Duct CFM Sizing Calculator
The airflow a duct must carry follows directly from the sensible heat equation, a standard relationship in HVAC engineering. The cubic feet per minute (CFM) needed equals the sensible heat load divided by the product of the air constant (1.08 at standard sea-level conditions) and the temperature difference between return and supply air. This calculator computes the required CFM from your load and temperature difference, and also shows the implied airflow per ton for comparison with the common 400 CFM-per-ton guideline. The air constant is editable for high-altitude or non-standard conditions.
Duct airflow formula
Required airflow (CFM) = sensible heat load / (air constant * temperature difference)
Load equivalent (tons) = sensible heat load / 12,000
Airflow per ton = required airflow / load equivalent (tons)
The air constant of 1.08 combines air's specific heat, density, and the minutes-per-hour factor at standard conditions. Comparing airflow per ton to the common 400 CFM-per-ton guideline is a useful sanity check.
Notes on duct airflow
- Enter the sensible load only; latent (moisture) load does not affect the air-temperature airflow.
- The air constant changes with altitude and temperature, so it is editable for non-standard conditions.
- Residential cooling often targets around 400 CFM per ton; compare your result to that.
- This gives the airflow target; sizing the duct itself needs friction rate and velocity (Manual D).
- Enter your own load and temperature difference; the tool assumes no fixed design value.
Duct CFM: frequently asked questions
How do I calculate the CFM a duct needs to carry?
Use the sensible heat equation: CFM = sensible heat load (BTU per hour) / (1.08 x temperature difference in degrees F). The constant 1.08 reflects the heat capacity and density of standard air at sea level. Rearranging the same equation lets you find load or temperature difference from the other two.
Where does the 1.08 constant come from?
It is the product of air's specific heat (about 0.24 BTU per pound per degree F), its density (about 0.075 pounds per cubic foot at standard conditions), and 60 minutes per hour: 0.24 x 0.075 x 60 = 1.08. At high altitude or unusual temperatures the constant changes, so it is editable here.
What is a typical supply temperature difference?
For cooling, the difference between return and supply air is often around 18 to 22 degrees F; for heating it can be larger. The exact value depends on the system and conditions, so enter your measured or design temperature difference rather than relying on a single fixed number.
What is the 400 CFM per ton rule of thumb?
Residential cooling systems are often designed for roughly 400 CFM of airflow per ton of cooling (12,000 BTU per hour). This calculator shows the airflow implied by your sensible load and temperature difference, which you can compare to the 400-per-ton guideline as a sanity check.
Does this size the duct itself?
It gives the required airflow in CFM. Translating CFM into a duct diameter or dimensions also needs the design friction rate and air velocity, typically from an ACCA Manual D calculation or a duct sizing chart. Use this tool for the airflow target, then size the duct to carry it.
Official sources
- U.S. Department of Energy, central air conditioning and ducts: energy.gov.
- Air Conditioning Contractors of America (Manual D duct design standard): acca.org.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.