{"id":100080,"date":"2026-02-15T20:15:31","date_gmt":"2026-02-15T20:15:31","guid":{"rendered":"https:\/\/vibromera.eu\/?post_type=calculator&#038;p=100080"},"modified":"2026-07-12T02:11:07","modified_gmt":"2026-07-12T02:11:07","slug":"duct-pressure-loss","status":"publish","type":"calculator","link":"https:\/\/vibromera.eu\/hi\/calculators\/duct-pressure-loss\/","title":{"rendered":"Round-Duct Pressure Loss | Darcy &#038; Colebrook"},"content":{"rendered":"\r\r\n<script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"WebApplication\",\"name\":\"Round-Duct Straight-Run Pressure Loss\",\"description\":\"Calculate fully developed straight circular-duct friction loss with Darcy-Weisbach,laminar64\/Re or iterative Colebrook,and explicit density\/viscosity.\",\"url\":\"https:\/\/vibromera.eu\/calculators\/duct-pressure-loss\/\",\"applicationCategory\":\"Engineering 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var(--vc-border);border-radius:var(--vc-radius-lg);display:flex;align-items:center;gap:16px}.vc-promo-text{flex:1;font-size:14px;color:var(--vc-ink-secondary)}.vc-promo-text strong{color:var(--vc-ink)}.vc-promo-link{padding:8px 20px;font-size:13px;font-weight:700;color:#fff;background:var(--vc-accent);border-radius:var(--vc-radius);text-decoration:none;white-space:nowrap;transition:background .15s}.vc-promo-link:hover{background:var(--vc-accent-hover)}\r\r\n.vc-footer{text-align:center;padding:32px 16px;font-size:13px;color:var(--vc-ink-muted)}.vc-footer a{color:var(--vc-accent);text-decoration:none}.vc-footer a:hover{text-decoration:underline}.vc-footer-links{margin-top:8px;font-size:12px}.vc-footer-links a{margin:0 8px}\r\r\n@media print{.vc-section-body{max-height:none!important}.vc-faq-a{max-height:none!important}.vc-results{max-height:none!important;padding:24px!important}.vc-copy-btn,.vc-section-chevron,.vc-faq-chevron,.vc-presets,.vc-promo{display:none!important}}<\/style>\r\r\n<div class=\"vc-calculator\" id=\"vc-duct\"><header class=\"vc-header\"><p class=\"vc-header-eyebrow\">Fully developed circular-pipe model<\/p><h1 class=\"vc-header-title\">Round-Duct Straight-Run Pressure Loss<\/h1><p class=\"vc-header-subtitle\">Calculate distributed wall-friction loss for a straight constant-diameter circular duct with supplied fluid density, viscosity and absolute roughness. Transitional flow and unmodelled fittings are not guessed.<\/p><div class=\"vc-badges\"><span class=\"vc-badge\">Darcy factor<\/span><span class=\"vc-badge\">Laminar\/Colebrook<\/span><span class=\"vc-badge\">Straight run only<\/span><\/div><\/header>\r\r\n<div class=\"vc-card\"><form class=\"vc-form\" id=\"vc-form\" autocomplete=\"off\"><div class=\"vc-form-grid\"><div class=\"vc-field\"><label class=\"vc-label\" for=\"vc-flow\">Volume flow Q <span class=\"vc-label-hint\">(m\u00b3\/h; &gt;0)<\/span><\/label><input class=\"vc-input\" id=\"vc-flow\" type=\"text\" inputmode=\"decimal\"><\/div><div class=\"vc-field\"><label class=\"vc-label\" for=\"vc-dia\">Internal diameter D <span class=\"vc-label-hint\">(mm; &gt;0)<\/span><\/label><input class=\"vc-input\" id=\"vc-dia\" type=\"text\" inputmode=\"decimal\"><\/div><div class=\"vc-field\"><label class=\"vc-label\" for=\"vc-length\">Straight length L <span class=\"vc-label-hint\">(m; &gt;0)<\/span><\/label><input class=\"vc-input\" id=\"vc-length\" type=\"text\" inputmode=\"decimal\"><\/div><div