DuctCalc: HVAC Ductulator

A ductulator that does the whole job, not just the wheel.

SIZE THE DUCT Enter CFM and friction rate, get the required round diameter. Or work the other way: enter a duct you already have and read back the velocity and the friction rate it will run at.

RECTANGULAR EQUIVALENTS Every round size comes with its rectangular equivalents, listed by aspect ratio, so you can pick the one that fits the joist bay instead of doing the conversion in your head on a ladder.

STOCK SIZES, NOT THEORY A calculator that answers 7.3 inches is a calculator you have to correct. Snap to the sizes the shop actually stocks.

MATERIAL ROUGHNESS Galvanized, flex, fiberglass duct board, spiral. The friction rate is not the same in all of them and the app does not pretend it is.

VELOCITY GUIDE Recommended velocity bands by application — main trunk, branch, return — so a number that is technically fine but noisy in a bedroom shows up as noisy in a bedroom.

HISTORY Calculations are kept with their inputs, so you can reopen the one you did before lunch instead of redoing it. The last ten, free.

IMPERIAL AND METRIC Both are first-class. Inches and CFM, or millimetres and L/s.

WHAT IS FREE AND WHAT IS NOT Everything above is free: both directions, rectangular equivalents, stock sizes, all four materials, the velocity guide, imperial and metric, and the last ten calculations.

DuctCalc Pro is one payment — no subscription — and adds unlimited history plus sending the day's duct schedule as a message.

No account. No sign-up. Works with no signal, which is the point of a tool for a mechanical room.

COMMON QUESTIONS Does it work in both directions? Yes. Enter CFM and friction rate to get the round diameter, or enter a duct you already have and read back the velocity and the friction rate it will actually run at.

Does it give me sizes the shop stocks? Yes. A calculator that answers 7.3 inches is one you have to correct on the ladder, so it snaps to the sizes a shop actually carries.

What about rectangular duct? Every round size comes with its rectangular equivalents, listed by aspect ratio, so you pick the one that fits the joist bay instead of doing the conversion in your head.

Does the material change the answer? It does, and the app does not pretend otherwise. Galvanised, flex, fibreglass duct board and spiral all have different friction rates, and each is a separate choice.

Will it warn me that a run will be noisy? Recommended velocity bands by application — main trunk, branch, return — sit next to the number, so something that is technically fine but noisy in a bedroom reads as noisy in a bedroom.

What is free and what does Pro add? Both directions, rectangular equivalents, stock sizes, all four materials, the velocity guide, imperial and metric and the last ten calculations are free. Pro is one payment, not a subscription, and adds unlimited history plus sending the day's duct schedule.

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