DuctWise HVAC Duct Calculator

You already know how to size a duct. This agrees with your wheel, and then answers the part the wheel cannot.

Type the airflow and the friction rate. DuctWise gives you a round size the supply house actually stocks, the velocity that size will really run at, and the static the whole run will cost you — off the ASHRAE friction chart for galvanised round duct, the same chart the ductulator in your door pocket is printed from.

THE PART A WHEEL CANNOT DO

The round size was never the hard part. What fits in a 2x10 joist bay is. Tap once and you get every stocked rectangular size that carries the same air, squarest first, with anything past 4:1 flagged before you order it — and the flat-oval sizes beside them, for the spaces where a rectangle will not go.

It is equal friction, not equal area. A 20x8 does not carry what its area suggests, because the air drags past more metal on the way through. DuctWise uses Huebscher's equation, so the equivalents are honest.

IT KNOWS WHAT THE DUCT IS FOR

A bedroom branch and a shop main want very different velocities. Say which one you are running and every size is checked against the design band for that job — in the band, or above it, or below it, with the band drawn so you can see how close to the edge you are.

Supply trunk, supply branch, return trunk, return branch, commercial main, commercial branch, commercial return. Typical values from Manual D and ASHRAE Fundamentals, with a line on each saying why the band is where it is.

THE REST OF IT

Static over the run. Enter the total equivalent length — straight duct plus the fittings — and see what the duct costs you in inches of water, which is the number that has to fit inside the blower table.

Altitude correction. Friction loss falls with air density. A job a mile up loses about a sixth less pressure per 100 ft than the chart says, and the app accounts for it.

Inches and CFM, or millimetres and litres per second. Friction in in.wg/100ft or pascals per metre. Switch at any time with nothing to re-enter.

The friction chart for your airflow, with two sizes either side of the pick, so you can argue with the answer instead of taking it.

WHAT IT IS NOT

It does not calculate a heat load or a total effective length. It sizes one duct against one target. A whole system still needs Manual J and Manual D, and the job's specification governs. The app says so on its own Reference screen.

Works offline. No account.