Ductwork CFM Calculator
CFM (cubic feet per minute) = BTU/hr × 0.07. Used for sizing ducts.
Result
CFM needed
1,680
4.20 tons (cooling).
- BTU/hr24,000
- Rule0.07 CFM/BTU
- CFM1,680
- Tons2.00
- Round-trunk size (10"-12" typical)~10-12" round, 8×16 or 10×20 rectangular
Step-by-step
- CFM = 24,000 × 0.07 = 1,680 CFM.
How to use this calculator
- Enter system BTU/hr.
- Pick the appropriate rule.
- Read CFM, then size ducts via Manual D.
About this calculator
CFM (cubic feet per minute) is the air-flow rate through a duct. Cooling: 400 CFM per ton (or 0.07 CFM/BTU). Heating: 0.06 CFM/BTU. Tight homes with low air-change rates may use 0.10. Trunk and branch duct sizes follow Manual D — this calculator gives the headline CFM number you need to size for.
Frequently asked
Standard ASHRAE design assumption for typical US cooling loads. Yields 12-15°F supply-air temperature differential.
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