FreeHVACCalculators

Duct Size Calculator

Size a round or rectangular duct for a given airflow by friction rate or maximum velocity — and see the velocity and friction loss at the standard size.

Method reviewed October 22, 2026. Version 1.0.0. How it works

Your inputs

Unit system

In ACCA Manual D the friction rate is calculated: available static pressure × 100 ÷ total effective length of the longest run. Residential designs often land between about 0.06 and 0.18 in. w.c. per 100 ft.

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

Result

Standard round duct size
12in

Use a 12-inch round duct (509 fpm, 0.035 in. w.c./100 ft) or an equivalent rectangular size.

Exact calculated diameter
10.14in
Velocity at standard size
509fpm
Friction rate at standard size
0.035in. w.c./100 ft
Velocity pressure
0.016in. w.c.

Alternative: one size smaller (10 in), within 10 % of the target friction

Round size
10in
Friction rate
0.086in. w.c./100 ft
Velocity
733fpm

Equivalent rectangular sizes (width for each height)

6 in high × width
16in
8 in high × width
11in
Show the working
  1. Solve Darcy–Weisbach + Colebrook for the diameter giving the target friction rate
    D = 10.14 in at 0.08 in. w.c./100 ft
  2. Velocity at chosen size
    V = 400 / (π × (12/12)² / 4) = 509 fpm
  3. Reynolds number (ASHRAE: Re = 8.56·D·V)
    52,315
  4. Friction rate at chosen size
    0.0352 in. w.c. per 100 ft

Equal friction method

Most residential duct systems are sized so every section loses roughly the same pressure per 100 feet. In ACCA Manual D that design friction rate is calculated from the static pressure available after the coil, filter, registers and grilles, divided by the total effective length of the longest run.

This calculator solves the Darcy–Weisbach equation, with the Colebrook friction factor and ASHRAE’s Reynolds number for standard air (Re = 8.56 × D × V), for the diameter that gives your friction rate. That is the same physics behind a paper ductulator wheel.

Standard sizes and rectangular equivalents

The exact diameter is rounded up to the next standard round size, and the velocity and friction rate are recalculated at that size. Rectangular options come from the Huebscher equation, which gives the rectangular duct with the same friction at the same airflow. Each width is rounded up to a whole inch, with aspect ratios kept to 4:1 or less.

Velocity limits

Velocity drives noise. ACCA Manual D’s residential table recommends about 700 fpm for rigid supply trunks (900 fpm maximum), 600 fpm for branches, and 600 fpm for flexible duct (700 fpm maximum). The result flags any size that exceeds them.

Worked example

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

Inputs

Airflow
400 CFM
Sizing method
Equal friction (friction rate)
Design friction rate
0.08 in. w.c./100 ft
Duct material
Galvanized steel (sheet metal)
Site elevation
0 ft

Result

Standard round duct size
12 in
Exact calculated diameter
10.14 in
Velocity at standard size
509 fpm
Friction rate at standard size
0.035 in. w.c./100 ft
Velocity pressure
0.016 in. w.c.

Use a 12-inch round duct (509 fpm, 0.035 in. w.c./100 ft) or an equivalent rectangular size.

Assumptions

  • Standard air (0.075 lb/ft³), corrected for site elevation. Friction uses the Darcy–Weisbach equation with the Colebrook friction factor, the method used to build ASHRAE duct-friction charts.
  • Absolute roughness: galvanized steel 0.0003 ft (0.09 mm, the ASHRAE “medium smooth” value); fully extended flexible duct and fibrous glass duct board 0.003 ft. Some references list galvanized steel as rough as 0.15 mm, which raises friction by only a few percent at residential sizes.
  • Rectangular equivalents use the Huebscher equation, De = 1.30·(a·b)^0.625 / (a + b)^0.25, so they have the same friction rate at the same airflow, rounded up to the next whole inch.

Limitations

  • Sizes a single straight section. Fittings, registers, filters and coils add pressure drop — a full Manual D design accounts for total effective length and available static pressure.
  • Equivalent rectangular ducts have equal friction, not equal velocity; the rectangular section’s velocity is lower.

Frequently asked questions

What size duct for 400 CFM?

At a 0.10 in. w.c. per 100 ft friction rate in sheet metal, about a 10-inch round duct, running near 730 fpm. At a lower friction rate, or in flexible duct, the size goes up.

Why is flex duct sized larger?

Its corrugated inner surface is far rougher than sheet metal, and any sag or compression adds much more loss. These results assume flex is pulled fully taut, which is the best case.

Is this a full Manual D design?

No. It sizes one straight section. A Manual D design accounts for every fitting through its equivalent length, the blower’s available static pressure, and room-by-room airflow from a Manual J load calculation.

Sources

  1. Manual D calculation checklist (Table 3-1 velocity limits) — ACCA (hosted by AHRI). Friction rate = available static pressure × 100 / total effective length; residential velocity limits (rigid trunk 700 recommended / 900 max; flex 600 / 700).
  2. Equivalent Diameter of rectangular and oval ducts — The Engineering ToolBox. Huebscher equation De = 1.30·(a·b)^0.625/(a + b)^0.25.

Spotted an error? Report a problem with this calculator — see our corrections policy.