| Type | Component | CFM | Size | Equiv. L (ft) | ΔP (in.w.c.) |
|---|
This calculator builds the external static pressure (ESP) for an air system by summing losses along the critical path. Add each duct segment on the worst-case run — its airflow, shape, size, and equivalent length — plus each piece of equipment with its rated pressure drop. The tool computes the friction in every duct segment, totals the equipment losses, and reports the duct friction, equipment losses, and the combined external static pressure in inches of water column.
External static pressure is the number you give the fan or air-handler vendor to confirm the unit can move the design airflow. Under-counting it leaves the system short of air; over-counting it oversizes the fan and wastes energy. Building the path component by component makes the budget auditable and easy to revise as the layout changes.
| Total ESP | ESP = Σ ΔPduct + Σ ΔPequipment |
| Segment friction | ΔP = (f · ρ · V² · 9.95×10⁻⁴ ÷ Dh) · (L ÷ 100) |
| Velocity | V (fpm) = Q (CFM) ÷ A (ft²) |
| Hydraulic dia. | Dh = 4 · W · H ÷ [2 · (W + H)] |
Each duct segment uses Darcy–Weisbach with the Colebrook–White friction factor (laminar f = 64 ÷ Re below Re ≈ 2,300). The constant 9.95×10⁻⁴ converts the Darcy result to in.w.c. per 100 ft in IP units, and that per-100-ft loss is scaled by the segment equivalent length L. Air density ρ = 0.075 · (530 ÷ (T + 460)) lb/ft³ corrects for temperature. Equipment drops are entered directly from manufacturer ratings. This follows standard ASHRAE/SMACNA duct-design practice.