Laboratory exhaust verification

Fume Hood Face Velocity Calculator

Calculate an opening-average velocity from documented hood exhaust airflow and the measured sash opening.

Created 2026-08-27 · Updated 2026-08-27

Airflow to face velocity

Use one sash position and airflow measurement from the same operating condition.

How to use this calculator

Set and record the sash position used for the airflow measurement.
Enter measured volume flow and the clear opening width and height.
Use the result as an average check, then follow the project traverse and containment-test procedure.

Formula

Face area = open width × open height
Average face velocity = volume airflow ÷ face area

Variables

Input	US customary	SI
Volume airflow	CFM	m³/h
Opening dimensions	inches	millimetres
Face velocity	ft/min	m/s

Worked example

A 48-inch-wide by 18-inch-high opening has an area of 6 ft². At 600 CFM, the opening-average velocity is 100 ft/min, or about 0.508 m/s. This arithmetic does not establish whether the hood contains a specific hazard.

Result interpretation

The result describes the average implied by one airflow and one rectangular opening. Cross drafts, sash position, obstructions, hood configuration, room balance, test instrument, and measurement grid can change field performance. ASHRAE identifies face velocity, flow visualization, tracer gas, and sash movement as different parts of a hood-performance review.

Frequently asked questions

Does an average face velocity certify the hood?

No. The owner’s test program and applicable procedures determine acceptance; a face-velocity calculation alone does not measure containment.

Can I use outside hood dimensions?

No. Use the clear open face at the documented sash position.

Should I use one anemometer reading?

A single point does not describe the whole opening. Use the required equal-area traverse and the Fume Hood Traverse Calculator.