TradeCalculator.co.uk
Duct Pressure Drop Calculator — System Resistance
Calculate total pressure drop through ductwork including straight runs, fittings, and terminals. Uses CIBSE Guide C loss coefficients for accurate fan selection.
Internal diameter of the round duct
Count of 90° bends at the radius selected above
ζ ≈ half the 90° value (0.13–0.45)
ζ ≈ 1.0 nominal (CIBSE/ASHRAE: 0.5–2.0 by split)
ζ ≈ 0.3 nominal (0.05–0.5 by taper angle)
~15 Pa each nominal — use maker's quoted drop
~10 Pa each (open balancing damper)
Next up: related pages
How We Calculate This
This calculator determines the total pressure drop through a ductwork system, which is essential for selecting the correct fan. The fan must generate enough static pressure to overcome the total system resistance.
Pressure drop components
- Straight friction: Pa/m × duct length, from Darcy–Weisbach with a Swamee–Jain (Colebrook–White) friction factor — depends on velocity, hydraulic diameter, and surface roughness
- Fitting losses: ζ × velocity pressure for each fitting (bends, tees, reducers)
- Terminal losses: nominal Pa values for grilles (~15 Pa) and dampers (~10 Pa)
Rectangular ducts
For a rectangular duct the calculator uses the hydraulic diameter Dh = 4A/P = 2ab/(a+b) for friction and the true area A = a×b for velocity, so you enter the actual width and height rather than working out an equivalent diameter yourself.
Typical loss coefficients (CIBSE Guide C 2007 / ASHRAE Duct Fitting Database)
- 90° bend: ζ ≈ 0.24 long-radius (1.5D), 0.33 standard (1.0D), 0.9 sharp/square
- 45° bend: ζ ≈ half the matching 90° value (0.13–0.45)
- Branch tee: ζ ≈ 1.0 nominal (varies 0.5–2.0 with the flow split)
- Reducer: ζ ≈ 0.3 nominal (0.05–0.5 by taper angle)
Terminal and damper figures are nominal defaults — grilles, diffusers and dampers vary widely, so confirm the pressure drop against the manufacturer's data for the final fan selection.
Working figures for this are collected in our Duct Sizing Tables reference table.
Frequently Asked Questions
Total pressure drop is the sum of three components: straight duct friction (Pa/m × length), fitting losses (loss coefficient × velocity pressure for each bend, tee, and reducer), and terminal losses (grilles, diffusers, dampers). The result in Pascals (Pa) determines the fan static pressure required. CIBSE Guide C provides detailed loss coefficients.
Velocity pressure (Pv) = 0.5 × air density × velocity², measured in Pascals. At 1.2 kg/m³ air density and 5 m/s velocity, Pv = 15 Pa. Fitting losses are expressed as multiples of velocity pressure using loss coefficients (zeta values). Higher duct velocities mean higher fitting losses.
CIBSE Guide B recommends: main ducts 4-6 m/s for low-velocity systems, 6-10 m/s for medium-velocity, and up to 20 m/s for high-velocity. Branch ducts should be 3-5 m/s. Higher velocities mean smaller ducts but more noise and higher pressure drops. For noise-sensitive areas (offices, bedrooms), keep below 4 m/s.
Flexible duct has a corrugated inner surface that creates much higher friction than smooth galvanised steel. Typical friction factors are 3-5 times higher than rigid duct. Always minimise flexible duct runs — use only for final connections (max 1.5m length). Fully stretched flexible duct has lower resistance than compressed duct.
The bend loss coefficient (zeta) depends mainly on the radius ratio (bend centreline radius ÷ duct diameter). Per CIBSE Guide C 2007 and the ASHRAE Duct Fitting Database, a smooth long-radius circular bend (r = 1.5D) is about 0.24, a standard-radius bend (r = 1.0D) about 0.33, and a sharp mitred/square elbow about 0.9 (up to ~1.5 with no turning vanes). A 45-degree bend is roughly half the matching 90-degree value. Choose the matching radius option in the calculator rather than relying on a single fixed figure.
Was this calculator helpful?
Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.