Exhaust Fan Sizing Calculator: engineer's guide
Exhaust fans are sized by ACH for enclosed rooms, by CFM per fixture for toilets, and by CFM per m² for car parks.
Open the Exhaust Fan Sizing calculator →
What this calculator does
The calculator applies the applicable code-based rate to the room dimensions or fixture count and reports the required fan CFM plus recommended static pressure.
Formulas used
- ACH-based:
CFM = V × ACH / 60 - Toilet exhaust:
CFM = Nfixtures × 50 to 75 CFM/fixture - Car park:
CFM = A × 1.5 to 3 CFM/ft²
Variables
- V = room volume, ft³
- ACH = air changes per hour
- A = floor area, ft²
Worked example
Scenario: Public washroom with 6 WCs and 4 urinals.
- CFM = 10 fixtures × 75 = 750 CFM
- Add 100 CFM for janitor closet transfer
- Total 850 CFM; select fan at 850 CFM and 1.0 in wg static
- Verify makeup air path (door undercut, transfer grille)
Common mistakes to avoid
- No makeup air path — the room goes negative and doors slam
- Sizing on room volume for a car park (must be per m²)
- Missing balancing damper for zone control
- Duct static too low — fan operates off its curve
Frequently asked questions
Kitchen ACH?
Not by ACH — sized by hood CFM per m of hood length.
Car park CO-controlled fans?
Two-speed or VFD, staged by CO sensor at 25 and 50 ppm typical.
Standards and references
ASHRAE 62.1, NFPA 88A, IMC
Related tools on this site
Open the calculator that pairs with this guide: Exhaust Fan Sizing Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.