DuctSizer: Duct Calculator

Airflow in, duct sizes out — live. Round, rectangular, flex and flat oval, with real ASHRAE math. One-time Pro unlock. No subscription.

Size ducts in seconds — right from the ladder.

DuctSizer turns airflow into duct sizes instantly. Enter CFM (or L/s) and a friction rate or velocity target, and read every valid size at a glance: round diameter plus rectangular options for the joist depth you actually have.

SIZING, FREE • Round and rectangular results, live as you type • Friction and velocity for every option, with clear limit warnings • Imperial and SI units (CFM, L/s, m³/h · in.wg/100 ft, Pa/m) • Material roughness shown for galvanized, spiral, duct board, and flex

DUCTSIZER PRO — ONE-TIME PURCHASE • Reverse mode: pick a size, get its max airflow — including flex with honest compression derating, and flat-oval ducts • Fittings & pressure loss: total effective length and Manual D friction rate (FR = ASP × 100 / TEL) • Altitude and temperature density correction, always visible • Saved jobs: group runs by project and export CSV or PDF

THE MATH, DONE RIGHT Calculations follow the ASHRAE friction chart with Darcy–Weisbach and Haaland for material roughness, and Huebscher equivalents for rectangular and flat-oval ducts. Results match the cardboard wheel you trained on — with the steps the wheel can't do.

BUILT FOR THE FIELD • Works 100% offline — no account, no sign-in • No analytics or tracking; your data stays on your device • Dark, glare-friendly interface with big touch targets • iPhone and iPad, iOS 17 or later

Pro is a single one-time purchase. No subscription, ever.

DuctSizer is an independent app and is not affiliated with or endorsed by ASHRAE, ACCA, or Trane.

Privacy Policy: https://ductsizer.app/privacy Terms of Use: https://ductsizer.app/terms