CFM Calculator
Calculate the airflow a room needs from its dimensions and target air changes per hour (ACH) — with a recommended ACH chart by room type for fast, accurate ventilation sizing.
CFM = (length × width × ceiling height × ACH) ÷ 60. The ton figure uses a ~400 CFM-per-ton rule of thumb for context only. Verify final airflow and duct sizing against manufacturer data and a Manual D calculation.
How to calculate CFM
CFM stands for cubic feet per minute — the volume of air a fan, register, or exhaust system moves each minute. To size airflow for a space, you start from how much air the room holds and how often you want to replace it. That replacement rate is called air changes per hour (ACH). A bedroom might only need 5–6 ACH, while a busy commercial kitchen can need 15–30 to clear heat and odor.
Multiply the room volume by the target ACH to get cubic feet per hour, then divide by 60 to convert to cubic feet per minute. That single figure tells you what your fan, exhaust, or supply register has to deliver.
The CFM formula and a worked example
Take a 15 ft × 12 ft bedroom with 8 ft ceilings, targeting 6 ACH. The volume is 15 × 12 × 8 = 1,440 cubic feet. Multiply by 6 air changes to get 8,640 cubic feet per hour, then divide by 60: that's 144 CFM. If you were sizing a bathroom exhaust or a supply register for that room, 144 CFM is your target. The calculator above runs this the moment you enter the numbers.
Recommended ACH chart by room type
Air change requirements vary widely by how a space is used. Use these common ranges as a starting point, then adjust for occupancy, heat load, and local code:
| Room type | Recommended ACH |
|---|---|
| Bedroom / living area | 5 – 6 |
| Kitchen | 7 – 8 |
| Bathroom | 6 – 8 |
| Office / classroom | 6 – 8 |
| Retail / restaurant dining | 8 – 12 |
| Commercial kitchen | 15 – 30 |
| Warehouse / storage | 3 – 6 |
| Garage / workshop | 6 – 8 |
CFM per ton — a quick sanity check
On the equipment side, HVAC airflow is often described in CFM per ton of cooling. The industry rule of thumb is roughly 400 CFM per ton, ranging from about 350 CFM/ton in humid climates (to pull more moisture) up to 450 CFM/ton in dry ones. So a 3-ton system typically moves around 1,200 CFM. The calculator shows an approximate ton-equivalent for the airflow you enter, which is handy for cross-checking a blower against a room load.
Common CFM mistakes
- Forgetting ceiling height. CFM scales with volume, not floor area — a room with 10 ft ceilings needs 25% more airflow than the same footprint at 8 ft.
- Using the wrong ACH. Applying a bedroom rate to a kitchen or bathroom under-ventilates the space and leaves odor and moisture behind.
- Ignoring duct losses. The register CFM and the fan rating differ once you account for duct length, fittings, and static pressure. Confirm with a Manual D design.
From airflow math to a booked job
A fast CFM estimate keeps a ventilation conversation moving, but the install still has to be quoted, scheduled, and invoiced. Service Storm keeps estimating, dispatch, and payments in one platform built for HVAC companies.
Size the airflow, quote the work, dispatch the tech, and collect payment on site — without juggling five different apps.
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