CFM & Air Changes (ACH) Calculator
How much airflow does a room need? Pick the room type and dimensions to get the required fan airflow (CFM), or enter a fan’s CFM to see the air changes per hour it delivers and whether that is enough.
Sizing Airflow for a Room
Ventilation is usually specified as air changes per hour (ACH) — how many times the entire volume of air in a room is replaced each hour. To size a fan you turn that target into an airflow: multiply the air changes you want by the room volume. The result is the CFM (cubic feet per minute) or cubic metres per hour the fan must move. This calculator does both directions: airflow needed for a target ACH, or the ACH a given fan actually delivers.
What Air Changes per Hour Means
An ACH of 8 means the room’s air is fully exchanged eight times an hour, once every 7.5 minutes. Higher rates suit spaces that generate moisture, heat, odours or fumes — bathrooms, kitchens, workshops — while bedrooms and offices need less. The rate is a guideline; building codes such as ASHRAE 62.1 and 62.2 set required ventilation for habitable and hazardous spaces.
Recommended Air Change Rates
| Space | Typical ACH |
|---|---|
| Bathroom | 8 |
| Kitchen | 15 |
| Bedroom | 5 to 6 |
| Living room | 6 |
| Home office | 6 |
| Garage | 8 |
| Workshop | 10 |
| Restaurant | 10 |
| Warehouse | 4 |
CFM, L/s and m³/h
Airflow is quoted as CFM in the US and as litres per second or cubic metres per hour elsewhere. One CFM is about 1.7 m³/h or 0.47 L/s. The calculator shows the metric equivalent alongside CFM so a spec reads the same either way. Remember that a fan’s rated airflow falls once it is fighting duct length, bends and grille resistance, so size up from the bare requirement.
Frequently Asked Questions
How do I calculate CFM for a room?
Multiply the air changes per hour you want by the room volume in cubic feet, then divide by 60. For a target of 8 ACH in a 320 cubic foot bathroom that is about 43 CFM.
What is a good number of air changes per hour?
Roughly 5 to 6 for bedrooms and offices, 8 for bathrooms and garages, 10 for workshops, and 15 for kitchens. Follow local code where it applies.
How do I convert CFM to air changes per hour?
Multiply CFM by 60 and divide by the room volume in cubic feet. The result is how many times per hour the air is replaced.
Should I oversize the fan?
Yes, a little. Rated CFM drops with duct runs and fittings, so choosing the next size up keeps the real delivered airflow on target.
Related calculators
- Duct Size Calculator — size the duct for that airflow.
- BTU Calculator — heating and cooling load for the room.
- Pipe Flow Calculator — velocity and flow in a duct or pipe.