Airflow CFM Calculator — Blower and Ventilation Requirements
ANA›Life Services Authority›National Calculator Authority›Airflow CFM Calculator — Blower and Ventilation Requirements
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Airflow CFM Calculator — Blower and Ventilation Requirements
Calculate required airflow (CFM — Cubic Feet per Minute) for blowers and ventilation systems using four industry-standard methods: Air Changes per Hour (ACH), Heat Load, Duct Velocity, or Room Volume with target ACH.
Calculation Method
Method 1: Air Changes per Hour (ACH) Method 2: Heat Load (Sensible Cooling/Heating) Method 3: Duct Velocity Method 4: Exhaust / General Ventilation (People-Based)
### Method 1: Air Changes per Hour (ACH)
Room Length (ft)
Room Width (ft)
Ceiling Height (ft)
Required Air Changes per Hour (ACH) Typical: Bedroom 4–6, Office 6–10, Kitchen 15–60, Server Room 20–30
### Method 2: Heat Load (Sensible)
Sensible Heat Load (BTU/hr)
Temperature Difference ΔT (°F) Supply air temp vs. room temp (typical: 15–25°F for cooling)
### Method 3: Duct Velocity
Duct Cross-Section Shape
Round Duct Rectangular Duct
Duct Diameter (inches)
Duct Width (inches)
Duct Height (inches)
Air Velocity (FPM — Feet per Minute) Typical: Supply 600–900 FPM, Return 400–600 FPM, Main trunk 1000–1500 FPM
### Method 4: People-Based Ventilation (ASHRAE 62.1)
Number of Occupants
CFM per Person ASHRAE 62.1: Office 5–20 CFM/person, Classroom 10–15, Gym 20–30
Floor Area (sq ft) (for area component)
CFM per sq ft (area component) ASHRAE 62.1 default: 0.06 CFM/sq ft for offices
Calculate CFM
#### Formulas Used
Method 1 — Air Changes per Hour (ACH):
CFM = (Room Volume × ACH) ÷ 60 Room Volume (ft³) = Length × Width × Height
Method 2 — Sensible Heat Load:
CFM = Q ÷ (1.08 × ΔT) Where Q = sensible heat (BTU/hr), ΔT = temperature difference (°F) 1.08 = 0.075 lb/ft³ (air density) × 0.24 BTU/lb·°F (specific heat) × 60 min/hr
Method 3 — Duct Velocity:
CFM = Duct Area (ft²) × Velocity (FPM) Round: Area = π × (D/2)² ÷ 144 [D in inches → ft²] Rectangular: Area = Width × Height ÷ 144
Method 4 — People-Based (ASHRAE 62.1):
CFM = (Occupants × CFM/person) + (Floor Area × CFM/ft²)
#### Assumptions & References
- The constant 1.08 in the heat load formula = 60 × 0.075 × 0.24 (specific heat of air at constant pressure).
(function() {
function airShowMethod() { var m = document.getElementById('air-method').value; ['ach','heat','duct','exhaust'].forEach(function(id) { document.getElementById('air-method-' + id).style.display = (id === m) ? '' : 'none'; }); document.getElementById('air-result').style.display = 'none'; }
function airDuctShape() { var s = document.getElementById('air-duct-shape').value; document.getElementById('air-duct-round').style.display = (s === 'round') ? '' : 'none'; document.getElementById('air-duct-rect').style.display = (s === 'rect') ? '' : 'none'; }
function airCalc() { var resultDiv = document.getElementById('air-result'); var method = document.getElementById('air-method').value; var errors = []; var cfm = 0; var details = '';
if (method === 'ach') { var L = parseFloat(document.getElementById('air-room-length').value); var W = parseFloat(document.getElementById('air-room-width').value); var H = parseFloat(document.getElementById('air-room-height').value); var ACH = parseFloat(document.getElementById('air-ach').value); if (isNaN(L) || L ' + 'Room Volume' + vol.toFixed(1) + ' ft³' + 'Air Changes per Hour' + ACH + ' ACH' + 'Formula(' + vol.toFixed(1) + ' × ' + ACH + ') ÷ 60' + ''; }
} else if (method === 'heat') { var Q = parseFloat(document.getElementById('air-heat-btu').value); var dT = parseFloat(document.getElementById('air-heat-dt').value); if (isNaN(Q) || Q ' + 'Sensible Heat Load (Q)' + Q.toLocaleString() + ' BTU/hr' + 'Temperature Difference (ΔT)' + dT + ' °F' + 'Formula' + Q.toLocaleString() + ' ÷ (1.08 × ' + dT + ')' + ''; }
} else if (method === 'duct') { var vel = parseFloat(document.getElementById('air-duct-vel').value); var shape = document.getElementById('air-duct-shape').value; var area = 0; var areaDesc = ''; if (isNaN(vel) || vel ' + 'Duct Cross-Section Area' + area.toFixed(4) + ' ft²' + 'Area Calculation' + areaDesc + '' + 'Air Velocity' + vel.toLocaleString() + ' FPM' + 'Formula' + area.toFixed(4) + ' ft² × ' + vel.toLocaleString() + ' FPM' + ''; }
} else if (method === 'exhaust') { var ppl = parseFloat(document.getElementById('air-people').value); var cfmPer = parseFloat(document.getElementById('air-cfm-person').value); var flArea = parseFloat(document.getElementById('air-floor-area').value); var cfmSqft = parseFloat(document.getElementById('air-cfm-sqft').value); if (isNaN(ppl) || ppl ' + 'People Component' + ppl + ' × ' + cfmPer + ' = ' + peopleCFM.toFixed(1) + ' CFM' + 'Area Component' + flArea + ' × ' + cfmSqft + ' = ' + areaCFM.toFixed(1) + ' CFM' + 'Total (People + Area)' + peopleCFM.toFixed(1) + ' + ' + areaCFM.toFixed(1) + ' CFM' + ''; } }
if (errors.length > 0) { resultDiv.style.display = 'block'; resultDiv.className = 'calc-result calc-error'; resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }
// Blower sizing recommendation var safetyFactor = cfm * 1.15; var cfmRounded = Math.ceil(cfm / 50) * 50;
// ACH back-calculation for context var achNote = ''; if (method !== 'ach') { achNote = ''; }
resultDiv.style.display = 'block'; resultDiv.className = 'calc-result calc-success'; resultDiv.innerHTML = 'Required Airflow: ' + cfm.toFixed(1) + ' CFM' + details + '' + 'Blower / Fan Sizing Recommendations:' + '' + '- Minimum rated CFM: ' + cfm.toFixed(1) + ' CFM' + '- With 15% safety factor: ' + safetyFactor.toFixed(1) + ' CFM' + '- Suggested blower selection (round up to nearest 50 CFM): ' + cfmRounded + ' CFM' + '- Equivalent: ' + (cfm * 1.699).toFixed(1) + ' m³/hr | ' + (cfm * 0.000472).toFixed(4) + ' m³/s' + '' + '' + 'Note: Add 10–20% for duct losses, filter resistance, and system inefficiencies in final blower selection.'; }
// Expose to global scope window.airShowMethod = airShowMethod; window.airDuctShape = airDuctShape; window.airCalc = airCalc;
})();
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