A free online ductulator for round and rectangular ductwork. Size by friction rate or velocity, in imperial or metric, with standard duct sizes, air density correction and a rectangular-to-round converter. Results update as you type. Also known as a ductulador online (Spanish).
Standard air is 0 ft and 70°F (21°C), density 0.075 lb/ft³ (1.2 kg/m³). Enter the temperature of the air inside the duct, not the outdoor temperature. Examples: at 2,000 ft (610 m), cooled supply air at 55°F (13°C) is about 4% below standard density and hot exhaust air at 113°F (45°C) about 14% below; at 5,000 ft (1,520 m), standard-temperature air is about 17% below. Friction loss scales with density; velocity does not.
This duct size chart is generated from the same equations the calculator above uses, for galvanised metal duct at standard air. Each cell shows the exact calculated diameter, with the recommended standard round size in brackets. Round duct steps in 1 in increments to 12 in and 2 in increments above, so odd sizes such as 13, 15, 17 and 19 in are not shown. The velocity column is at the exact calculated diameter. Need it on paper? Download the duct size chart PDF.
| Airflow (CFM) | Airflow (L/s) | 0.05 in.wg/100ft 0.41 Pa/m — quiet |
0.08 in.wg/100ft 0.65 Pa/m |
0.10 in.wg/100ft 0.82 Pa/m — standard |
Velocity at 0.10 |
|---|---|---|---|---|---|
| 100 | 47 | 6.7" (7") | 6.1" (7") | 5.9" (6") | 535 fpm / 2.7 m/s |
| 200 | 94 | 8.7" (9") | 7.9" (8") | 7.6" (8") | 636 fpm / 3.2 m/s |
| 300 | 142 | 10.2" (11") | 9.2" (10") | 8.8" (9") | 703 fpm / 3.6 m/s |
| 400 | 189 | 11.3" (12") | 10.3" (11") | 9.9" (10") | 754 fpm / 3.8 m/s |
| 600 | 283 | 13.2" (14") | 12.0" (12") | 11.5" (12") | 834 fpm / 4.2 m/s |
| 800 | 378 | 14.7" (16") | 13.4" (14") | 12.8" (14") | 895 fpm / 4.6 m/s |
| 1,000 | 472 | 16.0" (16") | 14.6" (16") | 13.9" (14") | 946 fpm / 4.8 m/s |
| 1,200 | 566 | 17.1" (18") | 15.6" (16") | 14.9" (16") | 990 fpm / 5.0 m/s |
| 1,600 | 755 | 19.1" (20") | 17.4" (18") | 16.6" (18") | 1,063 fpm / 5.4 m/s |
| 2,000 | 944 | 20.7" (22") | 18.9" (20") | 18.1" (20") | 1,123 fpm / 5.7 m/s |
| 2,500 | 1,180 | 22.5" (24") | 20.5" (22") | 19.7" (20") | 1,186 fpm / 6.0 m/s |
| 3,000 | 1,416 | 24.2" (26") | 22.0" (22") | 21.1" (22") | 1,241 fpm / 6.3 m/s |
| 4,000 | 1,888 | 26.9" (28") | 24.5" (26") | 23.5" (24") | 1,333 fpm / 6.8 m/s |
| 5,000 | 2,360 | 29.3" (30") | 26.7" (28") | 25.5" (26") | 1,408 fpm / 7.2 m/s |
| 6,000 | 2,832 | 31.4" (32") | 28.6" (30") | 27.3" (28") | 1,473 fpm / 7.5 m/s |
| 8,000 | 3,776 | 34.9" (36") | 31.8" (32") | 30.5" (32") | 1,582 fpm / 8.0 m/s |
| 10,000 | 4,719 | 38.0" (38") | 34.6" (36") | 33.1" (34") | 1,671 fpm / 8.5 m/s |
For duct board multiply the round diameter by about 1.06, and for fully stretched flexible duct by about 1.12, at the same airflow and friction rate. The calculator applies this automatically when you change duct material.
Rectangular sizing uses the Huebscher equivalent diameter for friction, but velocity is always calculated from the real rectangular area — the two are not the same number.
Prefer a walkthrough with screenshots? Follow the step-by-step online duct sizing tutorial on MEPBase.
A ductulator is the classic HVAC duct sizing calculator — traditionally a rotating cardboard wheel, made familiar by the Trane and McQuay versions, that engineers spin to line up airflow, friction rate, velocity and duct size. This online ductulator does exactly the same job digitally: enter CFM or L/s with your design friction rate or velocity and get the round diameter or rectangular equivalent instantly. No wheel, nothing to install, and it works in a phone browser on site.
You will see the tool spelled ductilator, ductalator or duct-u-lator in searches and on site. They all mean the same thing — ductulator is the correct spelling, and every version refers to the same duct sizing tool for air distribution.
Duct friction loss follows the Darcy-Weisbach equation with the Colebrook friction factor. For galvanised duct carrying standard air, ASHRAE gives a well-known power-law fit, which this calculator uses with a small size-dependent calibration:
Checked against a full Darcy-Weisbach / Colebrook solution, results fall within about 2% across 100–10,000 CFM and sit slightly on the conservative (larger duct) side — the same basis as the traditional ductulator wheels.
Because friction varies with diameter to the power of 5.02, a small size change has a large effect. Going from a 12" to a 14" duct at the same airflow cuts the friction loss to roughly half (800 CFM: 0.139 → 0.064 in.wg/100 ft).
For when to use each method on a real project, read equal friction vs velocity method for duct sizing.
| Application | Main ducts | Branch ducts |
|---|---|---|
| Residences | 700–900 fpm (3.5–4.5 m/s) | 600 fpm (3 m/s) |
| Offices, hotels, hospitals | 1,200–1,600 fpm (6–8 m/s) | 800–1,200 fpm (4–6 m/s) |
| Shops, cafeterias, commercial | 1,500–1,800 fpm (7.5–9 m/s) | 1,000–1,300 fpm (5–6.5 m/s) |
| Industrial | 1,800–2,500 fpm (9–12.5 m/s) | 1,200–1,800 fpm (6–9 m/s) |
| Return / exhaust ducts | 600–1,000 fpm (3–5 m/s) | 600–800 fpm (3–4 m/s) |
Noise-critical spaces — bedrooms, studios, conference rooms, places of worship — should stay at the lower end of each range.
| Duct system | Friction rate (imperial) | Head loss (metric) |
|---|---|---|
| Supply ducts — comfort HVAC | 0.08–0.10 in.wg/100ft | 0.65–0.85 Pa/m |
| Return ducts | 0.05–0.08 in.wg/100ft | 0.4–0.65 Pa/m |
| Exhaust ducts | 0.08–0.10 in.wg/100ft | 0.65–0.85 Pa/m |
| Low-noise / low-energy design | 0.05–0.08 in.wg/100ft | 0.4–0.65 Pa/m |
The industry default of 0.1 in.wg per 100 ft (about 0.82 Pa/m) balances duct size against fan energy for most comfort applications, and it is this tool's default. The total pressure drop result feeds straight into fan selection — take it to the fan static pressure calculator.
SMACNA recommends keeping rectangular duct aspect ratio at or below 4:1. A tall narrow duct carries the same air as a squarer one but needs more sheet metal, more insulation, more hangers and produces more friction than the equivalent diameter suggests. Where a ceiling void forces a high ratio, split the run into two parallel ducts instead of pushing past 4:1.
Last updated: 27 September 2026
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