Fan Static Pressure Calculator

Build a total external static pressure (TESP) estimate component by component, then compare it with the fan or unit rating.

Pressure drops

Pre-filled with typical values — replace them with your calculated duct losses and manufacturer data.

Supply duct (critical path: friction + fittings) (in. w.g.)

Return duct (critical path) (in. w.g.)

Cooling coil, wet (in. w.g.)

Heating coil / heat exchanger (in. w.g.)

Filters, mid-life (MERV 8–13) (in. w.g.)

Supply diffuser / grille (in. w.g.)

Return grille (in. w.g.)

VAV box / terminal (minimum inlet ΔP) (in. w.g.)

Dampers, sound attenuators, other (in. w.g.)

Design margin (%)

Unit rated external static (optional) (in. w.g.)

Results

Review the pressure drops, then select Calculate.

Worked example

An engineer is checking whether a packaged rooftop unit rated at 0.90 in. w.g. external static can serve a small office with a critical path of duct, one elbow-heavy return, a MERV 13 filter and VAV terminals.

1
Add the critical-path duct losses
Supply duct 0.28 + return duct 0.17 = 0.45 in. w.g.
Only the highest-loss path counts — branches are not added together.
2
Add the external components
Filter 0.25 + VAV terminal 0.12 + grilles 0.08 = 0.45 in. w.g.
Coil drops count only if the unit's rating excludes them — check the fan table basis.
3
Total and apply the design margin
TESP = 0.90; design = 0.90 × 1.10 = 0.99 in. w.g.
The calculator flags when design static exceeds the entered rating.

Takeaway: This unit is short by 0.09 in. w.g. before anyone touches the fan curve — the fix is usually cutting return losses or stepping up the unit, not hoping the fan finds the airflow.

Method and assumptions

TESP = sum of external component pressure drops on the critical (highest-loss) supply and return paths; design TESP = TESP × (1 + margin).
Duct values should come from the Duct Sizer, Equivalent Length or Fitting Loss calculators for the critical path only — not every run added together.
Pre-filled component values are typical rounded figures for comparison; coils, filters and terminals must use manufacturer data at the actual airflow.
Whether coil and filter losses count as 'external' depends on the unit's rating basis — check the manufacturer's fan table. Results shown to 0.01 in. w.g.

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Important Disclaimer

This calculator is for preliminary design guidance only. Final duct sizing, construction, and installation must be performed by qualified professionals and comply with applicable codes and standards.