Duct CFM Calculator

Airflow from anemometer velocity × inside duct area

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Anemometers & Backup ESP Meters

From the TestTalkHQ HVAC Tools / Duct Work & Air Flow catalog — in-duct and vane anemometers for the velocity reading this calculator uses, plus manometers for the static-pressure check when CFM is short. No capturing flow hood SKU was in the CSV at publish time, so none is invented here.

Fieldpiece STA2 In-Duct Hot Wire Anemometer

Hot-wire probe for a real duct traverse — the meter this CFM formula is built for. Average multiple points; do not trust a single centerline FPM.

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HoldPeak 866B Digital Anemometer

Handheld thermo-anemometer for register/grille face velocity. Convert FPM × free area carefully — face readings are not a duct traverse.

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BTMETER BT-100 Handheld Anemometer

Vane anemometer for supply-register FPM when you need a delivered-air check. Average the face; do not convert grille FPM with raw duct area.

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Testo 512-2 Differential Pressure Manometer

When measured CFM is short, check whether the blower is restricted. TESP vs nameplate is the next meter, not a second guess at velocity.

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Dwyer Series 475 Mark III Handheld Digital Manometer

Dwyer 475 Mark III Digital Manometer

Handheld in. WC manometer for supply/return static after a low-CFM traverse. Pair with the ESP calculator — static is not airflow.

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Fieldpiece SDMN5 Dual-Port Manometer

Dual-port HVAC manometer for filter/coil drop and TESP when the anemometer says the trunk is starving.

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How to Use This Duct CFM Calculator

Measure velocity with an in-duct anemometer using a multi-point traverse (not one center stab). Average those FPM values. Enter the average plus inside dimensions. The formula is:

CFM = Velocity (FPM) × Area (ft²) | Area_round = πd² / (4×144) | Area_rect = (W×H) / 144

Walkthrough: How to Measure Duct CFM. If the number is short of design: low/high CFM troubleshooting. In-duct vs capturing hood: Anemometer vs Flow Hood. Whether to grab a manometer first is already covered in Static Pressure vs Airflow (CFM) — use that instead of duplicating it here. High static after a low CFM? ESP calculator. Need to resize the run? Ductulator.

Duct CFM Calculator