What this calculator finds

From the width and height of a rectangular duct it first computes the equivalent diameter (De) — the round-duct diameter that gives the same friction — and then uses the ASHRAE empirical formula to find the friction loss per 100 ft (in.w.g.). For a round duct, its diameter is used directly as De.

Why it matters

• Sizing ducts and estimating the fan external static pressure
• Balancing airflow distribution using the equal-friction design method
• Checking that velocities stay within ranges acceptable for noise and energy

Air Duct Friction Loss

The rectangular section is first converted to an equivalent round section, then friction loss is found from the velocity and equivalent diameter.

How the formula works

• Friction loss rises with roughly the 1.9 power of velocity V, so even a modest speed increase sharply raises the loss.
• A larger equivalent diameter De (a bigger duct) reduces the loss (inversely with De^1.22).
• For the same area, a near-square duct has a larger De and less friction, while a flat duct has more loss.

Worked example

For a duct with W = 12 in and H = 12 in, De = 1.3 × (144)^0.625 / (24)^0.25 ≈ 1.3 × 21.8 / 2.21 ≈ 12.8 in. At V = 1500 ft/min, ΔP = 0.109136 × (1.5)^1.9 / 12.8^1.22 ≈ 0.109136 × 2.14 / 21.9 ≈ 0.011 in.w.g. per 100 ft.

Practical tips

• The equal-friction method typically targets 0.08–0.1 in.w.g./100 ft to size ducts.
• Keep main-duct velocities low to limit noise (e.g. 1000–1500 ft/min) and allow slightly higher speeds in branches.
• Add local losses from elbows, tees, and dampers separately (equivalent length or loss coefficients) to get total static pressure.

Limitations & cautions

• The empirical formula assumes standard air (~20°C, atmospheric) and smooth galvanized-steel duct.
• Flexible or rough-lined ducts have much higher roughness and loss — apply a correction factor.
• If air density differs from standard (hot air, high altitude), a density correction is required.