Duct Size Calculator

How the calculator sizes a duct

Type the airflow in CFM and either a friction rate or a velocity. A friction rate is the pressure the duct may use up for every 100 ft of its length, in inches of water column (in. w.c.). The calculator then finds the round duct that carries your airflow at exactly that rate.

It does this with physics, not with a chart. The duct friction chart in the ASHRAE Handbook is a plot of two equations: Darcy-Weisbach for the pressure loss and Colebrook for the friction factor, drawn for galvanized steel duct (roughness 0.0003 ft) and standard air (0.075 lb per cubic foot, about 70 °F at sea level). This page solves the same equations for your numbers:

• Velocity: CFM ÷ area, where the area of a round duct is π × (D ÷ 24)² sq ft with D in inches.
• Reynolds number: 8.56 × D × velocity, for standard air.
• Friction per 100 ft: (1,200 × f ÷ D) × 0.075 × (velocity ÷ 1,097)², with the friction factor f found from Colebrook’s equation by repeating it until it stops changing.

The calculator tries diameters until the friction matches your rate. In velocity mode it skips that search: the area is the airflow divided by the velocity you chose.

Why the answer is rounded up

The exact solve is rarely a size you can buy, so the page rounds it up to the next 0.1 in, then names the next whole inch and works out the velocity and friction at that size. A bigger duct always has less friction than your rate, so the size you see never asks the fan for more than you allowed.

For the default job, 400 CFM at 0.10 in. w.c. per 100 ft, the exact size is 9.684 in. The page shows 9.7 in, and the next whole inch, 10 in, moves the air at 733 fpm with 0.085 in. w.c. of friction per 100 ft.

Friction rate or velocity?

Most people size supply ducts by friction rate. The 0.10 default is the figure ACCA says people use everywhere because it sits in the middle of the chart, and ASHRAE puts low-pressure systems between 0.05 and 0.20. A better rate comes from your own system: available static pressure × 100 ÷ total effective length (HVAC School). Open Work out my friction rate, type both figures, and the rate fills the box, rounded down so the duct comes out a little bigger, never smaller. Energy Vanguard’s worked design lands on 0.073 this way.

Size by velocity when you want to hold a speed instead, such as a return you want quiet. Type the velocity and the page sizes the duct straight from the area.

Supply and return limits

Manual D lists 900 fpm as the highest velocity for a home’s supply ducts and 700 fpm for returns (per Energy Vanguard). Pick Supply or Return and the pilot lamp compares the velocity at the size on the plate with that limit: green at or below it, red above it, with a size that fits. The same 400 CFM that runs at 733 fpm in a 10 in duct is fine as a supply but too fast for a return, which needs 11 in.

To check a duct you already have, type its size in Check a size. The plate then shows the velocity and friction at that size, and the lamp reads it.

Rectangular ducts

A rectangle with the same area as a round duct has more friction, because more of its air rubs on the walls. The Huebscher equation gives the round size with the same airflow and friction: De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. Choose Rect, type one side, and the calculator solves for the other side so the rectangle’s equivalent round is at least the exact size, rounded up to the next 0.5 in (10 mm).

With an 8 in side, the default job needs an 8 × 10 in rectangle: its equivalent round is 9.762 in, the air moves at 720 fpm through the real 80 sq in of area, and the friction is 0.096 in. w.c. per 100 ft. That is why matching area alone is wrong.

Metric units

The Units band switches every box to litres per second, pascals per metre, metres per second and millimetres. The page still works in US units inside and converts with NIST factors: 1 CFM is 0.4719474 L/s and 1 in. w.c. per 100 ft is 8.1722 Pa/m. The exact size shows in millimetres, rounded up to the next 1 mm, and the next whole inch keeps its inch size with millimetres beside it.

What this does not cover

This sizes one straight section of galvanized duct in standard air. It leaves out fittings and their equivalent lengths, the blower’s pressure budget, altitude and temperature corrections, flex duct sag and compression, flat oval duct and grille sizing. A Manual J load calculation sets the airflow each room needs, and a duct design by a pro sizes the real system.

Worked example

1. Inputs: 400 CFM supply duct, friction rate 0.10 in. w.c. per 100 ft, round, US units.
2. Airflow = 400 CFM
3. Friction rate = 0.10 in. w.c. per 100 ft
4. Exact size: the round duct whose Darcy-Colebrook friction = 0.10 at 400 CFM is 9.684 in (782 fpm there), rounded up to 9.7 in
5. Velocity at 10 in = 400 CFM ÷ (π × (10 ÷ 24)²) = 400 ÷ 0.5454 sq ft = 733 fpm
6. Friction at 10 in = (1,200 × 0.0213 ÷ 10) × 0.075 × (733.4 ÷ 1,097)² = 0.085 in. w.c. per 100 ft (Re 62,778)
7. Next whole inch: 10 in · 733 fpm · 0.085 in. w.c. per 100 ft
8. The same duct as a rectangle with one 8 in side:
9. Other side: 1.30 × (8 × b)^0.625 ÷ (8 + b)^0.25 = 9.684 in at b = 9.836 in, rounded up to 10 in
10. Equivalent round of 8 × 10 in = 1.30 × (8 × 10)^0.625 ÷ (8 + 10)^0.25 = 9.762 in
11. Velocity in 8 × 10 in = 400 CFM ÷ (8 × 10 ÷ 144) = 400 ÷ 0.5556 sq ft = 720 fpm
12. Friction at the 9.762 in equivalent = 0.096 in. w.c. per 100 ft
13. 8 × 10 in rectangle · equivalent round 9.8 in · 720 fpm · 0.096 in. w.c. per 100 ft

Questions people ask