Ductwise: Duct Calculator

Ductwise sizes ductwork the way the standards do — and shows you the numbers behind the answer.

Type the airflow and the friction rate you design to. Ductwise gives you the exact round duct, the next size you can actually buy, the resulting velocity, and the rectangular equivalent for the same friction and flow. No chart, no cardboard slide rule, no guessing.

BUILT FOR THE JOB • Big, high-contrast numbers you can read on a roof in daylight • Large touch targets that work with gloves on • Dark mode for plant rooms and night shifts • Every calculation works offline — no account, no sign-in • Metric and imperial, switched with one tap

WHAT IT CALCULATES • Equal-friction and velocity duct sizing • Next manufactured size, so the answer is something you can order • Rectangular and flat-oval equivalents, with an aspect-ratio warning • Friction rate, velocity, velocity pressure, Reynolds number, friction factor • Total pressure loss: straight run plus fitting loss coefficients • Free area, hydraulic diameter, circular equivalent, surface area for insulation • Airflow from room volume and air changes per hour • Airflow from a sensible heating or cooling load

THE METHOD, IN THE OPEN Friction comes from the Darcy–Weisbach equation, with the friction factor solved iteratively from Colebrook–White using the hydraulic diameter and the actual mean velocity. Circular equivalents use the Huebscher relation for rectangular duct and the ASHRAE relation for flat oval. Duct roughness follows the standard material categories, from smooth PVC to flexible duct.

With Pro, air density and viscosity are computed from your air temperature and site altitude instead of being assumed. That matters: a mile above sea level, air is about 17 % lighter, and every rule-of-thumb constant quietly drifts with it. The app always shows you the density it used.

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 AND PRO Round duct sizing, airflow from air changes and the core analysis are free and complete — no time limit, no nag screens. The free version keeps three saved calculations and shows a small banner advert on the Projects screen.

Ductwise Pro is a single one-time purchase. 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

PRIVACY Nothing you type into Ductwise leaves your device. The advert in the free version is served by Google AdMob, which receives device and advertising data to show it. In the EEA, the UK and Switzerland you are asked for consent first. Ductwise then asks, through Apple's tracking prompt, whether ads may be personalised: if you allow tracking, Google may use your device's advertising identifier; if not, every ad is non-personalised. The app works exactly the same either way. Pro removes advertising entirely.

Ductwise is an engineering aid. It does not replace the designer's judgement or the codes that apply to your project — always check results against the standard in force.