← Back to Planner · All Guides
Fitting Equivalent Lengths
Typical Manual D allowances for elbows, tees and takeoffs — and how to adjust them
What Equivalent Length Means
An elbow, tee or takeoff disturbs airflow far more than the straight duct it replaces. Its equivalent length expresses that extra resistance as the feet of straight duct that would lose the same pressure — a corner worth 85 ft counts as 85 ft of duct, even though it occupies almost no space.
The effective length of a run is its straight length plus the equivalent lengths of every fitting along it, and it is what friction calculations use. Equivalent lengths are conditional values: each holds at a stated reference velocity and reference friction rate, and must be corrected when your design departs from them.
Typical Default Values
Manual D, Appendix 3 tabulates default equivalent lengths by fitting group. The selection below covers two groups that shape most residential layouts: supply fittings at the air handler and branch takeoffs at the supply trunk.
Supply fittings at the air handler (Group 1)
| Fitting | Equivalent length (ft) |
|---|---|
| Tapered head — gradual transition into the trunk (H/W 0.5 / 1.0) | 35 / 25 |
| Bull head tee, no vanes — air slams into a tee (H/W 0.5 / 1.0) | 120 / 85 |
| Tee with turning vanes | 20 |
| Mitered inside corner — sharp 90° turn | 85 |
| Radius elbow, no vanes — by bend radius (R/W 0.25 / 0.5 / 1.0) | 40 / 20 / 10 |
| Radius elbow with two vanes (by R/W) | 10–20 |
| Radius elbow — by number of vanes (0 / 1 / 2) | 60 / 40 / 30 |
H/W is the duct height-to-width ratio; R/W is the bend radius relative to duct width. All values are feet of straight duct at the reference conditions.
Branch takeoffs at the supply trunk (Group 2)
A takeoff's equivalent length grows with the number of branch fittings between it and the end of the trunk (or the next trunk reducer). The columns below span the two extremes:
| Takeoff type | 0 downstream branches (ft) | 5+ downstream branches (ft) |
|---|---|---|
| Best-case takeoff (2F) | 20 | 25 |
| Typical takeoff (2A) | 35 | 80 |
| Worst-case takeoff (2L, 2M) | 70 | 115 |
Adjusting for Other Conditions
Because the defaults hold only at the reference conditions, Manual D provides a correction for other velocities and friction rates:
Vr and FRr are the reference velocity and friction rate printed with the table; Vx and FRx are the values for your design.
Manual D's own example: a 65 ft default value (900 FPM, 0.08) used at 700 FPM and a 0.12 IWC friction rate becomes:
Why This Matters in the Planner
The planner's total pressure drop covers straight-duct friction along the critical path. To account for fittings, add each fitting's equivalent length to the length you enter for its segment — the friction total then reflects the true effective length of the run. Component pressure drops (filter, registers, grilles, dampers) still need to be budgeted separately.
How that total fits into the blower's pressure budget is covered in the companion guide: External Static Pressure and the Friction Rate Method
Sources
- ACCA (2016) — Manual D, Residential Duct Systems — 3rd Edition — Appendix 3 (default equivalent length values)
Ready to Design?
Open the planner set to residential supply at the 0.08 IWC per 100 ft reference friction rate — the same conditions the default equivalent lengths assume.
Open the Duct System Planner