Duct CFM Calculator
Calculate airflow volume (CFM) from velocity and duct size.
Calculated Airflow:
How to Calculate Duct CFM
CFM (Cubic Feet per Minute) is the measurement of air volume velocity. It tells you exactly how much air is moving through a heating or cooling system. To calculate it, you need two things: the size of the duct (Area) and how fast the air is moving (Velocity).
The Formula
The calculation is based on the fundamental airflow equation:
CFM = Area (sq ft) × Velocity (FPM)
Step 1: Calculate Cross-Sectional Area
Duct dimensions are usually measured in inches, but the formula requires Square Feet.
- Rectangular Ducts:
Area (sq ft) = (Width × Height) / 144 - Round Ducts:
Area (sq ft) = π × (Radius in inches)² / 144
(or: (π × Diameter²) / 576)
Step 2: Measure Velocity
Velocity is measured in FPM (Feet Per Minute) using a tool called an anemometer or a velometer. You insert the probe into the duct (traverse reading) or hold it against a grille to get the average air speed.
Why is this important?
Balancing an HVAC system requires ensuring each room gets the right amount of CFM. If a 12x12 room needs 100 CFM but is only getting 50 CFM, you can use this calculator along with an anemometer reading to diagnose if the duct is undersized or if the airflow velocity is too low.