About Ductulator Pro

Ductulator Pro does one thing: duct. Not a general calculator site with a duct page on it — a site where every tool, every chart and every article is about sizing, converting, specifying or taking off ductwork. That focus is deliberate. Duct design has enough depth to fill a site on its own, and the questions engineers actually ask about it are rarely answered well by a calculator built to cover forty unrelated subjects.

How the calculators work

The friction calculations follow ASHRAE Handbook — Fundamentals, Chapter 21. That means Darcy-Weisbach for pressure loss with the Altshul-Tsal approximation for the friction factor, the Huebscher relation for circular equivalent diameter, air density computed from the standard atmosphere at your site elevation, and Sutherland's law for viscosity. Hazen-Williams is deliberately absent: it is an empirical water correlation with no density or viscosity term, and applying it to air produces answers that look confident and are wrong.

The physics lives in one shared library that every tool and every generated chart calls into. This is not an implementation detail — it is the reason the duct size chart on this site cannot disagree with the ductulator, and the reason the gauge chart cannot disagree with the weight calculator. Published tables that drift out of step with the tool beside them are a common failure on calculator sites, and generating the tables from the same code removes that possibility entirely.

Checked against worked examples

Every tool is verified against hand-calculated cases before it goes live, and those cases are printed on the pages themselves so anyone can repeat them. A 300 mm round duct carrying 0.5 m³/s of standard air gives 7.07 m/s, a Reynolds number of 140,932, a friction factor of 0.01840 and a friction rate of 1.848 Pa/m. A 400 × 200 mm duct has a circular equivalent diameter of 304.7 mm, and 200 × 400 gives exactly the same answer. If a tool here ever disagrees with those numbers, it is broken and worth reporting.

Where this site's output differs from a traditional cardboard ductulator wheel, the difference is stated rather than hidden. Chart-read values typically land within about eight per cent of the computed answer, because reading a logarithmic nomogram by eye carries that much error. Every calculator shows its intermediate values — Reynolds number, friction factor, the branch of the Altshul-Tsal equation taken, the air density used — so the result can be checked by hand rather than trusted on faith.

Who builds these tools

The site is developed and maintained by a practising MEP engineer working in HVAC and fire protection design, with day-to-day involvement in shop drawings, duct layouts and hydraulic calculations to ASHRAE, SMACNA and NFPA standards. The tools started as spreadsheets built for real projects, which is why they ask for things like site elevation and duct material rather than assuming standard air and smooth steel.

What these tools are not

They are design aids, not a sealed design. A duct size from this site is a correct answer to the question asked, but a duct system is more than a sequence of straight runs: fittings carry losses that often exceed the straight-duct friction, acoustic criteria can override any velocity band, fire and smoke requirements shape routing, and a competent engineer has to look at the whole system. The gauge tables in particular are indicative — real SMACNA construction pairs sheet thickness with reinforcement spacing and joint class, and only the standard itself settles that.

If a number here looks wrong, it may well be. The contact page goes to a person who will check it against the source, and corrections are made and dated rather than quietly edited.