{"id":100083,"date":"2026-02-15T20:17:19","date_gmt":"2026-02-15T20:17:19","guid":{"rendered":"https:\/\/vibromera.eu\/?post_type=calculator&#038;p=100083"},"modified":"2026-07-12T02:29:24","modified_gmt":"2026-07-12T02:29:24","slug":"equivalent-duct-diameter","status":"publish","type":"calculator","link":"https:\/\/vibromera.eu\/de\/calculators\/equivalent-duct-diameter\/","title":{"rendered":"Rechteckige Rohrdurchmesser | Gleiches Reibung, hydraulischer und fl\u00e4chenm\u00e4\u00dfiger Durchmesser"},"content":{"rendered":"\r\n<script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"WebApplication\",\"name\":\"Rectangular Duct Diameter Definitions\",\"description\":\"Calculate ASHRAE equal-friction round equivalent, hydraulic diameter and equal-area diameter for a rectangular duct.\",\"url\":\"https:\/\/vibromera.eu\/calculators\/equivalent-duct-diameter\/\",\"applicationCategory\":\"Engineering 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class=\"vc-calculator\" id=\"vc-duct-diameters\"><header class=\"vc-header\"><p class=\"vc-header-eyebrow\">Do not mix diameter definitions<\/p><h1 class=\"vc-header-title\">Rectangular Duct Diameter Definitions<\/h1><p class=\"vc-header-subtitle\">Compare the ASHRAE equal-friction round equivalent with hydraulic diameter and equal-area diameter. These quantities answer different questions and are generally not interchangeable.<\/p><div class=\"vc-badges\"><span class=\"vc-badge\">Equal friction<\/span><span class=\"vc-badge\">Hydraulic diameter<\/span><span class=\"vc-badge\">Equal area<\/span><\/div><\/header>\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-a\">Inside side a <span class=\"vc-label-hint\">(mm; &gt;0)<\/span><\/label><input class=\"vc-input\" id=\"vc-a\" type=\"text\" inputmode=\"decimal\"><\/div><div class=\"vc-field\"><label class=\"vc-label\" for=\"vc-b\">Inside side b <span class=\"vc-label-hint\">(mm; &gt;0)<\/span><\/label><input class=\"vc-input\" id=\"vc-b\" type=\"text\" inputmode=\"decimal\"><\/div><\/div><p id=\"vc-error\" role=\"alert\" style=\"margin:.75rem 0 0;color:#b42318\"><\/p><\/form><div class=\"vc-results\" id=\"vc-results\"><div class=\"vc-results-head\"><h2 class=\"vc-results-title\">Geometric comparison<\/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\">Equal-friction round equivalent De<\/div><div class=\"vc-rcard-value\" id=\"vc-r-de\">\u2014<\/div><\/div><div class=\"vc-rcard\"><div class=\"vc-rcard-label\">Hydraulic diameter Dh<\/div><div class=\"vc-rcard-value\" id=\"vc-r-dh\">\u2014<\/div><\/div><div class=\"vc-rcard\"><div class=\"vc-rcard-label\">Equal-area diameter DA<\/div><div class=\"vc-rcard-value\" id=\"vc-r-da\">\u2014<\/div><\/div><div class=\"vc-rcard\"><div class=\"vc-rcard-label\">Rectangular area<\/div><div class=\"vc-rcard-value\" id=\"vc-r-area\">\u2014<\/div><\/div><div class=\"vc-rcard\"><div class=\"vc-rcard-label\">Aspect ratio<\/div><div class=\"vc-rcard-value\" id=\"vc-r-ratio\">\u2014<\/div><\/div><\/div><p id=\"vc-r-note\" style=\"margin:0;color:var(--vc-ink-secondary)\"><\/p><\/div><\/div>\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\udcd0<\/span><span class=\"vc-section-title\">Definitions and applicability<\/span><\/span><\/button><div class=\"vc-section-body\"><div class=\"vc-section-inner vc-theory\"><div class=\"vc-formula-box\">De = 1.30(ab)^0.625 \/ (a+b)^0.25<\/div><p><a href=\"https:\/\/handbook.ashrae.org\/Handbooks\/F17\/SI\/f17_ch21\/f17_ch21_si.aspx\" target=\"_blank\" rel=\"noopener\">ASHRAE Handbook, Duct Design<\/a> defines De as the circular equivalent of a rectangular duct for equal airflow, resistance and length. Its SI Table 3 gives 378 mm for 400 \u00d7 300 mm; the equation returns 377.7088 mm before rounding.<\/p><div class=\"vc-formula-box\">Dh = 4A\/Pw = 2ab\/(a+b)<\/div><p>Hydraulic diameter uses flow area and wetted perimeter. A <a href=\"https:\/\/ntrs.nasa.gov\/api\/citations\/19980206179\/downloads\/19980206179.pdf\" target=\"_blank\" rel=\"noopener\">NASA duct-flow derivation<\/a> gives Dh=4A\/Pw and, for a rectangle, Dh=2ab\/(a+b). It is the characteristic length used in Reynolds\/friction correlations when that model is applicable; it is not the ASHRAE equal-friction round size.