HVAC Duct Static Pressure Calculator
Estimate friction rate and duct + fitting static pressure from CFM, duct size, length, and elbows — for AHU/fan ESP checks after duct sizing.
Last reviewed:
Quick Calculator
Enter design airflow and duct geometry. Results update instantly.
Advanced options
About this calculator
Planning estimate for duct friction and fitting loss after you have a size from the HVAC duct size calculator. Browse all HVAC tools on the HVAC Calculators hub. Screening only—not Manual D or manufacturer fan curves. Add coil/filter ΔP separately.
Typical friction rate targets
Friction rate is usually expressed as in. w.g. per 100 ft of duct. Many commercial designs screen near 0.08–0.15; industrial trunks may run higher if noise and leakage allow.
| Application | Friction rate (in. w.g. / 100 ft) | Notes |
|---|---|---|
| Low-velocity comfort | 0.08–0.10 | Quieter supply mains |
| Commercial supply | 0.10–0.15 | Common office AHU design |
| Industrial / higher velocity | 0.15–0.25 | Check noise and leakage |
| Return / exhaust | 0.08–0.12 | Often longer effective length |
ESP building blocks (what the fan sees)
External static pressure (ESP) is the sum of resistances the blower must overcome outside the fan housing—duct friction, fittings, filters, coils, grilles, and terminals. This calculator screens duct + fitting loss; add the other rows from equipment data sheets before fan selection.
| Component | Typical allowance (in. w.g.) | In this tool? |
|---|---|---|
| Straight duct friction | From friction rate × length/100 | Yes — auto or override |
| Elbows / fittings | N × C × velocity pressure | Yes — count + C |
| Filter (clean → dirty) | ~0.1–0.5+ | Add separately |
| Cooling / heating coil | ~0.2–0.8+ | Add separately |
| Grille / diffuser / VAV box | Per catalog | Add separately |
ESP vs TSP: ESP usually means external system resistance for fan selection. Total static pressure (TSP) may include fan-internal losses depending on the manufacturer’s definition—always match the catalog’s ESP/TSP wording.
Formula & explanation
Hydraulic diameter (rectangular): Dh = 2ab / (a + b)
Velocity: V (FPM) = CFM / Area_ft²
Velocity pressure: VP ≈ (V / 4005)² in. wg
Friction rate (approx.): f ≈ 0.109136 × CFM1.9 / D5.02 (in. wg / 100 ft)
Straight duct loss: SP_duct = f × L / 100
Fitting loss: SP_fit = N_elbows × C × VP
Screening ESP (duct path): SP ≈ SP_duct + SP_fit
Best for: preliminary AHU fan ESP checks and comparing duct options after velocity sizing.
Not ideal for: final code submittals without manufacturer fitting data, or compressible/high-temperature specialty ducts.
Need CFM from air changes first? Use the ACH to CFM calculator. Need duct dimensions? Use HVAC duct size.
Worked example
Industrial trunk: 12,000 CFM, 32 in round, 80 ft straight, 2 elbows at C = 0.5.
- Velocity and VP update from area; friction rate comes from the CFM/diameter approximation (or your override).
- Straight duct loss = friction × 80/100; fitting loss = 2 × 0.5 × VP.
- Sum is a duct-path ESP screen—add filter/coil/terminal ΔP before selecting the fan.
If ESP exceeds the selected fan’s rated external static, enlarge the duct (lower velocity) or reduce fittings — then re-check with duct size and this calculator. Next, use fan laws if you change RPM or target CFM.
FAQ
What is HVAC duct static pressure?
Duct static pressure is the resistance the fan must overcome to move air through ducts and fittings. Designers often express friction as in. w.g. per 100 ft and sum straight-duct plus fitting losses for a screening external static pressure (ESP).
How do you calculate duct friction rate from CFM and duct size?
Use equivalent round diameter for rectangular ducts, then apply a friction-chart approximation from CFM and diameter. Multiply friction rate by length/100 for straight-duct loss, and add fitting loss as elbows × C × velocity pressure.
What is friction per 100 feet of duct?
It is the pressure drop (in. w.g.) expected along 100 ft of duct at the design CFM and size. Common screening targets are about 0.08–0.15 in. w.g./100 ft for commercial supply; see the friction rate table on this page.
What should my duct static pressure be?
There is no single “correct” number. Many light-commercial systems keep total external static under roughly 0.5–1.0 in. w.g. after filters and coils; industrial trunks can be higher. Match the selected fan’s rated ESP and re-check velocity/noise after sizing with the duct size calculator.
What is the difference between friction rate and external static pressure?
Friction rate is pressure drop per 100 feet of duct. External static pressure for a run sums straight-duct friction and fitting losses; coil, filter, and terminal losses are usually added separately for total fan ESP.
When should I use duct size instead of static pressure?
Use the duct size calculator when you need diameter or W×H from CFM and target velocity. Use this tool when you already have a duct size and need pressure drop for fan ESP. Neither replaces ACCA Manual D for residential stamped design.