Exhaust Fan Duct Size Calculator: What Duct Size Do I Need?

Calculate the proper duct size for your exhaust fan by entering the fan's CFM rating and the duct run length below. This will help ensure optimal performance and airflow in your ventilation system.

List of the Best Exhaust Fan:

# Image Product Link
1 Comfort Zone Window Fan for Bedroom, 9 inch 3 Speed Dual Fans with Auto-Locking Expanders, Window Exhaust Fan, Removeable Bug Screen, White Comfort Zone Window Fan for Bedroom, 9 inch 3 Speed Dual Fans with Auto-Locking Expanders, Window Exhaust Fan, Removeable Bug Screen, White View on Amazon
2 Maxx Air HVHF 12COMBO Heavy Duty Portable Exhaust Fan - 2000 CFM, 12 Inch, 2 Speed Extractor Fan with 20 ft Hose - Ideal Exhaust Fan for Efficient Ventilation Maxx Air HVHF 12COMBO Heavy Duty Portable Exhaust Fan - 2000 CFM, 12 Inch, 2 Speed Extractor Fan with 20 ft Hose - Ideal Exhaust Fan for Efficient Ventilation View on Amazon
3 Utility Blower,8 Inch Exhaust Fan,2500R/MIN High Velocity Portable Ventilator,Air Extractor Fan with 16 Ft / 5m Duct Hose for Paint Booth,Industrial Basement and More Utility Blower,8 Inch Exhaust Fan,2500R/MIN High Velocity Portable Ventilator,Air Extractor Fan with 16 Ft / 5m Duct Hose for Paint Booth,Industrial Basement and More View on Amazon
4 Air King AS54 Advantage Bath 50-CFM at 3.0-Sones, White Finish Exhaust Fans, Medium Air King AS54 Advantage Bath 50-CFM at 3.0-Sones, White Finish Exhaust Fans, Medium View on Amazon
5 Vornado TRANSOM Window Fan with 4 Speeds, Remote Control, Reversible Exhaust Mode, Weather Resistant Case, Black, Whole Room Vornado TRANSOM Window Fan with 4 Speeds, Remote Control, Reversible Exhaust Mode, Weather Resistant Case, Black, Whole Room View on Amazon

Complete Guide to Exhaust Fan Duct Sizing

Understanding Duct Size Requirements

Proper duct sizing is crucial for the performance of your exhaust fan. Incorrect duct size can reduce airflow, cause backdrafts, and lead to inefficient fan operation.

How Duct Size is Determined

The required duct size depends on the fan's CFM rating, the length of the duct run, and the type of duct being used (smooth or flexible). Longer duct runs and flexible ducts typically require larger diameters to maintain adequate airflow.

CFM and Duct Size Calculation

  • Fan CFM: The volume of air the fan moves each minute.
  • Duct Length: The distance the air travels from the fan to the vent exit.
  • Duct Type: Smooth ducts allow for more airflow than flexible ducts, meaning flexible ducts often require a larger diameter to accommodate the same airflow.