Duct Equivalent Length Calculator
Equivalent length is the length of straight duct that produces the same pressure loss as a fitting at the same airflow and duct conditions. Add it to the straight length to estimate a run’s total effective length and pressure loss.
Fittings (quantity)
Enter a positive airflow and diameter, then select Calculate.
What does the equivalent length of a duct fitting mean?
A fitting does not physically become longer. Its equivalent length expresses its resistance as feet of straight duct. A smooth-radius elbow and a sharp mitred elbow can have very different equivalent lengths even at the same diameter.
Equivalent length is not the same as equivalent diameter. For a rectangular-to-round size comparison, use the rectangular-to-round duct converter. This tool estimates resistance along a round, constant-size run.
How is the equivalent length of a duct determined?
C (also called K) is the fitting loss coefficient, D is the round diameter in feet, and f is the Darcy friction factor, not the Fanning factor. The calculator obtains f from the airflow, diameter, material roughness and air temperature. Do not substitute a friction rate in in. w.g./100 ft for f.
These equations express the same modeled fitting loss two ways. Calculate with unrounded values; displayed feet are rounded. A fixed “feet per elbow” table is only valid for its stated diameter, fitting geometry and friction factor.
At 70 °F and sea level, a 2,000 CFM run in 16 in spiral duct (ε = 0.09 mm) has 80 ft of straight pipe, four smooth-radius 90° elbows (r/D = 1.5, C = 0.15) and two 45° conical take-offs (C = 0.50). These are the exact fitting choices in this calculator.
- 1
Calculate the straight-duct conditions
D = 16 / 12 ft; f = 0.01725; velocity = 1,432 FPM
Friction rate = 0.165 in. w.g./100 ft — calculated, not an assumed 0.08 rate.
- 2
Convert 90° elbow, smooth radius, r/D 1.5
4 × (0.15 × 16/12 ÷ 0.01725) ≈ 46.4 ft
11.6 ft equivalent length per fitting.
- 3
Convert 45° conical take-off
2 × (0.5 × 16/12 ÷ 0.01725) ≈ 77.3 ft
38.7 ft equivalent length per fitting.
- 4
Total the effective length and pressure loss
80 + 123.7 = 203.7 ft; Δp ≈ 0.336 in. w.g.
Pressure is calculated before rounding the friction rate or equivalent feet.
Takeaway: The fittings add 1.55 times the straight length. Compare fitting geometry and duct size before selecting the fan; this run loss does not include filters, coils or terminals.
Equivalent length table for the stated example
| Fitting | C | Quantity | Equivalent ft each | Equivalent ft total |
|---|---|---|---|---|
| 90° elbow, smooth radius, r/D 1.5 | 0.15 | 4 | 11.6 | 46.4 |
| 45° conical take-off | 0.5 | 2 | 38.7 | 77.3 |
Only for 2,000 CFM, 16 in spiral duct and 70 °F at sea level. Change the inputs to calculate a different run; these are not universal fitting lengths.
Using equivalent length in a real duct path
- Choose the actual fitting geometry and count only fittings on the path being checked, not every branch in the system.
- When airflow, diameter or material changes, calculate each segment separately and add the pressure losses, not equivalent feet at different friction rates.
- For tees and take-offs, verify the coefficient’s reference velocity and flow split against the project fitting data. This tool applies its representative C-values to the entered run velocity pressure.
- Add equipment losses once, then compare the total path against available fan pressure. Do not count both C × Pv and the equivalent-length loss for the same fitting.
Use the duct fitting loss calculator for direct C × Pv calculations, the fan static pressure calculator to total component losses, and the critical duct path guide to choose the governing route.
References and coefficient limits
The pressure-loss relationships follow the ASHRAE Handbook—Fundamentals, Duct Design. Project-specific fitting coefficients, manufacturer pressure-drop data and actual flow splits take precedence over this calculator’s simplified fitting list.
Method and assumptions
- Equivalent length L = C × D / f, where C is the fitting loss coefficient, D the diameter (ft) and f the Darcy friction factor at this airflow — the same C-values used by the Fitting Loss calculator.
- Because f changes with velocity and roughness, a fitting's equivalent length changes with airflow; fixed 'feet per elbow' tables are only approximations.
- Friction via Darcy–Weisbach with the Altshul–Tsal friction factor; sea-level air at the entered temperature.
- Round, constant-size segments only. An equivalent round diameter approximates rectangular straight-duct friction, but does not reproduce rectangular velocity pressure or fitting coefficients; use actual section data for final design. Lengths shown to 1 ft, pressure to 0.001 in. w.g.
This calculator is for preliminary design guidance only. Final duct sizing, construction, and installation must be performed by qualified professionals and comply with applicable codes and standards.