Airflow / ACH Calculator
Enter room volume and target air changes per hour to get the required airflow in CFM, L/s and m³/h — or verify the ACH an existing fan delivers.
Open calculatorLast updated: 2026-09-15
How the calculation works
- One air change replaces the full room volume once; per minute that is volume/60, multiplied by the ACH target.
- The result is shown in CFM, litres per second and cubic metres per hour for international fans.
Worked example
Interpreting the result
The ACH target depends on the room's job: bathrooms and kitchens need high rates to remove moisture and odors quickly; bedrooms need moderate rates for air quality; whole-house ventilation runs at a low continuous rate. Exceeding the target wastes heating and cooling energy, so pick the target for the room type, not the maximum.
Assumptions
- Well-mixed room air — real rooms stratify, and effective ventilation is lower than the theoretical value.
- Fan rating at typical static pressure; real flow drops with ducting resistance.
Limitations
- Does not account for duct losses, backdraft dampers or filter loading.
- Codes (IMC/ASHRAE 62.2) may specify airflow directly rather than ACH — check local requirements.
Required airflow (CFM) by room volume and air changes per hour
Each cell is the calculator's result for that room volume and air-change target.
| Room volume | 4 ACH | 6 ACH | 8 ACH | 10 ACH | 12 ACH |
|---|---|---|---|---|---|
| 300 ft³ | 20 | 30 | 40 | 50 | 60 |
| 500 ft³ | 33 | 50 | 67 | 83 | 100 |
| 1,000 ft³ | 67 | 100 | 133 | 167 | 200 |
| 1,500 ft³ | 100 | 150 | 200 | 250 | 300 |
| 2,000 ft³ | 133 | 200 | 267 | 333 | 400 |
| 3,000 ft³ | 200 | 300 | 400 | 500 | 600 |
CFM = volume (ft³) × ACH ÷ 60. Round up to the next fan size that is available.
Common bathroom sizes at 8 air changes per hour (8 ft ceiling)
| Room | Volume (ft³) | Airflow (CFM) | Airflow (L/s) |
|---|---|---|---|
| 5 × 8 ft | 320 | 43 | 20 |
| 6 × 9 ft | 432 | 58 | 27 |
| 8 × 10 ft | 640 | 85 | 40 |
| 10 × 12 ft | 960 | 128 | 60 |
| 12 × 15 ft | 1,440 | 192 | 91 |
Worked scenario: an exhaust fan for a 5 × 8 ft bathroom
A 5 × 8 ft bathroom with an 8 ft ceiling holds 320 ft³. At 8 air changes per hour the calculator returns 42.7 CFM (20.1 L/s). Fans are sold in standard sizes, so the practical choice is the next size up.
Small-bathroom fans are commonly sold at 50 CFM, which matches the intermittent bathroom exhaust minimum in ASHRAE Standard 62.2. Check which edition your local code has adopted before relying on a single figure.
Mistakes that make the answer wrong
- Using floor area instead of volume. Ceiling height multiplies the airflow you need.
- Trusting the label rating. Ducts, elbows and a roof cap add resistance, and an installed fan delivers less than its free-air rating.
- Mixing up ACH with CFM per square foot. They are different ways of stating a ventilation requirement and are not interchangeable.
- Sizing exhaust without make-up air. In a tight house a strong exhaust fan can starve itself; the calculator sizes the exhaust only.
Frequently asked questions
How many CFM do I need for a bathroom?
The common rule is 1 CFM per ft² of floor area, which for a standard bathroom equals about 8 ACH. A 10 × 10 bathroom needs ~100 CFM.
What ACH should a bedroom have?
5–6 ACH with the door closed is a common target for air quality; whole-house continuous ventilation is much lower, around 0.35 ACH.