By Zencalcy Editorial TeamLast reviewed About us
Air changes per hour measures how many times the air in a room is replaced each hour. Bathrooms want 8 to 12 ACH for moisture control, workshops 4 to 6 for dust, and living spaces rely on infiltration plus a small mechanical rate. The number ties room volume to airflow in CFM.
ACH ventilation rate from room size and airflow.
volume = length × width × height ACH = CFM × 60 ÷ volume CFM = ACH × volume ÷ 60
CFM is cubic feet per minute of airflow; ACH multiplies it by 60 and divides by the room volume in cubic feet. Duct losses and filter pressure drop mean installed flow often lands 10 to 15% below the fan rating.
| Symbol | Meaning |
|---|---|
| V | Room volume in cubic feet. |
| CFM | Airflow, cubic feet per minute. |
| ACH | Air changes per hour. |
Volume is 1,152 cu ft; 120 × 60 ÷ 1,152 = 6.3 air changes per hour — right for a small workshop.
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8 to 12 air changes per hour, which for a typical 5×8×8 bath is a 50 to 80 CFM fan. Vent it outside, not into the attic.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026