<\/p><div class=\"vc-formula-box\">DA = \u221a(4ab\/\u03c0)<\/div><p>DA is only the round diameter with the same cross-sectional area. Equal area does not imply equal resistance. De alone also does not calculate pressure drop: the selected duct method still needs flow, length, roughness, fluid properties and applicable fitting losses.<\/p><div class=\"vc-warning-box\"><p style=\"margin:0\"><strong>Removed unsupported claims:<\/strong> universal aspect-ratio recommendations and an accuracy percentage without an applicability range\/source. Presets and prefilled dimensions were removed so the page opens without a hidden design assumption.<\/p><\/div><\/div><\/div><\/div><footer class=\"vc-footer\"><p>\u00a9 2024\u20132026 <a href=\"https:\/\/vibromera.eu\/\">Vibromera<\/a><\/p><p>Rectangular straight-duct geometry only; fittings and system pressure loss require a complete duct model. Scientific review: July 2026.<\/p><\/footer><\/div>\r\n<script>(function(){'use strict';function $(i){return document.getElementById(i)}function pos(v){var s=String(v).trim().replace(\/\\s+\/g,'');if(!\/^[+]?(?:\\d+(?:[.,]\\d*)?|[.,]\\d+)$\/.test(s))return NaN;var x=Number(s.replace(',','.'));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-de','vc-r-dh','vc-r-da','vc-r-area','vc-r-ratio'].forEach(function(id){$(id).textContent='\u2014'});$('vc-r-note').textContent='';$('vc-results').classList.remove('vc-visible')}function calc(){var a=pos($('vc-a').value),b=pos($('vc-b').value),any=$('vc-a').value.trim()||$('vc-b').value.trim();if(!Number.isFinite(a)||!Number.isFinite(b)){clearResult();$('vc-error').textContent=any?'Enter positive unsuffixed inside dimensions a and b.':'';return}var de=1.3*Math.pow(a*b,.625)\/Math.pow(a+b,.25),dh=2*a*b\/(a+b),da=Math.sqrt(4*a*b\/Math.PI),area=a*b,ratio=Math.max(a,b)\/Math.min(a,b);$('vc-error').textContent='';$('vc-r-de').textContent=fmt(de)+' mm';$('vc-r-dh').textContent=fmt(dh)+' mm';$('vc-r-da').textContent=fmt(da)+' mm';$('vc-r-area').textContent=fmt(area)+' mm\u00b2';$('vc-r-ratio').textContent=fmt(ratio)+':1';$('vc-r-note').textContent='De: equal airflow\/resistance\/length round equivalent;Dh:4A\/Pw;DA:equal area.';$('vc-results').classList.add('vc-visible');var u=new URL(location);u.searchParams.set('a_mm',$('vc-a').value.trim().replace(',','.'));u.searchParams.set('b_mm',$('vc-b').value.trim().replace(',','.'));history.replaceState(null,'',u.toString())}$('vc-form').addEventListener('input',calc);$('vc-copy-btn').addEventListener('click',function(){if(navigator.clipboard)navigator.clipboard.writeText($('vc-results').innerText)});document.querySelectorAll('.vc-section-toggle').forEach(function(b){b.addEventListener('click',function(){var s=this.closest('.vc-section');s.classList.toggle('vc-open');this.setAttribute('aria-expanded',s.classList.contains('vc-open'))})});var p=new URLSearchParams(location.search);if(p.has('a_mm'))$('vc-a').value=p.get('a_mm');if(p.has('b_mm'))$('vc-b').value=p.get('b_mm');calc()})();<\/script>\r\n","protected":false},"excerpt":{"rendered":"<p>Compare the ASHRAE equal-friction round equivalent with hydraulic diameter and equal-area diameter for a rectangular duct.<\/p>","protected":false},"featured_media":0,"template":"","meta":{"ai_generated_summary":"","footnotes":""},"categories":[],"tags":[],"class_list":["post-100083","calculator","type-calculator","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/vibromera.eu\/de\/wp-json\/wp\/v2\/calculator\/100083","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/vibromera.eu\/de\/wp-json\/wp\/v2\/calculator"}],"about":[{"href":"https:\/\/vibromera.eu\/de\/wp-json\/wp\/v2\/types\/calculator"}],"version-history":[{"count":2,"href":"https:\/\/vibromera.eu\/de\/wp-json\/wp\/v2\/calculator\/100083\/revisions"}],"predecessor-version":[{"id":102384,"href":"https:\/\/vibromera.eu\/de\/wp-json\/wp\/v2\/calculator\/100083\/revisions\/102384"}],"wp:attachment":[{"href":"https:\/\/vibromera.eu\/de\/wp-json\/wp\/v2\/media?parent=100083"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/vibromera.eu\/de\/wp-json\/wp\/v2\/categories?post=100083"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/vibromera.eu\/de\/wp-json\/wp\/v2\/tags?post=100083"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}