Ductly: Duct Size Calculator

Ductly is an HVAC duct calculator for the people who cut the metal: airflow in, duct size out, live as you type. When the answer would fall outside what the maths covers, it says so instead of quietly rounding it down.

It does what the cardboard sizing wheel in your toolbox does, in both directions, in the units you actually work in.

What it works out

- Round duct size from airflow and friction rate, with Darcy-Weisbach and Colebrook-White friction.
- Round duct size from airflow and a target velocity.
- The other way round: you already have the duct, and you want the airflow it carries at that friction rate.
- The rectangular equivalent, by the Huebscher method. Fix the side that has to fit the ceiling space, and Ductly gives you the other one.
- Galvanized, spiral, duct board and flex, each with its own roughness. The material stays on screen next to the result, so you always know what the number assumed.
- Velocity, friction rate and the cross-section area of the round duct, beside every result.

CFM, litres per second (L/s) or cubic metres per hour (m3/h)

Pick the flow unit you work in, and imperial or metric for everything else. You set it once on the first screen and it sticks. You can change it any time, on the calculator, without losing what you typed.

It tells you when it cannot answer

Ask for an airflow this tool does not cover and it says so, and gives you the range. A duct size is a cut in sheet metal. A capped answer that looks right costs more than no answer.

You can see what it is based on

One screen lists the published reference cases the engine is checked against, with the source of each, recalculated on the spot every time you open it. Nothing there is a stored table, so if a number ever drifted you would see it there first.

Starting points, not answers

Residential and commercial starting points for the friction rate, and typical branch and trunk velocities, are one tap away, with a note that the real friction rate comes from the static pressure your equipment has available. Above roughly 900 fpm a run starts to be heard in the room. Ductly tells you that instead of refusing to give you the size.

Keep the ones that matter

Save a calculation to the history and it stays on the phone. Export the history as a CSV file, or a single result as a PDF sheet, whenever you want.

No ads, no account, no connection

There is no advertising in Ductly, so it never asks you to agree to anything before it shows you a result. There is nothing to sign up for and no server behind it: it works in a crawl space with no signal, and what you type stays on your phone.

What it costs

7 days of full access to start, with no card. After that it is 19.99 USD, once. There is no subscription. Your local price is shown before you buy anything.

If you decide not to buy, nothing is deleted. Your history stays readable and the export stays free. You just cannot add new calculations.

What it does not do

Ductly does not do fitting losses, load calculations or equipment selection, and it has no catalogue of parts. It does the sizing part of ductwork design, both ways round, in the units you work in, and it does it properly.

Made by Arkadia Labs.