How to Calculate Fitting Loss Coefficients
Elbows, tees, take-offs and dampers are priced in velocity pressures. This guide covers the C × Pv method, where coefficients come from, a reference table for common commercial fittings, and a worked run showing how quickly the fitting total overtakes the straight duct.
Last updated September 2026
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The method
Every fitting loss is the same two-term product: a dimensionless coefficient for the geometry, multiplied by the velocity pressure of the air passing through it.
Because Pv scales with the square of velocity, the same elbow costs four times as much pressure at 2,000 FPM as it does at 1,000 FPM. That is the whole reason velocity control matters on fitting-heavy runs.
Worked example
A 14 in round branch carrying 1,800 CFM runs at 1,684 FPM, giving a velocity pressure of 0.176 in. w.g.
| Fitting | C | Loss (in. w.g.) |
|---|---|---|
| Conical take-off from the trunk | 0.50 | 0.088 |
| Two 90° smooth elbows, r/D 1.5 | 0.30 | 0.053 |
| One 3-piece segmented elbow | 0.34 | 0.060 |
| Volume damper, fully open | 0.20 | 0.035 |
| Fitting subtotal | 1.34 | 0.236 |
| 40 ft of straight 14 in duct | — | 0.105 |
14 in spiral galvanized duct, 1,800 CFM, 70 °F standard air.
On this branch the fittings cost more than the straight duct. Swapping the 3-piece segmented elbow for a smooth r/D 1.5 elbow saves 0.034 in. w.g. for the price of a better fitting — cheaper than upsizing the duct.
Build your own run in the fitting loss calculator.
Reference coefficients
| Fitting | Loss coefficient C |
|---|---|
| 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 |
Representative values for preliminary design. Manufacturer and ASHRAE Duct Fitting Database values for the exact geometry take precedence.
Converting to equivalent length
If you are working with an equivalent-length method, convert once you know the friction rate of the section:
The 3-piece elbow above converts to about 23 ft of 14 in straight duct. Note that this conversion is only valid at this velocity and this duct size.
Method and assumptions
- Method
- Loss coefficient method, Δp = C × Pv, ASHRAE Fundamentals Chapter 21 basis.
- Coefficient source
- Representative published values for common commercial fittings; not a substitute for the ASHRAE Duct Fitting Database entry for the exact geometry.
- Velocity pressure
- Computed from actual air density at the entered temperature and elevation, not fixed at 4005.
- Reference case
- Where a number is shown above, it is 14 in spiral galvanized duct at 1,800 CFM, 70 °F, sea level.
Limitations of this method
What the calculation on this page does not account for. Read these before using a number on a drawing or a submittal.
- Published coefficients describe idealised, isolated fittings. Two fittings within a few duct diameters of each other interact and lose more than the sum of their coefficients.
- Branch fittings have separate coefficients for the branch and the straight-through path, and both depend on the flow split. A single C value is an approximation of a two-dimensional table.
- Dampers are quoted fully open. A damper throttled for balancing can cost several times its open-position coefficient — that loss is intentional but must be accounted for.
- Field fabrication varies. A shop-built mitred elbow, a squashed flex bend or a take-off installed off-centre will not match the table.
- This method gives pressure loss only. It does not predict regenerated noise, which is often the binding constraint at take-offs and dampers near occupied space.
Frequently asked questions
How do you calculate fitting loss in ductwork?
Multiply the fitting's loss coefficient C by the velocity pressure of the air in the duct. Velocity pressure at standard air is the velocity in FPM divided by 4005, squared. Add up C times velocity pressure for every fitting on the run.
What is a loss coefficient?
A loss coefficient, written C or K, is a dimensionless number expressing how many velocity pressures a fitting costs. A C of 1.0 means the fitting loses one full velocity pressure; a smooth long-radius elbow at C 0.15 loses about a seventh of that.
Should I use loss coefficients or equivalent length?
Coefficients are more accurate because they scale with the actual velocity in that section. Equivalent length is quicker for hand estimating but hides the velocity dependence, so it drifts on runs whose velocity differs from the table's basis.