Duct Leakage Rate Calculator
ANA›Life Services Authority›National Calculator Authority›Duct Leakage Rate 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 Leakage Rate Calculator
Calculate duct leakage airflow (CFM) based on duct surface area, leakage class, and test pressure differential. Uses SMACNA and ASHRAE 90.1 standard leakage equations.
Duct Surface Area (ft²)
Leakage Class (CL)
Class 3 — Sealed (CL = 3) Class 6 — Sealed (CL = 6) Class 12 — Unsealed Low Pressure (CL = 12) Class 24 — Unsealed Medium Pressure (CL = 24) Class 48 — Unsealed High Pressure (CL = 48) Custom CL value
Custom Leakage Class (CL)
Test Pressure (in. w.g.)
Pressure Exponent (n)
0.65 — SMACNA / ASHRAE Standard 0.50 — Orifice Flow 0.60 — Intermediate 0.75 — Crack Flow
System Design Airflow (CFM) (optional — for % leakage)
Calculate
(function() { const clSelect = document.getElementById('duc-leakage-class'); const customGroup = document.getElementById('duc-custom-cl-group');
clSelect.addEventListener('change', function() { customGroup.style.display = (this.value === 'custom') ? 'block' : 'none'; });
window.ducCalc = function() { const resultDiv = document.getElementById('duc-result'); resultDiv.style.display = 'none'; resultDiv.innerHTML = '';
// --- Gather inputs --- const surfaceArea = parseFloat(document.getElementById('duc-surface-area').value); const testPressure = parseFloat(document.getElementById('duc-test-pressure').value); const n = parseFloat(document.getElementById('duc-pressure-exponent').value); const systemCFM = parseFloat(document.getElementById('duc-system-cfm').value);
let CL; if (clSelect.value === 'custom') { CL = parseFloat(document.getElementById('duc-custom-cl').value); } else { CL = parseFloat(clSelect.value); }
// --- Validation --- const errors = []; if (isNaN(surfaceArea) || surfaceArea 1) errors.push('Pressure exponent (n) must be between 0 and 1.'); if (!isNaN(systemCFM) && systemCFM 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Input Error:' + errors.map(e => '').join('') + ''; return; }
// --- Core Formula --- // SMACNA / ASHRAE 90.1: // F_L = CL × A × P^n // Where: // F_L = Leakage airflow (CFM) // CL = Leakage class (CFM/100 ft² at 1 in. w.g.) // A = Duct surface area (ft²) // P = Test pressure (in. w.g.) // n = Pressure exponent (dimensionless)
const leakageCFM = CL * (surfaceArea / 100) * Math.pow(testPressure, n);
// Leakage per 100 ft² at given pressure const leakagePer100ft2 = CL * Math.pow(testPressure, n);
// Percentage of system airflow (if provided) let pctHTML = ''; if (!isNaN(systemCFM) && systemCFM > 0) { const pct = (leakageCFM / systemCFM) * 100; const pctClass = pct Leakage as % of System Airflow \${pct.toFixed(2)}%
Assessment \${pctLabel} `; }
// Leakage at 1 in. w.g. reference (for comparison) const leakageAt1inWG = CL * (surfaceArea / 100);
resultDiv.style.display = 'block'; resultDiv.innerHTML = ` ### Results
ParameterValue
Duct Surface Area\${surfaceArea.toLocaleString()} ft² Leakage Class (CL)\${CL} CFM/100 ft² @ 1 in. w.g. Test Pressure\${testPressure} in. w.g. Pressure Exponent (n)\${n}
Duct Leakage Rate \${leakageCFM.toFixed(1)} CFM
Leakage per 100 ft² at Test Pressure \${leakagePer100ft2.toFixed(2)} CFM/100 ft²
Leakage at Reference (1 in. w.g.) \${leakageAt1inWG.toFixed(1)} CFM
\${pctHTML}
`; }; })();
#### Formula
FL = CL × (A / 100) × Pn
- FL — Duct leakage airflow (CFM)
- CL — Leakage class (CFM per 100 ft² of duct surface area at 1 in. w.g.)
- A — Total duct surface area (ft²)
- P — Test static pressure differential (in. w.g.)
- n — Pressure exponent (0.65 per SMACNA; range 0.5–0.75)
Leakage percentage: % = (FL / Qsystem) × 100
#### Assumptions & References
- Formula per SMACNA HVAC Air Duct Leakage Test Manual and ASHRAE Standard 90.1.
- Leakage class (CL) values are defined at a reference pressure of 1.0 in. w.g.
- Standard pressure exponent n = 0.65 is used per SMACNA for typical sheet metal duct joints and seams.
- ASHRAE 90.1 requires duct leakage ≤ 4% of system design airflow for sealed duct systems.
- For pressurization testing, typical test pressures are 0.5–2.0 in. w.g. depending on duct pressure class.
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