Duct Pressure Drop and Duct Size Calculator

How is duct pressure drop calculated?

Friction loss in straight duct follows the Darcy-Weisbach equation: Δp = f × (L / D) × ρ × v² / 2. The friction factor f depends on the Reynolds number and the relative roughness of the duct and is solved from the Colebrook-White equation. Losses in fittings such as elbows, branches and dampers are the sum of their loss coefficients times the velocity pressure.

In duct design friction is usually quoted per metre: R = (f / D) × ρ × v² / 2 (Pa/m). Printed friction charts are for standard air (20 °C, sea level). This calculator works out density and viscosity for your temperature and altitude.

Rectangular duct

Rectangular ducts use the Huebscher equivalent diameter: D_e = 1.30 × (a × b)^0.625 / (a + b)^0.25. It is the diameter of a round duct that carries the same flow at the same friction rate. A 600 × 300 mm duct, for example, has an equivalent diameter of 457 mm. The area of the equivalent circle is smaller than the rectangle, so it is not used to work out velocity.

Duct sizing: two methods

• By velocity: flow and allowable velocity are known, D = √(4 × Q / (π × v)). Used where noise matters.
• Equal friction: the whole run is sized at the same loss per metre, for example 1 Pa/m. The calculator iterates to the diameter that gives this loss.

Either way the diameter is theoretical: pick the next standard size up and check it in the pressure drop tab.

Example

Air at 3600 cubic metres per hour flows through a 400 mm galvanized duct. Velocity is 7.96 m/s, the Reynolds number 211 000, the friction factor 0.0171 and the friction rate 1.63 Pa/m. Over 10 metres of straight duct the loss is 16.3 Pa.

Limits

The result covers only the duct section you enter. For fan selection, add up all sections, terminals and equipment along the longest, most resistant path. Flexible duct that is not fully extended can lose several times more.