What is the formula for CFM from velocity?

CFM = velocity (FPM) × duct area (ft²). A 12 in × 8 in duct is 0.667 ft², so 800 FPM gives about 533 CFM.

How much airflow should a system move per ton?

Most residential cooling equipment is designed around 400 CFM per ton, with 350 CFM per ton in humid climates and up to 450 CFM per ton in dry climates or heat pump heating.

Where should I take the velocity reading?

Take a traverse in a straight run at least three duct diameters downstream and one diameter upstream of any fitting, and average the readings rather than using a single center-line value.

HVAC Airflow CFM Calculator — Tons & Delta-T

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Formula

CFM = tons × CFM/ton or CFM = Q_sensible ÷ (1.08 × ΔT)

Standard: 400 CFM/ton (350 dry climate, 450 humid). Formula uses 1.08 = 0.24 (specific heat) × 60 min × 0.075 lb/ft³ (sea-level air).

Method

Cooling capacity

tons

Target airflow

CFM/ton

Required airflow

1,200CFM

3 tons × 400 CFM/ton

Airflow within typical residential window (350–450 CFM/ton).

How to use this calculator

Convert duct velocity and cross-sectional area into airflow, or size airflow from a sensible load.

Measure average duct velocity in feet per minute with an anemometer or pitot traverse.
Enter the duct dimensions so the tool computes free area in square feet.
Read CFM as velocity multiplied by area.
Cross-check the result against the equipment's rated airflow, typically 350–450 CFM per ton.

Frequently asked questions

What is the formula for CFM from velocity?: CFM = velocity (FPM) × duct area (ft²). A 12 in × 8 in duct is 0.667 ft², so 800 FPM gives about 533 CFM.
How much airflow should a system move per ton?: Most residential cooling equipment is designed around 400 CFM per ton, with 350 CFM per ton in humid climates and up to 450 CFM per ton in dry climates or heat pump heating.
Where should I take the velocity reading?: Take a traverse in a straight run at least three duct diameters downstream and one diameter upstream of any fitting, and average the readings rather than using a single center-line value.