Duct Size Calculator
Size round ducts from cfm or m³/h by velocity or equal friction, see the next standard size and rectangular equivalents from the Huebscher formula.
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About this toolWhat is the Duct Size Calculator?
A duct has to carry a given airflow without being so small that it whistles and eats up fan pressure, or so large that it costs too much and will not fit. The two classic ways to choose a size are the velocity method, where you pick a target air speed, and the equal-friction method, where you pick a pressure loss per unit length and let the duct size follow. 1,000 cfm at 0.1 in.wg per 100 ft works out to a 13.65 in round duct running at about 980 fpm; the next standard size is 14 in.
This calculator does both methods in US or metric units. Friction is calculated with Darcy-Weisbach and the Colebrook equation for standard air, the same physics as the ASHRAE friction chart, with a choice of duct roughness from smooth PVC to flexible duct. It rounds up to the next standard round size and lists rectangular ducts that carry the same airflow at the same friction, from Huebscher's equivalent-diameter formula, with their aspect ratio and velocity.
- Velocity and equal-friction sizing in US or metric units
- Darcy-Weisbach with Colebrook friction and five roughness categories
- Next standard round size in inch or EN 1506 metric series
- Rectangular equivalents from the Huebscher formula with aspect ratio
- Typical design velocity table and velocity pressure for fitting losses
How to use the Duct Size Calculator
- Choose US units (cfm, fpm, in.wg/100 ft) or metric (m³/h, m/s, Pa/m).
- Enter the airflow the duct must carry, for example from a room load or the 400 cfm per ton rule.
- Pick equal friction and enter a friction rate, or pick velocity and enter a design velocity.
- Select the duct material to set the roughness used in the friction calculation.
- Read the exact round diameter, the next standard size with its velocity and friction, and the rectangular options.
Worked examples
Supply trunk by equal friction (page default)
Input
1,000 cfm, 0.1 in.wg/100 ft, galvanized steel
Output
13.65 in exact, 14 in standard, about 980 fpm; rectangular 13 x 12, 16 x 10 or 21 x 8 in
The 21 x 8 in option fits a shallow joist space but has an aspect ratio of 2.6 and carries the same friction because its equivalent diameter is still about 13.7 in.
Same airflow by velocity
Input
1,000 cfm at 900 fpm
Output
14.27 in exact diameter
At 900 fpm the duct is slightly bigger than the equal-friction answer, and its friction falls below 0.1 in.wg/100 ft.
Metric ventilation main
Input
1,700 m³/h at 0.8 Pa/m, galvanized
Output
348 mm exact, 355 mm standard, about 5.0 m/s
European round sizes follow the EN 1506 series, so the next size up from 348 mm is 355 mm.
Background and insight
The velocity method uses continuity alone: area equals airflow divided by velocity, so D = √(4Q/πV). It is simple and controls noise, but it ignores how much pressure the run will consume. The equal-friction method fixes the pressure loss per unit length instead, so every branch of a system loses pressure at a similar rate, which makes balancing easier. Residential designers commonly use 0.06 to 0.10 in.wg per 100 ft; ACCA Manual D derives the actual rate from the fan's available static pressure and the longest run.
Friction in a round duct follows the Darcy-Weisbach equation, Δp/L = (f/D)·ρV²/2. The friction factor f comes from the Colebrook equation, which depends on the Reynolds number and the relative roughness ε/D. ASHRAE classes galvanized steel ducts as medium smooth with ε = 0.09 mm (0.0003 ft), the basis of its friction chart, and lists smoother and rougher categories for other materials. Flexible duct, even fully stretched, is far rougher, and sagging or compressed flex can double the loss again.
A rectangular duct does not have the same friction as a round duct of equal area, because its corners add wetted perimeter. Huebscher (1948) gave the equivalent diameter that carries the same airflow at the same friction: De = 1.30 (ab)^0.625 / (a + b)^0.25. A 12 x 12 in duct is equivalent to 13.1 in round, not the 13.5 in its area would suggest. Aspect ratios above about 4:1 waste metal and pressure, and are best avoided.
Common use cases
Residential duct layouts
Size trunks and branch runs for a furnace or heat pump from room airflows and a target friction rate.
Ventilation and extract systems
Pick duct sizes for an MVHR unit, a bathroom fan run or a kitchen extract in metric or US units.
Swapping round for rectangular
Find a rectangular section that fits a ceiling void or joist bay without losing airflow or adding friction.
Checking an existing duct
Enter the airflow and see what friction and velocity a given round size produces, to explain noise or low airflow.
Tips and limitations
- Fittings often account for more pressure loss than straight duct. Add their equivalent lengths (or loss coefficients times velocity pressure) before checking against fan static pressure.
- Flexible duct should be pulled tight and kept short. Compressed flex can have several times the friction of the same diameter in sheet metal.
- Return ducts are often sized at lower velocity than supplies to keep grille noise down.
- At high altitude air is less dense, so friction for the same cfm is lower but the mass of air (and heat) moved is lower too.
- Round up to a standard size, then confirm the resulting velocity is still in the comfortable range for the space.
Frequently asked questions
How do I calculate duct size from CFM?
For a chosen velocity, area (ft²) = cfm / fpm and diameter (in) = 13.54 x √(cfm / fpm). For 1,000 cfm at 900 fpm that is 14.3 in. For equal friction, the diameter is found from the friction equation instead.
What size duct do I need for 1000 CFM?
About a 14 in round duct at a friction rate of 0.1 in.wg per 100 ft in galvanized steel, running near 950 to 1,000 fpm. A lower friction rate or a quieter target velocity needs 15 or 16 in.
What friction rate should I use for duct sizing?
Residential systems are commonly designed around 0.06 to 0.10 in.wg per 100 ft (about 0.5 to 0.8 Pa/m). ACCA Manual D calculates the rate from the blower's available static pressure divided by the total effective length.
How do I convert a round duct to rectangular?
Use the Huebscher equivalent diameter, De = 1.30 (ab)^0.625 / (a + b)^0.25, and find the a x b rectangle whose De matches the round size. A 14 in round is roughly 13 x 13, 16 x 11 or 22 x 8 in.
What is the maximum velocity in a residential duct?
Common guidance is 700 to 900 fpm in supply trunks and 500 to 700 fpm in branches and returns. Above about 900 fpm noise at registers becomes noticeable.
Why is a rectangular duct less efficient than round?
For the same cross-sectional area a rectangle has more perimeter in contact with the air, so more friction. The penalty grows with the aspect ratio.
Is there an in-depth guide to the Duct Size Calculator?
Yes. “Duct Sizing: The Velocity and Equal-Friction Methods Explained” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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