DUCT CFM
Duct CFM chart — airflow by heat load and temperature split
Airflow needed to move a given load, from the sensible heat equation. The 20–25°F columns are cooling, where the split runs between roughly 55°F supply air and 75°F return. The 60–70°F columns are forced-air heating, and they are why the same blower moves far less air in winter: a wider temperature split carries the same energy on less air.
| Heat load | Tons | ΔT 20°F | ΔT 25°F | ΔT 60°F | ΔT 70°F | 400 CFM/ton rule |
|---|---|---|---|---|---|---|
| 6,000 BTU/hr | 0.50 | 278 | 222 | 93 | 79 | 200 |
| 9,000 BTU/hr | 0.75 | 417 | 333 | 139 | 119 | 300 |
| 12,000 BTU/hr | 1.00 | 556 | 444 | 185 | 159 | 400 |
| 18,000 BTU/hr | 1.50 | 833 | 667 | 278 | 238 | 600 |
| 24,000 BTU/hr | 2.00 | 1,111 | 889 | 370 | 317 | 800 |
| 30,000 BTU/hr | 2.50 | 1,389 | 1,111 | 463 | 397 | 1,000 |
| 36,000 BTU/hr | 3.00 | 1,667 | 1,333 | 556 | 476 | 1,200 |
| 42,000 BTU/hr | 3.50 | 1,944 | 1,556 | 648 | 556 | 1,400 |
| 48,000 BTU/hr | 4.00 | 2,222 | 1,778 | 741 | 635 | 1,600 |
| 60,000 BTU/hr | 5.00 | 2,778 | 2,222 | 926 | 794 | 2,000 |
CFM = BTU/hr ÷ (1.08 × ΔT), the standard-air sensible heat equation used for Manual D duct layout. The last column is the equipment-side 400 CFM per ton rule of thumb.
These are theoretical airflows at zero static pressure. Real duct runs deliver 70–90% of this once friction is counted, and register and trunk sizing come from a Manual D layout off a room-by-room Manual J. Run your own load and split above.
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) = 926 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
Tool and material links below are affiliate links — we may earn a small commission if you buy, at no extra cost to you.