DUCT CFM
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. Duct sizing follows ACCA Manual D — match CFM per room from a Manual J load calc.
How to use this calculator
Enter the heat load in BTU/hr — for cooling, use the room's cooling load (~20 BTU/ft² rule of thumb or your Manual J number). For heating, use the room's heat loss. Set the supply temp differential (ΔT): 20°F for cooling (the standard delta between 55°F supply air and 75°F return), 50–70°F for forced-air heating, or whatever your equipment specs.
The calculator returns the airflow in CFM plus a comparison to the 400 CFM/ton rule of thumb (which is what equipment is rated for at standard cooling conditions). Use the result to size individual room registers and trunk ducts in a Manual D layout.
Worked example
For a 24,000 BTU/hr (2-ton) cooling load with 20°F supply ΔT:
CFM = 24,000 ÷ (1.08 × 20) = 24,000 ÷ 21.6 = 1,111 CFM. Rule of thumb: 2 tons × 400 = 800 CFM (lower because the rule assumes 22-25°F ΔT for proper dehumidification).
For a 60,000 BTU/hr forced-air heating load with 60°F ΔT:
CFM = 60,000 ÷ (1.08 × 60) = 925 CFM. Same blower, but the higher heating ΔT lets you move the same energy with less air than cooling.
For an individual bedroom with 4,000 BTU/hr cooling load: 4,000 ÷ 21.6 = 185 CFM. Two 6×12 supply registers (~95 CFM each at 0.10" w.c. drop) would handle this room.
Common mistakes & waste factors
Using cooling ΔT for heating. Cooling runs ~20°F ΔT; forced-air heating runs 50–70°F ΔT. Same load, different airflow. Mixing them up oversizes ducts by 3×.
Forgetting that 400 CFM/ton is a rule of thumb. Variable-speed equipment can run 350-450 CFM/ton depending on indoor humidity and conditions. The CFM = BTU ÷ (1.08 × ΔT) formula is more accurate for individual room sizing.
Ignoring static pressure. CFM is theoretical airflow at zero static. Real ducts have friction losses; actual CFM is 70-90% of theoretical for well-designed systems.
Sizing only for cooling. In cold climates, the heating CFM might be lower than cooling — but the duct still needs to be sized for the larger of the two loads.
Rules of thumb
CFM = BTU/hr ÷ (1.08 × ΔT). 1.08 = constant for standard air conditions.
Cooling ΔT: 20°F (high humidity climates) to 25°F (dry climates).
Forced-air heating ΔT: 50–70°F (entry-level furnaces 50°F, modulating 70°F).
400 CFM per ton of cooling is the equipment-side rule. Lower CFM/ton dehumidifies more; higher CFM/ton cools faster.
Individual room registers: 6×12 ~95 CFM, 4×10 ~70 CFM, 8×16 ~150 CFM at 0.10" w.c. friction loss.
Common questions
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