Duct Pressure Drop Calculator
Friction loss over a duct run.
Enter your values
Enter the friction rate in in wc/100 ft.
Enter the straight length in ft.
Enter the fitting equivalent length 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 equivalent length
- 75 ft
- Pressure drop
- 0.06 in wc
- In Pa
- 14.9
- Share of 0.5 in wc budget
- 12%
Quick answer
Duct pressure drop is the friction rate per 100 feet multiplied by the equivalent length of the run.
drop = rate per 100 ft x length / 100
At a glance
| What it does | Friction loss over a duct run. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Friction rate, Straight length, Fitting equivalent length |
| Main output | Total equivalent length |
| Formula | drop = rate per 100 ft x length / 100 |
| Cost | Free — no sign-up, no download |
How to use the Duct Pressure Drop Calculator
- 1Look up the friction rate for your duct size and airflow, or use 0.08-0.10 in wc per 100 ft.
- 2Add straight length plus the equivalent length of every elbow, tee and boot.
- 3Keep the total under the blower's available static pressure.
Inputs explained
- Friction rate(in wc/100 ft)
- Enter the friction rate in in wc/100 ft.
- Straight length(ft)
- Enter the straight length in ft.
- Fitting equivalent length(ft)
- Enter the fitting equivalent length in ft.
Worked example
Using the values the calculator loads with:
Inputs
- Friction rate0.08 in wc/100 ft
- Straight length40 ft
- Fitting equivalent length35 ft
Results
- Total equivalent length75 ft
- Pressure drop0.06 in wc
- In Pa14.9
- Share of 0.5 in wc budget12%
Frequently asked questions
What is a typical elbow worth?
A sheet-metal 90° elbow adds roughly 15-25 equivalent feet depending on radius.
Where does 0.08 come from?
It is a common residential design friction rate that balances duct size against blower capacity.
What do I need to enter into the Duct Pressure Drop Calculator?
Just 3 values: friction rate, straight length, fitting equivalent length. 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 Pressure Drop Calculator work out the answer?
Duct pressure drop is the friction rate per 100 feet multiplied by the equivalent length of the run. It applies the formula drop = rate per 100 ft x length / 100 and shows the working so you can check each step by hand.
Is the Duct Pressure Drop 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.