Ductwise › Duct Size Calculator
Duct Size Calculator
Enter the airflow and a design friction rate or velocity. You get the exact round duct diameter, the next standard size you can buy, and how fast and how hard the air moves through it. Free, no sign-up, the same numbers as the Ductwise app.
Need more than a round duct?
The Ductwise app does everything on this page offline, and Pro adds the rest. One payment, no subscription.
- Rectangular and flat-oval ducts
- Fitting losses and total system pressure
- Airflow from heating and cooling loads
- Real air density — altitude and temperature
- Unlimited saved projects
- Share a clean report with your client
- No ads, ever
Questions
What is the equal friction method?
You pick one friction rate for the whole system — often about 0.8 Pa/m or 0.1 in.wg per 100 ft — and size every duct so its pressure loss per metre matches it. This calculator finds the round diameter that carries your airflow at exactly that rate, then rounds up to a standard size.
Should I size by friction or by velocity?
Equal friction is the usual choice for supply and return branches. Velocity sizing suits places where noise or air speed is the limit, such as risers or ducts near occupied rooms. The calculator warns you when the standard duct runs below about 2 m/s or above about 9 m/s.
Which air density does the calculator use?
Standard air: 20 °C at sea level, 1.204 kg/m³. The Ductwise app with Pro corrects the density for your site's altitude and air temperature.
How is the friction rate calculated?
With the Darcy–Weisbach equation and a Colebrook–White friction factor, using the roughness of the duct material you choose. These are the relations the ASHRAE duct friction chart is drawn from.
Can I size a rectangular duct?
The Ductwise app does: with Pro it gives the rectangular or flat-oval duct with the same friction and airflow as the round one, rounded to the next 50 mm or 2 in side.
How it is calculated
- Air: ASHRAE standard air, 20 °C at 101.325 kPa (1.204 kg/m³), viscosity from Sutherland's law.
- Friction: Darcy–Weisbach with the Colebrook–White friction factor, solved by Newton iteration.
- Sizing: the round diameter is found by bisection to the target friction rate, or from the area for a target velocity.
- Standard sizes: EN 1506 spiral duct diameters (80–1250 mm), or 4–48 in for imperial.
- Velocity and friction are reported for the standard duct you would actually install.
The calculation runs in your browser; nothing you type is sent anywhere. It is the same engine as the Ductwise app.