Duct friction rate, from your static pressure budget.

Short answer

Design friction rate = available static pressure x 100 / total effective length. A blower rated for 0.50 in. w.g. external static pressure, minus 0.35 in. of component drops, leaves 0.15 in. over 200 ft of effective length, which is a friction rate of 0.075 in. w.g. per 100 ft.

Worked example

The blower is rated for 0.50 in. w.g. at the design airflow. The filter takes 0.10, the coil 0.20 and the registers and other parts 0.05, so 0.15 in. is left for ducts. The longest run is 100 ft of straight duct plus 100 ft of fitting equivalent length, a total effective length of 200 ft. The friction rate is 0.15 x 100 / 200 = 0.075 in. w.g. per 100 ft.

Formula, assumptions and sources

The DOE Building America guideline defines the pieces. Friction rate is the pressure loss between two points a set length apart, in inches of water per 100 ft. Static pressure is the resistance from coils, filters, elbows, grilles and duct friction; the available pressure is what remains after the equipment's other losses and is what the ducts and outlets can use. Effective length is straight duct plus the equivalent length of every fitting, and the guideline notes that ACCA Manual D sets the friction rate from the critical-path run with the longest effective length, then uses it for all other runs. Since friction rate is pressure per 100 ft, the rate is the available pressure times 100 over the total effective length. Equivalent lengths for fittings come from Manual D or the ASHRAE Handbook tables; this tool does not include them, so enter your own. ACCA's overview of Manual D gives the same steps: identify the longest supply and return runs to get the total effective length, subtract the total component pressure loss from the external static pressure to get the available static pressure, and use the two to calculate the friction rate. It describes a friction rate between 0.06 and 0.18 in. w.g. per 100 ft as the range to land in, and this tool flags results outside it.

Source: U.S. DOE Building America, Advanced Strategy Guideline: Air Distribution Basics and Duct Design (IBACOS, December 2011) (retrieved 2026-10-15)

Source: ACCA HVAC Blog, Definitive Design: ACCA's Manual D Revised (L. Escobar, 14 April 2014) (retrieved 2026-10-15)

Safety and limits

This is a planning aid, not professional advice. Duct, furnace and air-conditioner design affects comfort, combustion safety and code compliance; a licensed HVAC contractor should perform a load calculation, size the system and verify airflow on site.

FAQ

How do I calculate duct friction rate?

Subtract the pressure drops of the filter, coil and other components from the blower's external static pressure to get the available static pressure, multiply by 100, then divide by the total effective length of the longest run.

What is total effective length?

It is the straight duct length plus the equivalent length of every fitting along the run. For the design friction rate, ACCA's Manual D uses the longest supply run and the longest return run.

Should I just use 0.1 inches per 100 ft?

A fixed number ignores your blower, filter, coil and layout. Calculating the rate from the actual static pressure and effective length is the Manual D approach, and this tool shows that arithmetic.

What if available static pressure is zero or negative?

Then the components use up all the blower can deliver and the duct system has nothing left. Recheck the drops, pick a lower-resistance filter or coil, or choose a blower rated for more static pressure.