How to calculate duct external static pressure (ESP)

A step-by-step method for the external static pressure a fan must overcome: find the index run, add duct friction, fitting and accessory losses, then add a sensible margin.

J1 Pro · Updated 10 October 2026 · 8 min read

External static pressure (ESP) is the pressure the fan must develop outside its own casing to push the design airflow through the ductwork, fittings, dampers and terminals. Coils, filters and the casing inside an AHU or FCU are already included in the manufacturer's internal static pressure, so ESP covers only what you connect to the unit.

Under-estimate ESP and the far rooms starve of air. Over-estimate it badly and you buy a larger motor, waste energy for the life of the building and fight noise from dampers that have to be throttled.

Step 1: Find the index run

Air takes every path in parallel, so the fan only has to beat the single path with the highest total loss. That path is the index run, usually the longest one, or the one with the most fittings or a high-loss terminal such as a long-throw nozzle or a VAV box. Check two or three candidate runs if it isn't obvious. The other branches are balanced down with volume dampers.

Step 2: Duct friction

For each straight section, work out the velocity from the actual duct area, then the friction rate with the Darcy-Weisbach equation:

The friction factor f depends on the duct roughness and the Reynolds number. ASHRAE uses the Altshul-Tsal form, which is accurate for galvanised ductwork and easy to calculate by hand:

Section friction loss = friction rate × section length.

Step 3: Fittings

Every elbow, tee, transition and entry or exit loses a fraction of the velocity pressure at that fitting:

The loss coefficient C is not a constant. A smooth-radius elbow with r/W = 1.0 is roughly 0.2, while a mitred elbow without turning vanes is about 1.2, six times more. For tees and branches, C depends on the velocity ratio between the branch and the upstream main. A sharp 90° branch taking most of the flow can lose more than the velocity pressure itself. This is why using a single fixed C for "tee" or "transition" gives misleading results.

Step 4: Accessories and terminals

Fire dampers, volume dampers, sound attenuators, heaters, grilles and diffusers have pressure drops published by their manufacturers at a reference face velocity. Pressure drop varies with the square of velocity, so a catalogue value can be scaled to your actual velocity:

Always replace estimates with the selected product's data before final fan selection. Attenuators and terminal units are often the largest single items in the run.

Step 5: Total and margin

A 10 to 15% allowance covers construction changes and duct leakage. Round the result up to a practical selection step, usually 25 Pa, rather than piling margin on margin.

Worked example

A supply run of 1,800 L/s from an AHU, in four sections down to the last diffuser (galvanised duct, smooth-radius elbows, 10% margin):

The ductwork itself is only about a quarter of the total. The accessories, especially the attenuator and diffuser, dominate. That is typical for low-velocity comfort systems, and it is why accurate accessory data matters more than duct friction.

Common mistakes

• Adding up the losses of every branch instead of only the index run.
• Using the equivalent-diameter area to find the velocity. Use the actual W × H area.
• One fixed C value for all tees and transitions, regardless of flow split and size change.
• Counting the AHU's internal filter and coil losses twice, once by you and once by the manufacturer.
• Adding a safety factor at every step, then rounding up again at the end.

Guides are for preliminary design and learning. Always verify final designs against project specifications, local codes and manufacturer data.