Duct Size Calculator

How Duct Sizing Works

Most residential and light commercial ducts are sized with the equal friction method. You choose a friction rate, the pressure the air loses per 100 feet of straight duct, and size every section so it loses about that much. Bigger airflow at the same friction rate needs a bigger duct.

Velocity is the second check. A duct sized only by friction can still move air fast enough to be noisy, especially near registers. This calculator sizes for the friction rate, then makes sure the velocity stays under the limit you pick, and uses whichever gives the larger duct.

The airflow for each run comes from a room-by-room load calculation (ACCA Manual J), and the friction rate comes from the blower’s available static pressure and the length of the longest run (ACCA Manual D). This tool does the sizing step in between.

Choosing a Friction Rate

Manual D finds the design friction rate from the static pressure the blower can spend on straight duct. Start with the blower’s rated external static pressure, subtract the pressure drops of the coil, filter, supply registers and return grilles, then divide what is left by the total effective length of the longest supply and return path, including the equivalent length of fittings.

For example, a blower rated at 0.50 in. wg with a 0.20 coil, 0.10 filter, 0.03 register and 0.03 grille leaves 0.14 in. wg. Over 250 feet of total effective length, the friction rate is 0.14 × 100 ÷ 250 = 0.056 in. wg per 100 ft.

Residential systems usually land between 0.05 and 0.10. The old rule of thumb of 0.10 for everything often undersizes returns and long flex runs, which is one reason so many systems run at high static pressure.

Duct Sizing Formulas

Friction loss uses the Darcy-Weisbach equation with the Colebrook friction factor, the method used by ASHRAE and duct manufacturers, at standard air density (0.075 lb/ft³).

Q

Airflow in cubic feet per minute (CFM)

V

Velocity in feet per minute (fpm)

D

Round diameter in inches

f

Friction factor from the Colebrook equation

a, b

Rectangular duct sides in inches

ASP

Available static pressure for duct, in. wg

TEL

Total effective length of the longest run, ft

Rectangular duct uses its equivalent round diameter for friction but its actual area for velocity. Galvanized roughness is set so results match the standard galvanized friction chart within about 4%.

Worked Example: 3-Ton Supply Trunk

A 3-ton system moves 1,200 CFM through a galvanized supply trunk. The design friction rate is 0.08 in. wg per 100 ft and the trunk velocity limit is 900 fpm.

1. By friction alone, the trunk would need about 15.5 inches.
2. By velocity: 1,200 CFM ÷ 900 fpm = 1.33 sq ft, which is a 15.64 inch round. Velocity governs.
3. Next standard round size: 16 inches, at 859 fpm and 0.069 in. wg per 100 ft.
4. Rectangular equivalents: 30 x 8, 22 x 10, 18 x 12 or 16 x 14.

Result: a 16 inch round trunk, or 18 x 12 if you need a rectangular duct to fit the joist space.

Worked Example: Is a 6 Inch Duct Enough for 150 CFM?

A bedroom needs 150 CFM, and the existing branch is 6 inch galvanized round.

1. Velocity: 150 ÷ 0.196 sq ft = 764 fpm.
2. Friction: 0.188 in. wg per 100 ft, more than double a 0.08 design rate.
3. A 7 inch duct drops that to 0.087 in. wg per 100 ft at 561 fpm.

Result: no. The 6 inch run will starve the room or force the blower to work against high static. Use a 7 inch branch.

Round Duct CFM Chart (Galvanized)

Airflow each round size carries at common friction rates, with the velocity at 0.08. Check the velocity against your limit before using a size near the top of its range.

Velocity Limits and Flex Duct

Common residential design guidelines keep main supply trunks at or below about 900 fpm, return trunks around 700 fpm, and branch runs near 600 fpm. Commercial mains often run 1,000 to 1,500 fpm or more where noise matters less. Higher velocity is not wrong in itself, but it adds noise and pressure drop at every fitting.

Flex duct is rated fully stretched. Compressed, kinked or sagging flex can have several times the friction of the same duct pulled tight, which is the most common reason a correctly sized flex run underperforms. Pull flex taut, support it at close intervals, and keep bends gentle.

• Supply trunk: about 700 to 900 fpm (residential).
• Return trunk: about 600 to 700 fpm.
• Branch runs: about 500 to 600 fpm.
• Commercial mains: often 1,000 to 1,500 fpm or more.

Field Tips

Practical advice from HVAC pros on Energy Vanguard, HVAC-Talk and ACHR News.

1. 1

   Stretch the flex

   According to Energy Vanguard, flex duct compressed lengthwise by 15% roughly doubles its friction rate, and 30% compression roughly quadruples it. A run sized correctly on paper can deliver half the air if it's installed bunched up. Pull every run tight before you hang it.

2. 2

   Support flex properly

   ADC installation guidance cited by Energy Vanguard allows no more than 1/2 inch of sag per foot between supports, with supports no more than 4 feet apart. Use straps at least 1-1/2 inches wide; narrow straps pinch the duct and choke the airflow.

3. 3

   Check total static pressure before blaming the duct size

   Measure total external static pressure at the air handler and compare it to the nameplate rating, which is typically around 0.5 in. wg for residential equipment including the filter. A return that pulls in or flexes hard when the blower starts is a common field sign of an undersized return.

4. 4

   Don't size flex with a sheet-metal ductulator

   Flex duct has more friction than metal, so metal charts undersize it. Use the flex setting in this calculator, a flex-specific chart, or, as Energy Vanguard suggests, a conservative 0.05 friction rate if all you have is a metal ductulator.

5. 5

   Size returns and filters by velocity

   Manual D caps return ducts at 700 fpm, and techs often aim for about 300 fpm across the filter. At 1,200 CFM that means about 4 sq ft of filter. A single 20 x 25 filter is about 3.47 sq ft, which puts it near 346 fpm, so a larger or second return grille helps.

Sources: Energy Vanguard: How to install flex duct properly, Energy Vanguard: Duct design 5, sizing the ducts, ACHR News: Five tips to improve flexible duct installations, HVAC-Talk: Are my return ducts undersized?, HVAC-Talk: Return duct sizing