Ductulator: Duct Size Calculator

Enter the airflow and friction rate to size a round or rectangular duct, or enter a duct size to get its friction rate and velocity. It uses the same ASHRAE equations as the duct friction chart, with flex duct compression and other materials.

Units
Find

ASHRAE sizes low-pressure ducts at 0.05 to 0.20 in. w.c. per 100 ft (0.4 to 1.6 Pa/m).

Round duct size

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Fill in the boxes above to see the answer.

Next size up
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Velocity
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Rectangular, your height
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Rectangular, 6 in. high
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Rectangular, 8 in. high
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Rectangular, 10 in. high
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Rectangular, 12 in. high
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To get the width for a height you need, on top of the common ones.

What is a ductulator?

A ductulator is a duct sizing calculator. The name comes from the cardboard slide rules HVAC techs and designers have used for decades: you line up the airflow in CFM with a friction rate and read off the round duct size, the matching rectangular sizes and the air velocity.

This one does the same job and works both ways. Give it the airflow and friction rate to get a duct size, or give it a duct size to get its friction rate and velocity. It also corrects for flex duct, including how much it's compressed, and for duct board.

How it's calculated

Darcy: Δp per 100 ft = (1200 f ÷ Dh) × 0.075 × (V ÷ 1097)² in. w.c. (ASHRAE eqn 18; SI: 1000 f ÷ Dh × 1.204 V² ÷ 2 Pa/m)
Colebrook: 1/√f = −2 log(12ε ÷ 3.7Dh + 2.51 ÷ (Re √f)), solved by iteration (eqn 19)
Re = 8.50 Dh V for standard air (eqn 21; SI: 66.4 Dh V)
Rectangular: Dh = 2ab ÷ (a + b) with the actual velocity (eqn 24)
Equivalent round = 1.30 (ab)^0.625 ÷ (a + b)^0.25 (eqn 25, Huebscher)
Flex compression: × [1 + 0.58 × % compressed × e^(−0.126 D)] (eqn 22; SI: e^(−0.00496 D))
Sizing: the diameter (or width) that gives the friction rate, solved by bisection

Sources

If a number doesn't match your chart, email me what you entered and what you expected, and I'll check it against the source. You can also read how the numbers are checked.

Checked by Paul Williams · Updated

Questions

How does a ductulator size a duct?

It finds the duct whose friction loss at your airflow equals the friction rate you picked. This one uses the same equations as ASHRAE's duct friction chart (Darcy and Colebrook, standard air, galvanized steel at 0.0003 ft roughness). For example, 700 CFM at 0.10 in. w.c. per 100 ft needs an 11.9 in. round duct, so a 12 in. duct at about 890 fpm, or about 15.3 × 8 in. rectangular.

What friction rate should I use?

ASHRAE sizes low-pressure ducts by equal friction at 0.05 to 0.20 in. w.c. per 100 ft. For a residential system, ACCA Manual D works the rate out from the blower's available static pressure and the longest supply and return runs, so use that if you have it.

How much does flex duct compression matter?

A lot. Fully stretched flex already has about 50% more friction than metal duct. ASHRAE's correction for a 10 in. duct is about 1.7 times at 4% compression, 3.5 times at 15% and 5.9 times at 30%, and more for smaller ducts. The flex duct industry's rule of thumb (ADC) is lower, about 2 times at 15% and 4 times at 30%, so the two disagree; both say to install flex fully stretched, and ADC adds supports no more than 4 ft apart.

How are the rectangular sizes worked out?

Each width gives the same friction rate as the round duct at the same airflow, using the duct's hydraulic diameter as ASHRAE's equation does. The equivalent round shown in friction mode uses Huebscher's formula; a 12 × 8 in. duct, for example, is equivalent to a 10.7 in. round. Ratios beyond about 4:1 use a lot more sheet metal for the same airflow.

Why does it give an exact size and a next size up?

Round duct comes in fixed sizes, so you'd pick the next size up. In inches that's the next whole inch, the way ASHRAE's own sizing tables round. In millimetres it's the next size in the EN 1506 series (100, 125, 160, 200, 250, 315 mm and so on). Check what your supplier stocks: above 14 or 20 in., some lines go in 2 in. steps.

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