CFM Calculator
CFM = volume × ACH ÷ 60. Not a Manual J BTU size. Click any i for detail.
Room
Length (ft)
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Length
Room length. |
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|---|---|
Width (ft)
i
Width
Room width. |
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Height (ft)
i
Height
Ceiling height. |
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Air changes / hour
i
ACH
How many times the room air is replaced per hour. Bath exhaust is often sketched near 8; whole-house ventilation is much lower. |
How to use this calculator
- Enter room L × W × H.
- Set ACH for the job (bath vs whole house differ a lot).
- Read CFM.
- Mini-split / BTU sizing is a different page.
Results explained
Volume = L × W × H. CFM = volume × ACH / 60. Dew point is weather math, not airflow.
Quick reference: CFM
A 12×12×8 ft room at 4 ACH is 76.8 CFM.
| Item | Detail |
|---|---|
| 12×12×8 ft | 1,152 ft³ |
| 4 ACH | 76.8 CFM |
| Bath sketch | Often ~8 ACH — confirm locally |
| BTU / mini-split | Mini-split sizing calculator |
Duct leakage and filter restriction are not in this number.
How the estimate is built
CFM = (L × W × H × ACH) / 60.
Example scenario
12×12×8 ft, 4 ACH → 77 CFM.
FAQ
Kitchen range hood?
Capture CFM is a hood spec, not this room ACH model.
Whole-house 0.35 ACH?
Enter 0.35 — the formula does not care.
Cooling load?
Use mini-split sizing / HVAC cost pages.