Bathroom Fan Duct Size Calculator
Duct diameter and run length limits for an extractor.
Enter your values
ⓘ Live results
Results
- Total equivalent length
- 48 ft
- Recommended maximum
- 45 ft
- Verdict
- Too long — go up to 6 inch duct
- Duct velocity
- 917 FPM
- Suggested diameter
- 4 in
Quick answer
Rated CFM is only achieved when the duct is large enough and short enough; every elbow adds significant equivalent length.
equivalent length = straight run + elbows x per-elbow allowance
How to use the Bathroom Fan Duct Size Calculator
- 1Enter the fan's rated CFM and the duct diameter you plan to use.
- 2Measure the straight run and count the 90° elbows.
- 3Shorten the run, reduce elbows or upsize the duct if you exceed the limit.
Worked example
Using the values the calculator loads with:
Inputs
- Fan rating80 CFM
- Duct diameter4 in
- Straight run18 ft
- Number of 90° elbows2
Results
- Total equivalent length48 ft
- Recommended maximum45 ft
- VerdictToo long — go up to 6 inch duct
- Duct velocity917 FPM
- Suggested diameter4 in
Frequently asked questions
Can I vent into the attic?
No — moist air must exhaust to the outside through a roof or wall cap, or you will grow mould in the roof space.
Is flexible duct acceptable?
Rigid smooth pipe is far better; if you must use flex, pull it tight and add 25% to the equivalent length.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.