How to use this calculator
- Get the velocity at the fitting. Flow ÷ duct area — the duct size calculator reports it from CFM and dimensions.
- Look up C for the exact geometry. ASHRAE Duct Fitting Database or SMACNA tables; radius ratio, vanes and aspect ratio all matter.
- Multiply by velocity pressure. ΔP = C × ρV²/2, totaled over identical fittings.
- Feed the static budget. Add fitting and straight-run losses into the total external static and compare against the blower table.
How it works
Air pays for every direction change in the currency of velocity pressure. The loss coefficient C says how many velocity pressures a given geometry costs:
VP = ρV²/2 ≈ (V/4005)² in. w.g. · ΔP = C × VP
Because VP grows with the square of velocity, the same elbow that is invisible at 800 fpm becomes the biggest line item at 2,000. Straight runs are priced separately by friction — the duct friction loss calculator handles those — and everything lands in the total external static budget the blower has to overcome. Velocity itself comes from the duct size calculator.
Worked example
Verified against the live calculator
A square mitered 90° elbow — no vanes, C = 1.2 — in a
trunk running 1,500 fpm at standard air:
VP = 0.140 in wg · ΔP = 1.2 × 0.140 = 0.168 in wg
Drop in single-thickness turning vanes (C ≈ 0.2) and the
same corner costs 0.028 in wg — the vanes hand back 0.14 in wg of
static, roughly a quarter of a typical residential blower's whole
budget. Eight such hard corners without vanes would burn 1.34 in wg,
which is how duct layouts starve systems long before any friction
table says they should.
Frequently asked questions
How do you calculate pressure loss through a duct fitting?
Multiply the fitting’s loss coefficient by the velocity pressure: ΔP = C × VP, where VP = ρV²/2 — in imperial terms VP ≈ (V/4005)² in. w.g. at standard air. At 1,500 fpm the velocity pressure is about 0.140 in wg, so a C = 1.2 mitered elbow costs 0.168 in wg.
How much pressure drop do turning vanes save?
A square mitered 90° elbow without vanes commonly tabulates near C = 1.2; with single-thickness turning vanes the same elbow runs roughly C = 0.2–0.35. At 1,500 fpm that is the difference between 0.168 and about 0.028 in wg — one set of vanes buys back ~0.14 in wg of static.
Where do duct fitting C coefficients come from?
Published tables for the exact geometry: the ASHRAE Duct Fitting Database and the SMACNA duct design manuals. Aspect ratio, radius ratio, takeoff angle and entry conditions all change C, which is why this calculator asks for your table value instead of guessing one.
Why do fitting losses matter more than straight duct?
Because they scale with velocity squared and stack per fitting. A run with eight hard elbows at 1,500 fpm burns over 1.3 in wg — far more than typical straight-run friction — which is why layout and vanes, not bigger blowers, fix most starved systems.
Method & assumptions
- Standard local-loss model (ASHRAE Fundamentals duct design chapter): ΔP = C·ρV²/2 with C user-entered for the exact geometry — no coefficient tables are embedded, and the tooltip magnitudes are orientation only.
- C assumes developed flow into the fitting; closely coupled fittings interact and can lose more than the sum of their table values.
- Identical-fitting totaling assumes each sees the same velocity — re-run per section where the trunk steps down.
- Density default 1.2 kg/m³ (0.075 lb/ft³); altitude and temperature scale every dynamic loss linearly through ρ.