Equal-friction and velocity duct sizing
Enter airflow and your target friction rate. Ductwise returns the exact calculated diameter, the next round manufactured size you can order, and the resulting mean velocity.
For the building-services trade
Ductwise the way the standards do.
Ductwise sizes ductwork the way the standards do — and shows you the numbers behind the answer.
What it does
Rectangular and flat-oval equivalents
Converts any round duct to a rectangular or flat-oval section carrying the same flow at the same friction rate, with an aspect-ratio warning when the shape becomes impractical.
Total pressure loss with fittings
Add straight duct runs and fittings using standard loss coefficients. The app sums the system pressure and shows every contributing section separately.
Real air density from altitude and temperature
Pro computes air density and viscosity from your site altitude and air temperature. At a mile above sea level, air is about 17 % lighter — every standard constant shifts with it.
Offline — no account, no sign-in
Every calculation runs on-device. No internet connection is required and no account is needed. Your project data stays on your phone.
Metric and imperial
Switch between SI and imperial units with one tap. The app converts all displayed values at once so you can work in the units your project requires.
A look inside
Calculations you can follow and verify
The method, in the open
Friction comes from the Darcy–Weisbach equation; the friction factor is solved iteratively from Colebrook–White using hydraulic diameter and mean velocity. Circular equivalents use the Huebscher relation for rectangular duct and the ASHRAE relation for flat oval. You can read the full method inside the app. A calculator that will not tell you what it is doing does not deserve your trust.
Free to use — one-time Pro purchase
Round duct sizing, airflow from air changes, and core analysis are free with no time limit and no nag screens. Pro adds rectangular and flat-oval ducts, fitting losses, real air density, unlimited saved projects, report sharing, and no ads. Single purchase, no subscription.
Your data stays on your device
Nothing you type leaves your phone. The free version shows a Google AdMob banner. In the EEA, UK and Switzerland you are asked for consent before ads load. Ads are never personalised. Pro removes advertising entirely.