Static Pressure Calculator (Duct Friction & ESP)
Instant duct pressure-drop screen: enter CFM, duct size, length, and fittings for friction rate (in. w.g./100 ft) and external static pressure. Fan ESP planning—not a full Manual D spreadsheet.
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Enter design airflow and duct geometry. Results update instantly.
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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.
CFM → pressure-drop screen (100 ft metal round)
Order-of-magnitude friction near common design bands—run the live calculator for your exact diameter and fittings. Related: duct pressure drop calculator, CFM to static pressure.
| CFM | Round Ø (in) | ~Velocity (fpm) | ~Friction / 100 ft |
|---|---|---|---|
| 400 | 10 | ~730 | ~0.08–0.12 |
| 1,000 | 14 | ~930 | ~0.10–0.15 |
| 2,000 | 18 | ~1,130 | ~0.12–0.18 |
| 5,000 | 24 | ~1,590 | ~0.15–0.25 |
| 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
Friction rate vs total static pressure (ESP)
Friction rate is a per-length metric—typically in. w.g. per 100 ft of straight duct at design CFM and size. Multiply by (length ÷ 100) to get straight-duct loss. External static pressure (ESP) for fan selection is the sum of all external resistances: duct friction, fittings, filters, coils, grilles, and terminals. Total static pressure (TSP) may include fan-internal losses depending on the manufacturer’s catalog definition—always match the rating label (ESP vs TSP) on the fan curve.
Fitting equivalent lengths (screening)
Many duct designers convert fittings to equivalent straight length for friction budgeting. Values below are typical galvanized round-duct screens at moderate velocity—verify with SMACNA or manufacturer data for stamped design.
| Fitting type | Equivalent length (ft of straight duct) | Notes |
|---|---|---|
| 90° smooth elbow (radius) | 15–25 | Depends on R/D; tighter = higher |
| 90° mitered elbow (vanes) | 25–35 | Without vanes can be 40–60+ |
| 45° elbow | 10–15 | Lower than 90° |
| Tee (branch takeoff) | 30–50 | Varies with tap geometry |
| Transition (enlarge/reduce) | 5–15 | Gradual taper lowers loss |
| Entry / exit (open) | 0–10 | Grilles add catalog ΔP |
This calculator uses velocity-pressure fitting loss (N × C × VP) instead of equivalent length—both are first-pass methods. Cross-check critical runs with a duct calculator or fitting database.
How to calculate ESP (4 steps)
- Size the duct — set CFM and pick diameter or W×H from velocity targets (duct size calculator).
- Find friction rate — from a friction chart, SMACNA table, or this tool’s CFM/diameter approximation (in. w.g./100 ft).
- Sum straight-duct + fitting loss — friction × L/100 plus elbows × C × VP (or equivalent-length method).
- Add equipment allowances — filter, coil, VAV box, and terminal ΔP from submittals; compare the total to the fan’s rated ESP.
References
- ACCA Manual D — residential duct design and friction targets.
- ASHRAE duct design guidance — friction, fitting loss, and fan selection fundamentals.
- SMACNA — HVAC duct construction and pressure-class references for commercial/industrial work.
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
Is this a duct pressure drop or CFM to static pressure calculator?
Yes for first-pass duct pressure drop and CFM → static pressure screening: enter airflow and geometry to get friction rate and duct+fitting ESP. It is not an Excel Manual D workbook and does not auto-add filter/coil ΔP—add those from submittals.
How do I calculate HVAC duct static pressure step by step?
(1) Size the duct with the duct size / ductulator, (2) enter CFM and size here for friction rate, (3) add length and fitting losses, (4) add filter/coil/terminal allowances, (5) compare to the fan’s rated ESP.
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.