Duct CFM Airflow Calculator
Every room in a forced-air HVAC system requires a specific volume of conditioned air expressed in cubic feet per minute (CFM). The required CFM is calculated from the room's sensible heat load and the temperature difference between the supply air and the room's design temperature. The standard HVAC formula is CFM = BTU/h divided by (1.08 times the temperature difference in degrees Fahrenheit). The constant 1.08 accounts for air density and specific heat at standard conditions. This calculator lets you solve for required CFM given a BTU/h load and a supply air temperature, or check an existing system's airflow capacity.
CFM airflow formula
CFM = BTU/h ÷ (1.08 × dT)
dT = Room temperature (F) − Supply air temperature (F)
The constant 1.08 = air density (0.075 lb/ft3) x specific heat (0.24 BTU/lb-F) x 60 min/h. This formula applies to sensible (dry) heat only. For spaces with significant humidity loads (latent heat), a separate latent CFM calculation is needed and total CFM is the larger of the two. For heating, dT is supply temperature minus room temperature (reversed sign but same magnitude).
Ductwork design principles
- Trunk ducts serving multiple zones should be sized for the total CFM they carry at velocities of 600 to 900 FPM to minimize noise and pressure drop.
- Branch ducts to individual rooms are typically sized for 400 to 600 FPM to keep noise acceptable at registers.
- Flex duct is more restrictive than sheet metal; add 10-15% to equivalent length calculations when using flex duct runs.
- Duct leakage is a major efficiency loss; Energy Star requires ducts to leak less than 4% of system airflow in conditioned space installations.
- ACCA Manual D is the industry standard method for residential duct system design, covering friction rate, fitting losses, and system balancing.
Duct CFM airflow: frequently asked questions
What is CFM in HVAC?
CFM stands for cubic feet per minute, the volume flow rate of air through HVAC ductwork. Higher CFM delivers more conditioned air per minute. Each room or zone in a system has a required CFM based on its heat load and the temperature difference between supply air and room air.
Where does the 1.08 constant come from?
The constant 1.08 equals the product of air density (0.075 lb/ft3 at standard conditions), specific heat of air (0.24 BTU/lb-F), and the unit conversion factor 60 minutes/hour: 0.075 x 0.24 x 60 = 1.08. It simplifies the sensible heat equation to CFM = BTU/h / (1.08 x temperature difference in F).
What supply air temperature should I use for cooling?
Residential central AC systems typically supply air at 55 to 60 degrees F. If room design temperature is 75 F and supply air is 55 F, the temperature difference (dT) is 20 F. Using a dT of 18 to 22 F is common in residential design. Use the actual temperature difference for your system.
How do I convert CFM to duct size?
For rectangular ducts, CFM = velocity (FPM) x area (sq ft). Typical residential trunk velocity is 600 to 900 FPM; branch ducts are 400 to 600 FPM. For a 100 CFM branch at 500 FPM, the area is 100/500 = 0.2 sq ft, or about a 6-inch round duct. ACCA Manual D gives complete duct sizing tables.
Can I use CFM to check if my ductwork is correctly sized?
Yes. Measure the actual airflow at each register with an airflow meter or anemometer, then compare to the design CFM for that room. Rooms that receive significantly less than design CFM may have undersized ducts, damper restrictions, or excessive duct leakage.
Official sources
- ACCA: Manual D Residential Duct Systems.
- ASHRAE: Fundamentals of HVAC Design.
- U.S. DOE Energy Saver: Duct Sealing and Insulation.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.