Room Air Changes Calculator
Turn a room’s size and its fan or vent airflow into air changes per hour (ACH), and find the CFM you need to hit a target ventilation rate.
How it works
Volume = L × W × H · ACH = CFM × 60 ÷ volume · CFM needed = target ACH × volume ÷ 60
Air changes per hour (ACH) counts how many times a room’s entire air volume is replaced each hour — the standard measure of ventilation. Because fan ratings are in cubic feet per minute, you multiply by 60 to get an hourly figure and divide by the room’s volume. Rearranged, the same relationship tells you the CFM a fan must move to hit a recommended rate: about 8 ACH for bathrooms, 7–8 for kitchens, and 5–6 for living spaces.
Worked example
A 12 × 10 × 8 ft room holds 960 ft³. A 100 CFM bath fan moves 100 × 60 = 6,000 ft³ per hour, so ACH = 6,000 ÷ 960 = 6.25. To reach the 8 ACH bathroom target you would need 8 × 960 ÷ 60 = 128 CFM, so the 100 CFM fan is slightly undersized.
Frequently asked questions
What is a good number of air changes per hour?
It depends on the room. Bathrooms are usually specified at 8 ACH, kitchens 7–8, and bedrooms or living rooms 5–6. Higher rates suit spaces that generate moisture, odours or fumes.
How do I convert ACH back to a fan size?
Multiply the target ACH by the room volume in cubic feet, then divide by 60. The result is the CFM your exhaust fan or supply vent must deliver. This calculator shows that figure alongside your current ACH.
Does duct length affect the result?
Yes in practice — long or restrictive ducting reduces a fan’s effective airflow below its rated CFM. If your runs are long or have several bends, size up a step so the delivered airflow still meets your target.
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This calculator is for educational purposes. Double-check important results before acting on them.