Room Air Changes & CFM Calculator
Ventilation airflow for a room from its size and target air changes per hour — recommended ACH by room type (kitchen, bathroom, office, classroom, lab, hospital), CFM, m³/h and L/s, the ACH your fans deliver after duct losses, contaminant clearance times, HVI bathroom sizing and ASHRAE 62.1 outdoor air per person.
How it works
volume = length × width × height (or floor area × height) · m³/h = volume (m³) × ACH · CFM = m³/h ÷ 1.699 = volume (ft³) × ACH ÷ 60 · L/s = m³/h ÷ 3.6 · ACH delivered = fan CFM × fans × (1 − duct loss) × 1.699 ÷ volume (m³) · removal time = ln(1 ÷ (1 − efficiency)) ÷ ACH × 60 min
Air changes per hour express ventilation relative to room size: at 8 ACH a fan moves a volume of air equal to the room every 7.5 minutes. Recommended rates scale with how much a space generates — a bedroom needs less than one air change of fresh air an hour, a home bathroom or kitchen 6–8 to clear moisture and odours, a conference room 8–12, a commercial kitchen 20 or more, and health-care rooms follow their own standards (12 for airborne isolation, 20 for operating rooms). Because a well-mixed room is diluted exponentially, three air changes remove about 95% of what was in the air and 4.6 remove 99%, which is how the CDC sets clearance times for isolation rooms. Fan ratings are optimistic: they assume very little duct resistance, so a bathroom fan on a long flexible duct may deliver half its nameplate; the duct-loss field and the number of fans let you check what actually arrives. For occupied rooms such as offices and classrooms, ASHRAE 62.1 also sets a minimum of outdoor air per person plus per square foot of floor, and HVI sizes bathroom fans at 1 CFM per square foot of floor (or by fixtures in large bathrooms).
Worked example
A 6 × 4.5 × 2.7 m kitchen (72.9 m³) at 8 ACH needs 72.9 × 8 = 583 m³/h, about 343 CFM. A 150 CFM range hood alone gives 150 × 1.699 ÷ 72.9 = 3.5 ACH, so a supplementary fan or a bigger hood is needed. At 8 ACH, 95% of smoke and odour clears in about 22 minutes and 99% in about 35 minutes; at the hood’s 3.5 ACH it takes about 79 minutes to reach 99%.
Sources: CDC — Guidelines for Environmental Infection Control, Appendix B: Air (Table B.1 clearance times, Table B.2 ACH) · Home Ventilating Institute — bathroom ventilation sizing · Home Ventilating Institute — how much ventilation do I need (range hoods)
Frequently asked questions
How do I calculate CFM from air changes per hour?
Multiply the room volume in cubic feet by the ACH and divide by 60. A 12 × 10 ft bedroom with an 8 ft ceiling is 960 ft³; at 6 ACH that is 960 × 6 ÷ 60 = 96 CFM. In metric, volume in m³ × ACH gives m³/h, and m³/h ÷ 1.699 gives CFM.
How many air changes per hour does a bathroom need?
About 6–8. HVI sizes bathroom fans at 1 CFM per square foot of floor with a 50 CFM minimum (about 8 ACH with an 8 ft ceiling); for bathrooms over 100 sq ft add 50 CFM for each toilet, shower and tub and 100 for a jetted tub. Choose Bathroom to get both figures.
Is more ACH always better?
For contaminant removal yes, but each air change costs heating or cooling energy and can create drafts. Heat- or energy-recovery ventilators let you ventilate well without the full energy penalty.
How long does it take to clear the air in a room?
In a well-mixed room, time = ln(1 ÷ fraction left) ÷ ACH × 60 minutes. At 6 ACH, 99% removal takes 46 minutes and 99.9% takes 69 minutes, the figures the CDC uses for clearing patient rooms; at 12 ACH it halves.
How do I measure actual ACH?
Measure airflow at the grille with an anemometer or balometer, or do a tracer-gas or CO₂ decay test: ACH = ln(C₁/C₂) ÷ hours between readings.
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This calculator is for educational purposes. Double-check important results before acting on them.