How do I calculate duct pressure drop?

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.

What is velocity pressure in ductwork?

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.

What duct velocity should I use?

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.

Why does flexible duct have higher pressure drop?

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.

What loss coefficient should I use for a 90-degree bend?

Per CIBSE Guide C, a 90-degree bend with 1.5D radius has a loss coefficient (zeta) of approximately 0.15-0.5 depending on the ratio of bend radius to duct diameter. Sharp 90-degree bends (square elbows) can have zeta values of 1.0-1.5. This calculator uses 0.5 for standard 90-degree bends — a reasonable mid-range value.

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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.

Duct Type

Surface roughness affects friction loss

Duct Diameter

Flow Rate

Straight Duct Length

90° Bends

Zeta = 0.5 per bend

45° Bends

Zeta = 0.2 per bend

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 — depends on velocity, diameter, and surface roughness
Fitting losses: Zeta × velocity pressure for each fitting (bends, tees, reducers)
Terminal losses: Fixed Pa values for grilles (~15 Pa) and dampers (~10 Pa)

Typical loss coefficients (CIBSE Guide C)

90° bend: zeta = 0.5
45° bend: zeta = 0.2
Branch tee: zeta = 1.0
Reducer: zeta = 0.3

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.