Exhaust fans, range hoods, HVAC supply registers and air purifiers are rated in CFM. To size one, decide how many times per hour the air should be replaced (ACH) and multiply by the room volume. Bathrooms often need at least 50 CFM (or 1 CFM per ft² of floor) and kitchens 100 CFM or more.
How to use the CFM Calculator
- Room length
- Room width
- Ceiling height
- Air changes per hour
- Every result field is editable: type into Required airflow and Air changes per hour plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the room length, width and ceiling height and the air changes per hour you want. Type a fan’s CFM rating into the result to see the ACH it delivers.
Tips for an accurate estimate
- Keep units consistent – the calculator handles conversions, but check every field’s unit before reading a result.
- Use values from standards, data sheets or test pieces whenever you have them; typical values are only a starting point.
- Apply appropriate safety factors and have structural, electrical and pressure work checked by a qualified professional.
- Record the inputs you used so the calculation can be reviewed later.
Common mistakes to avoid
- Confusing diameter and radius, or nominal and actual sizes.
- Using allowable stresses and ultimate strengths interchangeably.
- Treating rule-of-thumb results as a substitute for code tables or an engineered design.
Inputs explained
| Field | What to enter |
|---|---|
| Room length | Units: ft, m, in, yd, cm |
| Room width | Units: ft, m, in, yd, cm |
| Ceiling height | Units: ft, m, in, yd, cm |
| Air changes per hour | A number |
What each result means
- Room volume = [Room length]×[Room width]×[Ceiling height]
- Required airflow = [Room volume]×[Air changes per hour]÷3600
Example values
How the main result changes when Air changes per hour changes and everything else stays at the example values above:
| Air changes per hour | Required airflow (CFM) |
|---|---|
| 4 | 64 |
| 6 | 96 |
| 8 | 128 |
| 10 | 160 |
| 12 | 192 |
| 16 | 256 |
Formula
CFM = room volume (ft³) × ACH ÷ 60Worked example
Inputs — Room length: 12 ft; Room width: 10 ft; Ceiling height: 8 ft; Air changes per hour: 8.
Results — Room volume = 960 ft³; Required airflow = 128 CFM.
Frequently asked questions
How many CFM do I need for a bathroom?
HVI recommends 1 CFM per ft² of floor (minimum 50 CFM) for bathrooms up to 100 ft².
How do I convert CFM to m³/h?
Multiply by 1.699.
What ACH should I use?
6–8 for bathrooms and kitchens, 4–6 for living spaces with a purifier, 10–12 for workshops and garages.
What formula does the CFM Calculator use?
CFM = room volume (ft³) × ACH ÷ 60.