Free Room Exhaust Fan CFM Calculator

Calculate required ventilation airflow (CFM and m³/h) based on room dimensions, target air changes per hour (ACH), and ductwork static pressure losses.

🌀 Room Dimensions & Application

Room Application Preset

Length (ft)

Width (ft)

Ceiling Height (ft)

Air Changes / Hr (ACH) Recommended exchanges/hr

Duct Type

Duct Run Length (ft)

90° Duct Elbows Each elbow ≈ 10 ft equiv run

📊 Recommended Fan Sizing

Recommended Fan Rating 0 CFM 0 m³/h (inc. duct friction)

Minimum Theoretical CFM 0 CFM Free air (0.0" static)

Room Total Volume 0 cu ft 0 sq ft area

Equivalent Duct Run 0 ft ~0.15" static pressure

HVI Bathroom Standard: 80 CFM min

Recommended Round Duct Diameter: 4-inch (up to 90 CFM)

Target Sound Rating: ≤ 1.5 Sones (Ultra-quiet)

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Frequently Asked Questions

What CFM exhaust fan do I need for an 8x10 bathroom?

An 8x10 bathroom is 80 square feet with 640 cubic feet of volume. Under HVI guidelines (1 CFM per sq ft plus duct friction), a fan rated at 80 to 110 CFM is ideal for fast steam evacuation.

What is a good Sone rating for an exhaust fan?

1.0 Sone equals the sound of a quiet refrigerator humming. Ratings below 1.5 Sones are considered ultra-quiet. Builder-grade fans typically measure 3.0 to 4.5 Sones (loud and intrusive).

Why should I avoid flexible ducting for exhaust fans?

Flexible ribbed vinyl or foil ducting causes roughly 2 to 3 times the air friction resistance of smooth galvanized steel or rigid PVC ducting. Flex ducting also sags over time, allowing moisture condensation to pool inside low spots.

How long should a bathroom exhaust fan run after a shower?

Run your exhaust fan for at least 20 minutes after completing a shower to fully exhaust residual humidity from drywall, ceilings, and towels. Using a countdown timer switch or humidity sensor switch ensures complete moisture removal.