Duct Fitting Loss Calculator
Elbows, tees, take-offs and dampers often lose more pressure than every foot of straight duct combined. Count the fittings, add the straight run, and get the total the fan has to overcome.
Run Parameters
Airflow and section for this path.
Fittings on this run
Enter a quantity for each fitting present.
| Fitting | C | Qty |
|---|---|---|
| 90° elbow, smooth radius, r/D 1.5 | 0.15 | |
| 90° elbow, smooth radius, r/D 1.0 | 0.22 | |
| 90° elbow, 5-piece segmented | 0.24 | |
| 90° elbow, 3-piece segmented | 0.34 | |
| 45° elbow, smooth radius | 0.10 | |
| 90° mitred elbow, no vanes | 1.20 | |
| Tee or cross, flow into the branch | 1.00 | |
| Tee or cross, straight through | 0.10 | |
| 45° conical take-off | 0.50 | |
| Gradual reducer, 30° included | 0.05 | |
| Gradual expander, 20° included | 0.25 | |
| Sharp-edged duct entry | 0.50 | |
| Abrupt duct exit | 1.00 | |
| Volume control damper, fully open | 0.20 | |
| Fire damper, curtain type | 0.50 | |
| Flexible connector at the plant | 0.30 |
Total Pressure Drop
Enter valid airflow and duct dimensions, then select Calculate.
Calculation basis and data sources
- Coefficient source
- Representative, rounded loss coefficients based on the ASHRAE Handbook—Fundamentals (2021), Chapter 21 duct-design method. They describe generic fitting geometries rather than edition-specific fitting IDs; use project- or manufacturer-specific data when available.
- Fitting-loss method
- ΔP = ΣC × Pv. Velocity pressure Pv is calculated from the section velocity and calculated air density; each fitting quantity multiplies its listed coefficient before the values are summed.
- Straight-duct method
- Darcy–Weisbach friction using the Altshul–Tsal friction-factor approximation and the roughness of the selected duct material. Rectangular sections use the Huebscher equivalent diameter.
- Air assumptions
- The default is 70 °F dry air at sea-level pressure. Temperature can be changed; altitude remains 0 ft in this calculator. Density and dynamic viscosity are recalculated from the entered temperature.
- Units and rounding
- Inputs use CFM, inches, feet and °F. Pressure is reported in inches of water gauge; totals are shown to 0.001 in. w.g., friction rate to 0.0001 in. w.g./100 ft, equivalent diameter to 0.1 in. and velocity to the nearest FPM. Calculations retain full precision before display rounding.
- Estimate boundary
- Results support option comparison and preliminary critical-path estimates. Final design should replace generic coefficients with the actual fitting geometry and verify fan and equipment losses, leakage, balancing, acoustics and adopted-code requirements.
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Important Disclaimer
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.