Duct Pressure Drop Calculator
Friction loss over a duct run.
Enter your values
ⓘ Live results
Results
- 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
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.
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.
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.