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).
Available static pressure is the blower's rated total external static pressure minus the coil, filter, and register/grille drops (all from OEM data). What is left pushes air through the ducts.
Total effective length is the longest supply-plus-return run measured, plus the equivalent lengths of its fittings from ACCA Manual D Appendix 3. Those fitting lengths are your input; this tool does not publish them.
This rate is above the 0.06-0.15 usable range: ducts will be small and noisy. Check the available static pressure and the total effective length before sizing from it.
Show the math
Friction rate = available static pressure × 100 / total effective length
= 0.5 × 100 / 150 = 0.333 in. wc per 100 ft (ACCA Manual D equal-friction)
Where this number comes fromIntry Verified
- Calculated from
- ACCA Manual D friction rate = ASP x 100 / TEL. Computed from the ACCA Manual D relation friction rate = available static pressure x 100 / total effective length; the target range is advisory design guidance. See the full receipt ↓
- Run with
- Available static pressure0.5 in. wc
- Total effective length150 ft
- Friction rate0.333 in. wc/100 ft
- Checked
- Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 31163 automated checks, one suite for the whole site rather than a per-tool count, re-derive the numbers before each deploy from the tables this site transcribed from the ACCA Manual D friction-rate standard it was computed from. This is our own deterministic gate, not a third-party audit.
- Final say
- The blower's measured external static pressure and a full ACCA Manual D duct design have final say.
Intry Verified · Build 2731D30 · 2026-10-15
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. A rate of 0.333 is above the usable range, so check the static pressure and effective length before you size ducts from it. Once the rate is in range, carry it 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?
Most residential designs land at about 0.06 to 0.10 in. wc per 100 ft (the tool flags anything outside 0.06 to 0.15), 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 still above the usable range. Much above 0.10 means small, noisy ducts; much below means larger ducts than you need.