HVAC · Air side
Static Pressure & Friction Rate
Turn available static pressure and total effective length into the design friction rate the duct size calculator needs. Friction rate = available static pressure × 100 / total effective length (ACCA Manual D equal-friction method).
The friction-rate formula
Friction rate (in. wc per 100 ft) = available static pressure (in. wc) × 100 / total effective length (ft). Available static pressure is the blower's total external static pressure minus the coil, filter, and register drops (OEM data). Total effective length is the longest supply-plus-return run plus its fitting equivalent lengths (ACCA Manual D Appendix 3, your input). This is the equal-friction design rate used to size every duct in the system.
Friction Rate FAQs
How do I calculate the friction rate for duct design?
Friction rate = available static pressure x 100 / total effective length (ACCA Manual D equal-friction method). For 0.5 in. wc of available static pressure over a 150 ft total effective length that is 0.333 in. wc per 100 ft. That rate is what you carry into the duct size calculator.
What is available static pressure?
Available static pressure is the blower's rated total external static pressure minus the pressure drops across everything that is not duct: the coil, the filter, and the supply registers and return grilles. All of those drops come from OEM data. What is left is the push available to move air through the duct runs.
What is total effective length?
Total effective length is the measured length of the longest supply-plus-return run, plus the equivalent lengths of the fittings on that run (elbows, boots, takeoffs). The fitting equivalent lengths come from ACCA Manual D Appendix 3; this tool takes your total effective length as an input and does not publish the fitting table.
What is a good friction rate?
The usable residential range is about 0.06 to 0.10 in. wc per 100 ft, and 0.08 is the common design target that Manual D Appendix 3 tables assume. A rate of 0.4 in. wc over 200 ft comes out to 0.200 in. wc per 100 ft, which is well within range. Much above 0.10 means small, noisy ducts; much below means larger ducts than you need.