DUCT CFM
DUCT CFM
CFM = BTU ÷ (1.08 × ΔT)
BTU/hr
°F
RESULT
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FILL IN ABOVE
CFM = BTU ÷ (1.08 × ΔT). Cooling ΔT ≈ 20°F. Forced-air heating ΔT ≈ 60°F.
About this calculator
This duct CFM calculator gives the airflow needed (in cubic feet per minute) to deliver a given heating or cooling load. The sensible heat formula CFM = BTU/hr ÷ (1.08 × ΔT) is what HVAC contractors use for room-by-room duct sizing in a Manual D layout. ΔT is the difference between the supply air temperature and the return air temperature — typically 20°F for cooling and 50–70°F for forced-air heating. The industry rule of thumb is 400 CFM per ton of cooling (1 ton = 12,000 BTU/hr), which lines up with a 20°F cooling ΔT.
Common questions
What is the typical CFM per ton of cooling?
400 CFM per ton is the residential rule of thumb, which lines up with a 20°F supply-vs-return ΔT. High-humidity climates sometimes target 350 CFM/ton for better dehumidification; dry climates may run 450 CFM/ton for sensible-only cooling.
How do I figure CFM for a single room?
Two ways. (1) Run a Manual J load on that room, then apply CFM = BTU ÷ (1.08 × ΔT). (2) Proportional: room CFM = (room load ÷ total load) × system CFM. Method 2 is what most installers do once Manual J gives them the per-room loads.
Why does correct duct sizing matter?
Undersized ducts starve airflow — coils freeze in cooling mode, heat exchangers crack in heating mode, and equipment lifespan drops. Oversized ducts waste fan energy, leak more, and can produce uncomfortable air velocity. Manual D sizing pays back in efficiency and longevity.