Formula
Room CFM is the air changes per hour times the cubic feet, divided by 60. Reverse that and the fan’s CFM times 60, divided by the volume, is the air changes. A round duct’s area is π times the radius in feet, squared. A rectangle is width times height, in feet. CFM is that area times the velocity in feet per minute.
CFM = ACH × ft³ ÷ 60
ACH = CFM × 60 ÷ ft³
CFM = velocity × area
sensible CFM = BTU/h ÷ (1.08 × ΔT)
1.08 = 0.075 × 60 × 0.24
The 1.08 is standard air at sea level: about 0.075 lb/ft³, 60 minutes in an hour, and 0.24 BTU per pound per degree Fahrenheit. This page’s factor is 1.08. Altitude and a different temperature change the density, so the factor moves. Four hundred CFM per ton is a cooling rule of thumb. Humid air is often planned closer to 350 so the coil stays cold long enough to dry the air. Supply branches are often kept around 600–900 feet per minute because faster air is louder. Those speeds are typical, not a code limit.
Ventilation replaces room air. Circulation moves the same air around. A bathroom fan under the HVI guideline used here is sized on floor area, not on air changes. The living, kitchen, bathroom, and workshop presets are labelled guidelines. They are not ASHRAE 62.2 and they are not a local exhaust code.
Worked examples
12 × 14 × 8 ft at 6 ACH. 1,344 ft³, 134.4 CFM, about 228.3 m³/h and 63.4 L/s.
200 CFM in that room. About 8.93 air changes per hour.
8 in round at 700 FPM. About 0.3491 ft² and 244.3 CFM. 10 × 6 in at 600 FPM. 0.4167 ft² and 250.0 CFM.
36,000 BTU/h, ΔT 50°F. About 666.7 CFM. 3 tons × 400. 1,200 CFM.
Bathroom, 80 sq ft. 80 CFM. 40 sq ft. 50 CFM, because the guideline’s floor is 50.
Reference
Cubic metres per hour are CFM × 1.699011. Litres per second are CFM × 0.4719474. The bathroom line is 1 CFM per square foot up to 100 sq ft, and not less than 50 CFM.
Questions
How do you calculate CFM for a room?
Multiply the air changes per hour by the room volume in cubic feet and divide by 60. A 12 × 14 × 8 ft room is 1,344 ft³. At 6 air changes that is 134.4 CFM.
How many CFM per ton?
A common planning figure is 400 CFM per ton of cooling. Humid climates are often planned nearer 350. Three tons at 400 is 1,200 CFM. That is not a Manual J load.
How do you convert CFM to air changes per hour?
Multiply CFM by 60 and divide by the room volume. Two hundred CFM in 1,344 ft³ is about 8.93 air changes per hour.
What size bathroom fan do I need?
The Home Ventilating Institute guideline used here is 1 CFM per square foot for a bathroom up to 100 sq ft, and at least 50 CFM. An 8 × 10 ft bath is 80 CFM. A 5 × 8 ft bath is 40 sq ft, so the floor of 50 CFM applies. It is a guideline, not a code section.
How do you convert BTU to CFM?
Divide BTU per hour by 1.08 times the temperature difference in Fahrenheit. The 1.08 is standard air at sea level: 0.075 lb/ft³ × 60 × 0.24. Thirty-six thousand BTU/h across 50°F is about 666.7 CFM. Altitude and a different air temperature change the 1.08.
How many CFM go through an 8 inch duct?
Area is π times the radius in feet, squared. An 8 inch round duct is about 0.3491 ft². At 700 feet per minute that is about 244.3 CFM. The velocity is what you type. A quieter branch is often slower.
What air changes should a room have?
The presets are typical guidelines: living space about 4–6, a kitchen about 7–8, a bathroom about 8, a workshop or garage about 6–10. They are not code. ASHRAE 62.2 and the local code govern required ventilation.
Sources
- ASHRAE Handbook — Fundamentals, the sensible-heat equation for standard air. The 1.08 factor is 0.075 lb/ft³ × 60 min/h × 0.24 BTU/lb·°F. It is a sea-level standard-air assumption. This page does not correct it for altitude.
- Home Ventilating Institute, bathroom exhaust guideline: 1 CFM per square foot for bathrooms up to 100 square feet, with a 50 CFM minimum. Labelled here as a guideline.
- ASHRAE Standard 62.2, ventilation for residential buildings. Named as the place required ventilation is specified. The air-change presets on this page are not that standard, and they are not a local code.
- NIST Special Publication 811 for the cubic-foot conversions: 1.699011 cubic metres per hour per CFM, and 0.4719474 litres per second per CFM. Duct diameter from friction is the duct size calculator.