Ductspec: HVAC Duct Sizer

Duct calculator for HVAC installers, techs and sheet metal shops. 800 CFM at 0.08 friction rate: 14 in round at 750 fpm, or 16 × 10 in rectangular.

Pay once. No subscription, no account, no ads. Works offline. No data collected.

WHAT YOU CAN DO • Size a duct: enter the airflow and a friction rate or a velocity. You get the exact and the standard round diameter, the rectangular equivalent for the side you choose, and the velocity and friction at that size. • Convert round to rectangular and back, with equivalent sizes for several heights and an aspect-ratio check. • Check velocity and pressure drop for a trunk, branch or runout over a run length, against typical residential and commercial velocity guidelines. Straight duct only: fittings, coil and filter are not included. • Work out room CFM from air changes per hour, or the air changes a fan gives. • Apply the fan laws: new CFM, static pressure and power at a new speed, or the speed needed for a CFM. • Convert units: airflow, velocity, pressure, friction rate, length, area, temperature and air density. • Save results by job with a note, search them, reopen them in their tool and share them as text.

BUILT FOR THE JOB SITE • Imperial (CFM, inches) or metric (L/s or m³/h, Pa, mm), with EN 1506 round sizes in metric mode. Units follow your region; change them in Settings. • English and Spanish. • Duct material, site temperature and altitude, round sizes in whole inches or ½ in steps up to 15 in, rectangle step of 1 or 2 inches. • Dark Mode and larger text sizes. • For iPhone.

METHOD AND ACCURACY Ductspec uses the equal-friction method of ASHRAE Fundamentals: Darcy–Weisbach friction loss with the Altshul–Tsal friction factor, Huebscher's equivalent diameter for rectangular ducts, and ASHRAE roughness values for four duct materials. Sizes always round up: to whole inches or ½ in steps, or to EN 1506 sizes in metric. The formulas, values and sources are listed in the app, under Settings > How calculations work.

Ductspec sizes straight duct. Fitting losses, static regain and flat oval ducts are not calculated.

IMPORTANT Results are engineering estimates for planning. Check them against the project design and local codes, and for residential work against ACCA Manual D. Ductspec does not replace the calculation of a qualified engineer. Do not rely on Ductspec alone for life-safety systems such as smoke control or hazardous exhaust. Compressed or sagging flex duct loses much more pressure than shown.

ASHRAE and ACCA are named only to identify a method (ASHRAE Fundamentals) and a reference to check against (ACCA Manual D). Ductspec is an independent app and is not affiliated with or endorsed by them.

Found a number that looks wrong? Write to us from the Contact row in Settings.

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