Duct Static Pressure

Critical-path external static pressure — duct segments, fittings, and equipment losses summed for fan selection.

System Settings
Critical Path Components
Summary
Duct Friction
—
in.w.c.
External Equipment
—
in.w.c.
External Static (ESP)
—
in.w.c.
Inside AHU Casing
—
in.w.c. (internal)
Total Static (TSP)
—
in.w.c.
Path Breakdown
Type Component CFM Size Equiv. L (ft) ΔP (in.w.c.)
Inputs, results, and method — as an Excel workbook or plain CSV.

Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.

Book a Demo →
About This Calculator

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 external static pressure (ESP — ducts plus field components outside the unit) and the total static pressure (TSP — ESP plus the coils, filters and other items you mark as inside the AHU casing), in inches of water column.

External static pressure is the number you give a packaged or catalogued air-handler vendor, whose rating already covers the unit’s internal losses; a built-up fan must be selected for the total static. 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.

Formula & Method
External staticESP = Σ ΔPduct + Σ ΔPexternal equipment
Total staticTSP = ESP + Σ ΔPinside AHU casing
Segment frictionΔP = (f · ρ · V² · 9.95×10⁻⁴ ÷ Dh) · (L ÷ 100)
VelocityV (fpm) = Q (CFM) ÷ A (ft²)
Hydraulic dia.Dh = 4 · W · H ÷ [2 · (W + H)]

Each duct segment uses Darcy–Weisbach with the Colebrook 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 (straight duct plus the equivalent length of its fittings). Rectangular segments use the hydraulic diameter with the actual face velocity. Air density ρ = 0.075 · (530 ÷ (T + 460)) lb/ft³ and viscosity are corrected for temperature at sea-level pressure; at altitude, duct friction at the same airflow is lower roughly in proportion to density (about 15–17% at 5,000 ft), so keep the duct losses and the fan rating on the same density basis. Equipment drops are entered directly from manufacturer ratings. Roughness: galvanized and galvanized spiral 0.0003 ft, stainless 0.00015 ft, duct board 0.0025 ft, flexible duct (fully extended) 0.012 ft, concrete 0.005 ft — the same values as the Duct Sizer.

Frequently Asked Questions
What is external static pressure (ESP)?
External static pressure (ESP) is the resistance the fan must overcome outside its own unit casing, in inches of water column: duct friction along the critical (highest-loss) path plus field-installed components such as terminals, diffusers, grilles, louvers, dampers and duct-mounted coils. Coils, filters and other components inside a packaged or catalogued air handler are internal static, which the manufacturer accounts for. Mark those items as inside the AHU casing: the tool reports ESP without them — what you give the vendor for a packaged unit — and total static pressure (TSP) with them, which is what a built-up fan must deliver.
Which path do I add components for?
Add the components along the critical path, the single route from the fan to the most remote outlet that carries the highest total pressure loss. The fan must satisfy this worst-case path, so it sets the external static pressure. Use the equivalent length of each duct run, which folds straight duct plus fittings and elbows into one length the friction formula can act on.
What pressure drop should I enter for coils and filters?
Use the manufacturer's rated clean and dirty pressure drops at your design airflow. Typical placeholders are roughly 0.3 to 0.6 in.w.c. for a cooling coil, 0.3 to 0.5 in.w.c. for a clean filter bank rising as it loads, and 0.2 to 0.3 in.w.c. for a terminal or diffuser, but always confirm against the submittal. Enter each component's rated drop directly, and mark coils and filters that sit inside the air handler as internal so they are kept out of the external static.
How is the duct friction in each segment calculated?
Each duct segment uses the Darcy-Weisbach equation with a Colebrook friction factor, then scales the per-100-ft loss by the segment's equivalent length. Air density and viscosity are corrected for the air temperature you set at sea-level pressure, and duct roughness comes from the selected material. Duct losses plus external equipment give the external static pressure; adding the items inside the AHU casing gives the total static pressure.
Related Tools
Duct Sizer Static Pressure Budget Fan Power Calculator Filter Pressure Drop Velocity Calculator Duct Leakage Calculator Fan Affinity Law VAV Box Sizing
AIM Works · launches Oct 15, 2026

Connect the calculation to the rest of the project.

AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.

  • Peak loads (RTS or CLTD/CLF) and ASHRAE 62.1-2019 ventilation
  • Equipment and plant sizing against ASHRAE 90.1-2022 efficiency floors
  • Design Review that cites the check behind every finding
  • An AI co-pilot that drafts changes — you approve each one

Duct and pipe distribution sizing is planned for V2 — at launch, keep using this calculator for that step.

Book a Demo →
A walkthrough first, then a 14-day free trial. No credit card.
How the calculations work →

Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.