Airflow CFM Calculator
Heat, Airflow, and Temperature Rise Are Locked Together
The same BTU/hr of heating or cooling capacity can be delivered by a small volume of air at a large temperature swing, or a large volume of air at a gentle one — and HVAC designers use the standard sensible heat formula to move between the two. This calculator rearranges that formula to find the airflow, in CFM, required to deliver a known heat load at a chosen supply-to-room temperature difference.
The Formula
CFM = BTU/hr / (1.08 × Delta T)
1.08 is the standard sensible heat factor for air at sea-level density and specific heat, combining air's density and specific heat capacity into a single constant. Delta T is the temperature difference, in °F, between supply air and room air.
A Worked Example
| Input | Value |
|---|---|
| Heat Load | 36,000 BTU/hr |
| Delta T | 20°F |
| Required Airflow | 1,666.7 CFM |
CFM = 36,000 ÷ (1.08 × 20) = 1,666.7 CFM.
Where This Calculation Matters
- Blower and equipment selection — confirming a unit's rated CFM output can deliver the required airflow at a realistic delta T.
- Duct system design — the CFM figure this produces is the direct input to duct sizing for each branch.
- Diagnosing comfort complaints — a system delivering less CFM than required at its design delta T will underperform even if the rated tonnage looks adequate on paper.
- Commissioning and balancing — technicians compare measured CFM against the calculated requirement when balancing airflow to each room.
How to Use This Calculator
- Enter the Heat Load in BTU/hr.
- Enter the Temperature Difference (Delta T) in °F between supply and room air.
- Select Calculate to see the required airflow in CFM.
Related Calculations
Turn the resulting CFM into a duct size with the HVAC Duct Sizer Calculator, or check how many air changes per hour that airflow produces with the Air Changes Per Hour Calculator.