class=\"vc-field\"><label class=\"vc-label\" for=\"vc-rough\">Absolute roughness \u03b5 <span class=\"vc-label-hint\">(mm; \u22650)<\/span><\/label><input class=\"vc-input\" id=\"vc-rough\" type=\"text\" inputmode=\"decimal\"><\/div><div class=\"vc-field\"><label class=\"vc-label\" for=\"vc-rho\">Fluid density \u03c1 <span class=\"vc-label-hint\">(kg\/m\u00b3; &gt;0)<\/span><\/label><input class=\"vc-input\" id=\"vc-rho\" type=\"text\" inputmode=\"decimal\"><\/div><div class=\"vc-field\"><label class=\"vc-label\" for=\"vc-mu\">Dynamic viscosity \u03bc <span class=\"vc-label-hint\">(Pa\u00b7s; &gt;0)<\/span><\/label><input class=\"vc-input\" id=\"vc-mu\" type=\"text\" inputmode=\"decimal\"><\/div><\/div><div style=\"padding:0 24px 24px\"><div class=\"vc-field\"><label class=\"vc-label\" for=\"vc-source\">Flow, geometry, roughness and fluid-property source\/temperature\/pressure basis<\/label><input class=\"vc-input\" id=\"vc-source\" type=\"text\" maxlength=\"300\"><\/div><p id=\"vc-error\" role=\"alert\" style=\"margin:.75rem 0 0;color:#b42318\"><\/p><div class=\"vc-warning-box\"><p style=\"margin:0\"><strong>Scope:<\/strong> distributed friction in a fully developed straight circular run only. Add entrances, exits, bends, dampers, transitions, branches and other local losses separately from sourced K values. Do not use this incompressible model when density changes materially along the run.<\/p><\/div><\/div><\/form><div class=\"vc-results\" id=\"vc-results\"><div class=\"vc-results-head\"><h2 class=\"vc-results-title\">Straight-run result<\/h2><button type=\"button\" class=\"vc-copy-btn\" id=\"vc-copy-btn\">Copy<\/button><\/div><div class=\"vc-result-grid\"><div class=\"vc-rcard vc-rcard-primary\"><div class=\"vc-rcard-label\">Pressure drop \u0394P<\/div><div class=\"vc-rcard-value\" id=\"vc-r-dp\">\u2014<\/div><\/div><div class=\"vc-rcard\"><div class=\"vc-rcard-label\">Pressure gradient<\/div><div class=\"vc-rcard-value\" id=\"vc-r-dppm\">\u2014<\/div><\/div><div class=\"vc-rcard\"><div class=\"vc-rcard-label\">Mean velocity v<\/div><div class=\"vc-rcard-value\" id=\"vc-r-vel\">\u2014<\/div><\/div><div class=\"vc-rcard\"><div class=\"vc-rcard-label\">Reynolds number<\/div><div class=\"vc-rcard-value\" id=\"vc-r-re\">\u2014<\/div><\/div><div class=\"vc-rcard\"><div class=\"vc-rcard-label\">Darcy friction factor f<\/div><div class=\"vc-rcard-value\" id=\"vc-r-f\">\u2014<\/div><\/div><div class=\"vc-rcard\"><div class=\"vc-rcard-label\">Regime<\/div><div class=\"vc-rcard-value\" id=\"vc-r-regime\">\u2014<\/div><\/div><\/div><p id=\"vc-r-note\" style=\"margin:0;color:var(--vc-ink-secondary)\"><\/p><\/div><\/div>\r\r\n<div class=\"vc-section vc-open\"><button type=\"button\" class=\"vc-section-toggle\" aria-expanded=\"true\"><span class=\"vc-section-toggle-text\"><span class=\"vc-section-icon\">\ud83d\udcd8<\/span><span class=\"vc-section-title\">Equations and regime gate<\/span><\/span><\/button><div class=\"vc-section-body\"><div class=\"vc-section-inner vc-theory\"><div class=\"vc-formula-box\">A=\u03c0D\u00b2\/4; &nbsp; v=(Q\/3600)\/A; &nbsp; Re=\u03c1vD\/\u03bc<\/div><div class=\"vc-formula-box\">\u0394P=f(L\/D)\u03c1v\u00b2\/2; &nbsp; f=64\/Re for Re&lt;2300<\/div><div class=\"vc-formula-box\">1\/\u221af=\u22122log\u2081\u2080[\u03b5\/(3.7D)+2.51\/(Re\u221af)] for Re\u22654000<\/div><p>An <a href=\"https:\/\/www.energy.gov\/sites\/prod\/files\/2014\/03\/f11\/vss045_rugh_2011_o.pdf\" target=\"_blank\" rel=\"noopener\">NREL\/US DOE HVAC modelling presentation<\/a> documents Darcy-Weisbach with laminar and turbulent friction-factor treatment. <a href=\"https:\/\/www.grc.nasa.gov\/WWW\/K-12\/airplane\/reynolds.html\" target=\"_blank\" rel=\"noopener\">NASA defines Re=\u03c1vL\/\u03bc<\/a> as the inertial-to-viscous similarity parameter.