Bathroom Vent Fan Calculator
Calculates bathroom exhaust fan airflow capacity in CFM (Cubic Feet per Minute) under HVI (Home Ventilating Institute) standards (minimum 50 CFM or 1 CFM per square foot for bathrooms up to 100 sq ft, plus fixture adders for jetted tubs, showers, and water closets).
Formula
CFM = Max(50, Area Sq Ft) × (Height > 8 ? Height/8 : 1) × (1 + Duct Resistance %)
1. Bath Floor Area = Length × Width 2. HVI Rule: For bathrooms <= 100 sq ft: CFM = 1 CFM per sq ft (min 50 CFM) 3. For bathrooms > 100 sq ft: CFM = Sum of fixtures (Shower: 50 + Tub: 50 + Toilet: 50 + Jetted Tub: 100) 4. Ceiling Height Multiplier = (Height > 8 ? Height ÷ 8 : 1.0) 5. Total CFM with Duct Friction = Base CFM × Height Multiplier × (1 + Waste %) 6. Recommended Noise Rating: <= 1.0 Sones (Ultra-Quiet)
Inputs
- Bathroom Length
- Bathroom Width
- Ceiling Height
- Special Fixtures
Worked example
Sizing an exhaust ventilation fan for an 80 sq ft bathroom with shower/tub combo.
Outcome: 80 sq ft requires 80 base CFM. Adding 15% duct backpressure allowance equals 92 CFM. Select a standard 110 CFM ultra-quiet (< 0.8 sones) bath exhaust fan with 4" or 6" rigid ducting.
What does a "Sone" rating mean on a bathroom fan?
A Sone is an international unit of perceived loudness. 1.0 Sone is roughly equivalent to the quiet hum of a refrigerator. Fans rated at 0.5 to 1.0 Sones are whisper-quiet, whereas older builder-grade fans rated at 3.0 to 4.0 Sones sound like an airplane taking off.