Ductline

Every number shows its working.

An HVAC duct calculator for iPhone and iPad where every result opens its equation, with your numbers inside.

Size a duct. Then check it.

Change the numbers, then press ƒ(x).

Imperial Metric (SI)

Airflow

cfm

Friction rate Velocity

Target friction rate

in. w.g./100 ft

Aim 700–900 fpm, max 1000 fpm

Air: 70 °F at sea level.

14.3in

Press ƒ(x) for the working.

EquationSolve Δp₁₀₀(D) = target for D

With your numbersΔp₁₀₀ = 0.080 in. w.g./100 ft at Q = 1000 cfm → D = 14.28 in

SourceFriction equation inverted numerically (bisection, tolerance 1e-7)

15.0 in ø

Velocity815fpm

Friction0.063in. w.g./100 ft

EquationV = Q / A

With your numbersV = 1000 cfm / 1.2272 ft² = 815 fpm

SourceContinuity. A is the actual cross-sectional area, not the equivalent round area.

EquationΔp₁₀₀ = f · (100 / D) · Pv, 1/√f = −2·log₁₀( ε/(3.7D) + 2.51/(Re√f) )

With your numbers

Q = 1000 cfm, Dₑ = 15.00 in → V = 815 fpm, Re = 104874, f = 0.01905, Pv = 0.0413 → Δp₁₀₀ = 0.06296 = 0.0630 in. w.g./100 ft

SourceDarcy–Weisbach with the Colebrook–White friction factor. Roughness from the ASHRAE Handbook — Fundamentals duct roughness categories.

Darcy friction factor f = 0.01905 at Re = 104874, relative roughness 2.40e-04 (galvanised steel, ε = 0.00030 ft).

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1000 cfm · 0.080 in. w.g./100 ft

Ductline snaps to real fabrication sizes, squarest first, each with its velocity and aspect ratio.

Build each path segment by segment, and Ductline grades the system against the static pressure you have.

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