<\/p><p>A <a href=\"https:\/\/ntrs.nasa.gov\/api\/citations\/19810011966\/downloads\/19810011966.pdf\" target=\"_blank\" rel=\"noopener\">NASA pipe-flow reference<\/a> places transition usually between Re 2300 and 4000 depending on disturbances. No single regime equation is asserted in that interval, so this calculator reports it as out of scope. The <a href=\"https:\/\/www.energy.gov\/sites\/default\/files\/2026-04\/DOE-HDBK-1012-92_VOL3.pdf\" target=\"_blank\" rel=\"noopener\">DOE fluid-flow handbook<\/a> notes viscosity is a fluid property affected by temperature.<\/p><div class=\"vc-warning-box\"><p style=\"margin:0\"><strong>Removed claims:<\/strong> the former page hard-coded \u03c1=1.2 kg\/m\u00b3 and \u03bc=1.81\u00d710\u207b\u2075 Pa\u00b7s without conditions, called a Haaland explicit approximation \u201cColebrook\u201d, and applied it immediately above Re 2300. Unsourced roughness\/velocity\/equivalent-length presets and rectangular-duct shortcut were removed.<\/p><\/div><\/div><\/div><\/div><footer class=\"vc-footer\"><p>\u00a9 2024\u20132026 <a href=\"https:\/\/vibromera.eu\/\">Vibromera<\/a><\/p><p>Straight circular-run friction only; verify fluid properties and local losses. Scientific review: July 2026.<\/p><\/footer><\/div>\r\r\n<script>(function(){'use strict';function $(i){return document.getElementById(i)}function raw(v){var s=String(v).trim().replace(\/\\s+\/g,'');if(!\/^[+]?(?:\\d+(?:[.,]\\d*)?|[.,]\\d+)$\/.test(s))return NaN;return Number(s.replace(',','.'))}function pos(v){var x=raw(v);return Number.isFinite(x)&&x>0?x:NaN}function nonneg(v){var x=raw(v);return Number.isFinite(x)&&x>=0?x:NaN}function fmt(x){var y=Math.round(x*1e9)\/1e9;if(Math.abs(x-y)<=2e-12*Math.max(1,Math.abs(x)))x=y;return Number(x.toPrecision(12)).toString()}function clearResult(){['vc-r-dp','vc-r-dppm','vc-r-vel','vc-r-re','vc-r-f','vc-r-regime'].forEach(function(id){$(id).textContent='\u2014'});$('vc-r-note').textContent='';$('vc-results').classList.remove('vc-visible')}function colebrook(Re,rr){var f=0.02;for(var i=0;i<60;i++){var z=-2*Math.log10(rr\/3.7+2.51\/(Re*Math.sqrt(f))),next=1\/(z*z);if(Math.abs(next-f)<1e-14){f=next;break}f=next}return f}function calc(){var Q=pos($('vc-flow').value),Dmm=pos($('vc-dia').value),L=pos($('vc-length').value),emm=nonneg($('vc-rough').value),rho=pos($('vc-rho').value),mu=pos($('vc-mu').value),source=$('vc-source').value.trim(),any=Array.from(document.querySelectorAll('#vc-form input')).some(function(x){return x.value.trim()}),bad=![Q,Dmm,L,emm,rho,mu].every(Number.isFinite);if(bad||!source){clearResult();$('vc-error').textContent=any?'Enter positive unsuffixed Q, D, L, \u03c1, \u03bc; non-negative \u03b5; and the property\/geometry basis.':'';return}var D=Dmm\/1000,e=emm\/1000,A=Math.PI*D*D\/4,v=Q\/3600\/A,Re=rho*v*D\/mu;if(Re>=2300&&Re<4000){clearResult();$('vc-error').textContent='Re='+fmt(Re)+' is in the 2300\u20134000 transition interval. 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