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Engineering disclaimer
Every result this site produces is an engineering estimate — verify it before fabrication. Read this page so you know exactly what the numbers assume and where their limits are.
This calculator is an educational aid for sizing clean galvanized steel round duct carrying standard air (about 0.075 lb/ft³, 70 °F, at sea level). It does not replace a licensed mechanical engineer, a certified ACCA Manual D contractor, the current ASHRAE Handbook of Fundamentals, or your local mechanical code.
What the results assume
Sizes are computed on the ASHRAE friction-chart basis for fully turbulent flow in smooth galvanized steel. The tool sizes straight duct only — it does not add the resistance of elbows, takeoffs, transitions, balancing dampers, registers, the coil, or the filter, all of which a complete design must include.
Material and air-condition limits
Flexible duct, fiberglass ductboard, lined, spiral, and non-galvanized duct have a higher effective roughness and lose more pressure — flexible duct can carry two to three times the friction of rigid metal, and far more when it is compressed or sagging — so it must be sized larger with the correct roughness applied. High altitude or hot and cold air change the air density and have to be recomputed; the standard-air basis here is the default, not a universal one.
The friction rate is not a constant
The friction rate is not a fixed number such as 0.10. A proper ACCA Manual D design derives it from the blower's Available Static Pressure divided by the Total Effective Length of the run — which includes the equivalent length of every fitting, elbow, register, coil, and filter. Fittings often dominate that length, and because this tool sizes straight duct only, the friction rate you enter must already account for them.
Shape conversions match friction, not velocity
The round-to-rectangular and flat-oval conversions use circular-equivalent relationships that give equal friction loss and equal air-carrying capacity — not equal velocity. A rectangular duct of the same equivalent diameter runs faster than the round one it replaces. Keep rectangular aspect ratios at or below about 4 to 1.
Why correct sizing matters
Undersized duct drives up static pressure and noise and can overheat a heat exchanger, freeze a coil, or shorten blower life; oversized duct wastes money and can invite condensation. Treat this tool as a way to estimate and sanity-check a design, and confirm every figure against the current ASHRAE Handbook of Fundamentals (Duct Design chapter) and your local mechanical code (IMC) — with a qualified professional — before anything is built.
No warranty
The calculator carries no warranty of accuracy, fitness, or merchantability of any kind. You are responsible for verifying results and for how they are used. As far as the law allows, Worklifted Technologies accepts no liability for loss or damage arising from use of this site. See the terms of use.