Duct Airflow Velocity Calculator
ANA›Life Services Authority›National Calculator Authority›Duct Airflow Velocity Calculator
.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }
Duct Airflow Velocity Calculator
Calculate the air velocity inside a duct given the volumetric flow rate and duct cross-sectional dimensions (rectangular or circular).
Volumetric Flow Rate (CFM)
Duct Shape
Circular Rectangular
Duct Diameter (inches)
Duct Width (inches)
Duct Height (inches)
Calculate
function ducToggleShape() { var shape = document.getElementById("duc-duct-type").value; document.getElementById("duc-circular-inputs").style.display = (shape === "circular") ? "block" : "none"; document.getElementById("duc-rectangular-inputs").style.display = (shape === "rectangular") ? "block" : "none"; document.getElementById("duc-result").innerHTML = ""; }
function ducCalc() { var resultDiv = document.getElementById("duc-result"); resultDiv.innerHTML = "";
var cfm = parseFloat(document.getElementById("duc-flow-rate").value); var shape = document.getElementById("duc-duct-type").value;
// Validate flow rate if (isNaN(cfm) || cfm Please enter a valid positive volumetric flow rate (CFM)."; return; }
var areaSqFt;
if (shape === "circular") { var dIn = parseFloat(document.getElementById("duc-diameter").value); if (isNaN(dIn) || dIn Please enter a valid positive duct diameter."; return; } // Area = π/4 × d² (in²) → convert to ft² var areaSqIn = (Math.PI / 4) * dIn * dIn; areaSqFt = areaSqIn / 144; } else { var wIn = parseFloat(document.getElementById("duc-width").value); var hIn = parseFloat(document.getElementById("duc-height").value); if (isNaN(wIn) || wIn Please enter a valid positive duct width."; return; } if (isNaN(hIn) || hIn Please enter a valid positive duct height."; return; } var areaSqIn = wIn * hIn; areaSqFt = areaSqIn / 144; }
if (areaSqFt Calculated duct area is zero or negative. Check your inputs."; return; }
// Velocity (FPM) = CFM / Area (ft²) var velocityFPM = cfm / areaSqFt; var velocityMPS = velocityFPM * 0.00508; // 1 FPM = 0.00508 m/s
// ASHRAE recommended range: 500–2500 FPM for supply ducts var recommendation = ""; if (velocityFPM ⚠ Velocity is below the typical minimum of 500 FPM — risk of poor air distribution."; } else if (velocityFPM > 2500) { recommendation = "⚠ Velocity exceeds the typical maximum of 2,500 FPM — may cause excessive noise and pressure drop."; } else { recommendation = "✔ Velocity is within the recommended ASHRAE range (500–2,500 FPM)."; }
resultDiv.innerHTML = "Air Velocity: " + velocityFPM.toFixed(1) + " FPM" + " (" + velocityMPS.toFixed(2) + " m/s)" + "Duct Cross-Sectional Area: " + areaSqFt.toFixed(4) + " ft²" + " (" + (areaSqFt * 144).toFixed(2) + " in²)" + recommendation; }
#### Formula
V = Q / A
- V = Air velocity (FPM — feet per minute)
- Q = Volumetric flow rate (CFM — cubic feet per minute)
- A = Duct cross-sectional area (ft²)
Circular duct area: A = (π / 4) × d²
Rectangular duct area: A = W × H
Dimensions in inches are divided by 144 to convert in² → ft².
Unit conversion: 1 FPM = 0.00508 m/s
#### Assumptions & References
- Recommended velocity range of 500–2,500 FPM is based on ASHRAE Fundamentals Handbook (Chapter 21 — Duct Design) for low-velocity HVAC supply ductwork.
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