Duct Friction Loss Calculator
Pressure drop along a duct run.
Enter your values
Enter the airflow in CFM.
Enter the duct diameter in in.
Enter the duct length in ft.
Enter the equivalent length of fittings in ft.
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 friction loss
- 0.197 in w.c.
- Friction rate
- 0.281 in w.c. per 100 ft
- Total equivalent length
- 70 ft
- Velocity
- 1,146 FPM
Working
- 10.281 per 100 ft x 70 ft = 0.197 in w.c.
Quick answer
Friction loss rises roughly with the square of velocity and falls sharply as duct diameter grows.
ΔP ≈ 0.109136 x L x Q^1.9 / d^5.02 (in w.c. per 100 ft basis)
At a glance
| What it does | Pressure drop along a duct run. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Airflow, Duct diameter, Duct length, Equivalent length of fittings |
| Main output | Total friction loss |
| Formula | ΔP ≈ 0.109136 x L x Q^1.9 / d^5.02 (in w.c. per 100 ft basis) |
| Cost | Free — no sign-up, no download |
How to use the Duct Friction Loss Calculator
- 1Enter airflow and the duct diameter.
- 2Add the straight length plus the equivalent length of elbows and fittings.
- 3Aim for a friction rate near 0.08–0.10 in w.c. per 100 ft on residential systems.
Inputs explained
- Airflow(CFM)
- Enter the airflow in CFM.
- Duct diameter(in)
- Enter the duct diameter in in.
- Duct length(ft)
- Enter the duct length in ft.
- Equivalent length of fittings(ft)
- Enter the equivalent length of fittings in ft.
Worked example
Using the values the calculator loads with:
Inputs
- Airflow400 CFM
- Duct diameter8 in
- Duct length40 ft
- Equivalent length of fittings30 ft
Results
- Total friction loss0.197 in w.c.
- Friction rate0.281 in w.c. per 100 ft
- Total equivalent length70 ft
- Velocity1,146 FPM
- 0.281 per 100 ft x 70 ft = 0.197 in w.c.
Frequently asked questions
What equivalent length is an elbow?
A smooth 90° elbow is roughly 15 ft of equivalent length on 8 inch duct; a sharp one is far worse.
Does flex duct differ?
Yes — compressed flex can triple the loss, so use fully stretched flex or rigid pipe.
What do I need to enter into the Duct Friction Loss Calculator?
Just 4 values: airflow, duct diameter, duct length, equivalent length of fittings. 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 Duct Friction Loss Calculator work out the answer?
Friction loss rises roughly with the square of velocity and falls sharply as duct diameter grows. It applies the formula ΔP ≈ 0.109136 x L x Q^1.9 / d^5.02 (in w.c. per 100 ft basis) and shows the working so you can check each step by hand.
Is the Duct Friction Loss 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.