), pressure in bar is more useful than head in meters because the rating doesn\u2019t change with fluid density. Conversion: 1 bar = 10.197 m of water column = 14.504 psi. xconvert has a <a href=\"https:\/\/www.xconvert.com\/unit-converter\/bar-to-psi\">bar\/psi converter<\/a> for the inevitable mixed-unit datasheets.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Try it now<\/h2>\n\n\n\n<p>Convert any pump-curve number directly: <a href=\"https:\/\/www.xconvert.com\/unit-converter\/gallons-per-minute-to-cubic-meters-per-hour\">GPM to m\u00b3\/h<\/a>, <a href=\"https:\/\/www.xconvert.com\/unit-converter\/feet-to-meters\">feet to meters<\/a>, <a href=\"https:\/\/www.xconvert.com\/unit-converter\/horsepower-(british)-to-kilowatts\">HP to kW<\/a>, <a href=\"https:\/\/www.xconvert.com\/unit-converter\/pounds-per-square-inch-to-bar\">psi to bar<\/a>. For an overview of which flow-rate units each industry uses, see <a href=\"\/blog\/flow-rate-conversion-hvac-plumbing-gpm-lpm-cubic-meters\/\">Flow Rate Conversion for HVAC and Plumbing<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Step-by-step guide to matching pump specifications across imperial and metric flow-rate units. Covers Grundfos, Pentair, Goulds, and Wilo capacities with side-by-side conversions and real selection examples.<\/p>\n","protected":false},"author":3,"featured_media":529,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5,14],"tags":[],"class_list":["post-391","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-how-to-guides","category-tools"],"_links":{"self":[{"href":"\/blog\/wp-json\/wp\/v2\/posts\/391","targetHints":{"allow":["GET"]}}],"collection":[{"href":"\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"\/blog\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"\/blog\/wp-json\/wp\/v2\/comments?post=391"}],"version-history":[{"count":1,"href":"\/blog\/wp-json\/wp\/v2\/posts\/391\/revisions"}],"predecessor-version":[{"id":531,"href":"\/blog\/wp-json\/wp\/v2\/posts\/391\/revisions\/531"}],"wp:featuredmedia":[{"embeddable":true,"href":"\/blog\/wp-json\/wp\/v2\/media\/529"}],"wp:attachment":[{"href":"\/blog\/wp-json\/wp\/v2\/media?parent=391"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"\/blog\/wp-json\/wp\/v2\/categories?post=391"},{"taxonomy":"post_tag","embeddable":true,"href":"\/blog\/wp-json\/wp\/v2\/tags?post=391"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}