About this calculator

Duct Size Calculator is a free tool from Worklifted Technologies for HVAC technicians, installers, and serious DIYers who need a fast, honest answer to a deceptively fiddly question: how big does this duct have to be?

Most duct calculators answer only part of it — they give you a round size and stop, or they make you re-run the math for every shape and never tell you whether the result is actually safe. We built this one to return every answer at once: from a single airflow you get the round diameter, the rectangular and flat-oval equivalents, the resulting air velocity and friction rate, and a clear verdict on whether the size stays under your velocity cap. Flat-oval is treated as a first-class shape, not an afterthought, and you can size by friction rate or by target velocity.

What it's built on

The math follows established, published methods rather than rules of thumb. Round sizing uses the Darcy-Weisbach friction relationship on the ASHRAE friction-chart basis; shape conversions use the ASHRAE circular-equivalent (Huebscher) relationship for round and rectangular duct and the Heyt & Diaz correlation for flat-oval; and the velocity and friction-rate figures come straight from the airflow and the cross-section. Every formula and constant lives in one typed module with locked test cases, so the calculator, the charts, and the converter pages can never disagree with one another.

Sources

• ASHRAE Handbook of Fundamentals — Duct Design

  The primary basis: friction-chart method, circular-equivalent (Huebscher) relationship, velocity-pressure constants.

• ACCA Manual D — Residential Duct Systems

  Velocity guidance and the Available-Static-Pressure ÷ Total-Effective-Length friction-rate method.

• AMCA / United McGill duct design guides

  Equivalent-diameter and flat-oval correlations (Heyt & Diaz) with worked sample problems.

Honest about the limits

This is an estimating and teaching tool, not a stamped design. It sizes straight, clean galvanized round duct on the standard-air basis and does not model fittings, the coil and filter, or the blower curve — that is the work of a full Manual D design. Read the engineering disclaimer for exactly what the numbers assume, and the sizing guide for the full method and worked examples.

Contact

Found an error or have a suggestion? We'd genuinely like to hear it — accuracy is the whole point of this tool. Email Worklifted Technologies at support@ductsizecalculator.com, and we'll review any correction against the source standards.