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Duct Pressure Loss Calculator

Enter the duct you have — size, length, material and airflow — to see how much static pressure it uses, including fittings if you know their loss coefficients.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system
Duct shape

Add up the loss coefficient of every fitting in this section, from the fitting manufacturer or a fitting-loss reference. Each fitting loses C × velocity pressure. Leave 0 for straight duct only.

Thinner air at altitude carries less heat per CFM. Leave at 0 for sea-level (standard-air) results.

Result

Total pressure loss
0.128in. w.c.
Friction rate
0.256in. w.c./100 ft
Velocity
1,146fpm

400 CFM through this duct loses about 0.128 in. w.c. (0.256 in. w.c. per 100 ft at 1,146 fpm).

Straight-duct loss
0.128in. w.c.
Fitting loss
0in. w.c.
Velocity pressure
0.082in. w.c.
Show the working
  1. Friction rate: Darcy–Weisbach with the Colebrook friction factor
    0.2558 in. w.c. per 100 ft (Galvanized steel (sheet metal), ε = 0.091 mm)
  2. Straight-duct loss = friction rate × length ÷ 100
    0.2558 × 50 ÷ 100 = 0.1279 in. w.c.
  3. Velocity pressure = density ratio × (V ÷ 4,005)²
    1 × (1,146 ÷ 4,005)² = 0.0819 in. w.c.
  4. Fitting loss = ΣC × velocity pressure
    0 × 0.0819 = 0 in. w.c.
  5. Total = straight + fittings
    0.1279 in. w.c.

How it is calculated

Straight duct friction uses the Darcy–Weisbach equation with the Colebrook friction factor, the method in the current ASHRAE duct design chapter, with standard air corrected for your elevation. Rectangular ducts use their Huebscher equivalent diameter for friction and their real area for velocity.

Fittings are handled with loss coefficients: each fitting loses C times the velocity pressure, where velocity pressure is (V ÷ 4,005)² in inches of water for standard air. This is the component equation DOE’s fan system sourcebook uses.

Where loss coefficients come from

Loss coefficients depend on the exact shape of a fitting — the radius of an elbow, the ratio of branch to main in a tee — so they are not built in. Use the fitting manufacturer’s data or a fitting-loss reference, add up the coefficients for the section and enter the total. Leave it at 0 for straight duct only.

Reading the result

Compare the total with the static pressure your blower has left for the ducts. The friction rate lets you compare with a design friction rate, and the velocity helps you judge noise. Flexible duct is assumed pulled tight; compressed flex can lose several times more.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Duct shape
Round
Inside diameter
8 in
Airflow
400 CFM
Straight duct length
50 ft
Duct material
Galvanized steel (sheet metal)
Fittings: sum of loss coefficients (C)
0
Site elevation
0 ft

Result

Total pressure loss
0.128 in. w.c.
Friction rate
0.256 in. w.c./100 ft
Velocity
1,146 fpm
Straight-duct loss
0.128 in. w.c.
Fitting loss
0 in. w.c.
Velocity pressure
0.082 in. w.c.

400 CFM through this duct loses about 0.128 in. w.c. (0.256 in. w.c. per 100 ft at 1,146 fpm).

Assumptions

  • Friction: Darcy–Weisbach with the Colebrook friction factor (the method in the ASHRAE 2021 duct chapter), standard air corrected for elevation, Re = 8.56·D·V.
  • Rectangular ducts use the Huebscher equivalent diameter for friction, and their actual area for velocity and velocity pressure.
  • Fitting loss = C × velocity pressure, the component equation in DOE’s fan system sourcebook (Δp = C·ρ·(V/1,097)², which is (V/4,005)² for standard air).

Limitations

  • Fitting loss coefficients are not built in; they come from the fitting manufacturer or a fitting-loss reference and vary with geometry.
  • Roughness values are for clean duct. Flexible duct is assumed fully stretched.

Frequently asked questions

How much pressure does 50 feet of 8-inch duct lose at 400 CFM?

About 0.13 in. w.c. in galvanized steel at sea level — a friction rate near 0.26 in. w.c. per 100 ft at roughly 1,150 fpm. That is a high rate for a residential branch.

Why does altitude lower the pressure loss?

Thinner air is lighter, so it loses less pressure at the same velocity. The calculator scales by the standard-atmosphere density at your elevation.

Can I enter fittings as equivalent lengths instead?

Add their equivalent lengths to the straight length. Use the total effective length calculator to add them up first.

Sources

  1. Exhaust Duct Design (ASHRAE Distinguished Lecture; friction per ASHRAE 2021 Handbook p. 21.7) — P. J. Brooks, SMACNA, for ASHRAE (hosted by ETEK). Duct friction loss in the ASHRAE 2021 Handbook uses Darcy–Weisbach with the Colebrook friction factor; absolute roughness is determined experimentally.
  2. Determining the Absolute Roughness of Phenolic Duct — S. Idem & A. Paruchuri, ASHRAE eSociety (April 2019). Galvanized steel duct “average” absolute roughness 0.0003 ft (0.09 mm), ASHRAE Handbook—Fundamentals (2017), Duct Design, Table 1.
  3. Equivalent Diameter of rectangular and oval ducts — The Engineering ToolBox. Huebscher equation De = 1.30·(a·b)^0.625/(a + b)^0.25.
  4. Improving Fan System Performance: A Sourcebook for Industry — U.S. DOE (Industrial Technologies Program) with AMCA International. Component pressure loss Δp = C·ρ·(V/1,097)² (in. wg, V in fpm, ρ = 0.075 lb/ft³ standard), giving a parabolic system curve; system curves can have a static component; sheave changes alter fan speed; airflow is linear with speed and power is cubed.
  5. Compression Effects on Pressure Loss in Flexible HVAC Ducts — B. Abushakra, I. S. Walker & M. H. Sherman, Lawrence Berkeley National Laboratory (2002). ASHRAE classes fabric-and-wire flexible duct as “rough”, 0.0035–0.015 ft (1.0–4.6 mm) fully extended; compression raises effective roughness far above that range.

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