Laboratory exhaust reference

Fume Hood Airflow Formulas

Equations, units, examples, and record fields for preliminary airflow and conditioned-exhaust checks—not containment certification.

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

Formula table

Check	Formula	Output
Open face area	A = width × height	ft² or m²
Average face velocity	V = Q/A	ft/min or m/s
Traverse average	V̄ = ΣVᵢ/n	Arithmetic mean
Traverse range	R = Vmax − Vmin	Velocity spread
Connected exhaust	Qconnected = Σ(quantity × full flow)	CFM or m³/h
Operating scenario	Qscenario = Σ(active × full + inactive × standby)	CFM or m³/h
US thermal load	q = CFM × 60 × ρ × \|Δh\|	BTU/h
SI thermal load	q = m³/h × ρ × \|Δh\| / 3600	kW

Unit conversion table

From	To	Multiply by
CFM	m³/h	1.69901082
ft/min	m/s	0.00508
inch	mm	25.4
lb/ft³	kg/m³	16.0184634
Btu/lb	kJ/kg	2.326
BTU/h	kW	0.00029307107

Worked values

Input condition	Calculation	Result
600 CFM, 48 × 18 in opening	600 ÷ (48 × 18 ÷ 144)	100 ft/min
9 readings totaling 900 ft/min	900 ÷ 9	100 ft/min average
4 hoods, 2 at 600 and 2 at 150 CFM	2 × 600 + 2 × 150	1,500 CFM
1,000 CFM, 0.075 lb/ft³, Δh 10 Btu/lb	1,000 × 60 × 0.075 × 10	45,000 BTU/h

Field record checklist

Category	Record	Why it matters
Identification	Building, room, hood ID, hood type	Links results to the tested device
Configuration	Sash type and height, baffle position, equipment load	Defines the opening and internal condition
Room	Door/window positions, supply/exhaust status, nearby disturbances	Documents external airflow influences
Instrument	Type, serial, calibration status, measurement unit	Preserves traceability
Traverse	Grid, point locations, every reading, average, minimum, maximum	Preserves the distribution, not only one number
Other tests	Visualization, tracer-gas, sash movement, applicable procedure	Separates velocity from containment evidence
Decision	Owner criteria, result, tester, date, corrective action	Prevents a generic calculator from defining acceptance

How to use this reference

Use the face formulas for a single opening condition, the traverse formulas for measured distribution, the scenario formulas for connected controls planning, and the thermal formulas for replacement-air load. Then return to the owner’s procedure, applicable requirements, instrument instructions, hood data, control sequence, and qualified laboratory ventilation review.

Important limits

These equations do not prescribe a face-velocity target or acceptable variation.
Average velocity does not demonstrate contaminant containment.
Multi-hood simultaneity and room makeup-air fractions must be documented project inputs.
Thermal load uses matched air-state data and does not equal plant electrical input.
This scope applies to laboratory chemical fume hoods, not biosafety cabinets or other containment devices.