How this CFM calculator works
Ventilation starts with one identity — flow equals volume times how often you want that volume replaced:
L/s = CFM × 0.4719 · m³/h = CFM × 1.699
Worked example: CFM for a 12 × 12 room
1,152 ft³ of room at 6 air changes per hour needs 6,912 ft³ moved every hour — divide by 60 and that's 115 CFM (54 L/s, 196 m³/h). Halve the ACH target and the fan halves with it.
Air changes per hour by room
| Room | ACH |
|---|---|
| Bedroom | 5–6 |
| Living room | 6–8 |
| Kitchen | 7–8 |
| Bathroom | 6–8 |
| Office | 6–8 |
| Basement | 3–4 |
CFM FAQ
How do you calculate CFM for a room?
CFM = room volume (ft³) × air changes per hour ÷ 60. A 12 × 12 ft room with an 8 ft ceiling is 1,152 ft³; at 6 ACH that needs 115 CFM. The calculator does the division and the metric conversions.
How many air changes per hour should a room have?
Typical comfort targets: bedrooms 5–6, living rooms 6–8, kitchens 7–8, bathrooms 6–8, basements 3–4. Code-driven ventilation (ASHRAE 62.2 whole-house rates, commercial occupancy tables) and equipment specs override these rules of thumb.
What size bathroom exhaust fan do I need?
The HVI rule is 1 CFM per square foot of floor with a 50 CFM minimum — an 8 × 10 bath wants 80 CFM. By air changes, the same room at 8 ACH computes to 85 CFM; size to the larger and vent it outdoors, never into the attic.
What is the difference between CFM and ACH?
CFM is a flow rate — cubic feet moved per minute. ACH counts how many times the room's whole volume is replaced in an hour. They convert through volume: ACH = CFM × 60 ÷ ft³, which is why the same fan gives a small room far more air changes than a big one.
How many CFM per square foot?
At 8 ft ceilings, 1 ACH is 0.133 CFM/ft² — so the common 6 ACH target is about 0.8 CFM per square foot of floor. Taller ceilings need proportionally more; that is why the calculator works from volume, not floor area.
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