Ductwork CFM Calculator
CFM (cubic feet per minute) = BTU/hr × 0.07. Used for sizing ducts.
Result
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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
Why 400 CFM per ton?+
Standard ASHRAE design assumption for typical US cooling loads. Yields 12-15°F supply-air temperature differential.
Heating vs cooling CFM?+
Heating uses lower CFM (0.06) because supply air is hotter (20-30°F differential vs 15° for cooling).
Trunk vs branch sizing?+
Trunk: total CFM. Branches: room-by-room CFM. Manual D handles the full duct sizing.
Is this Manual D?+
No — this is the input. Manual D uses your CFM and friction-loss budget to compute exact duct dimensions.
Round vs rectangular ducts?+
Round: less friction, easier insulation. Rectangular: fits in walls/joists easier. Both work; pick based on space.
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