Bathroom Fan Duct Size Calculator
Duct diameter and run length limits for an extractor.
Enter your values
Enter the fan rating in CFM.
Enter the duct diameter in in.
Enter the straight run in ft.
Enter the number of 90° elbows you are working with.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Duct diameter and run length limits for an extractor. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Fan rating, Duct diameter, Straight run, Number of 90° elbows |
| Main output | Total equivalent length |
| Formula | equivalent length = straight run + elbows x per-elbow allowance |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Fan rating(CFM)
- Enter the fan rating in CFM.
- Duct diameter(in)
- Enter the duct diameter in in.
- Straight run(ft)
- Enter the straight run in ft.
- Number of 90° elbows
- Enter the number of 90° elbows you are working with.
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.
What do I need to enter into the Bathroom Fan Duct Size Calculator?
Just 4 values: fan rating, duct diameter, straight run, number of 90° elbows. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Bathroom Fan Duct Size Calculator work out the answer?
Rated CFM is only achieved when the duct is large enough and short enough; every elbow adds significant equivalent length. It applies the formula equivalent length = straight run + elbows x per-elbow allowance and shows the working so you can check each step by hand.
Is the Bathroom Fan Duct Size Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.