Disclaimer

Last updated: 24 July 2026

Every result on Ductulator Pro is a design aid produced by a published equation and the assumptions printed on the tool's own page. A duct size is the start of a design decision, not the decision itself. This page sets out exactly where the outputs stop.

Duct sizing results

The calculators solve friction and velocity for straight duct using the methods in ASHRAE Handbook — Fundamentals, Chapter 21. They do not include losses at elbows, tees, transitions, take-offs, dampers, coils, filters or terminal units, and on a congested run those fitting losses routinely exceed the straight-duct friction. A duct schedule produced from these tools is not a system pressure calculation and must not be used on its own to select a fan.

Air properties and site conditions

Density is computed from the standard atmosphere at the temperature and elevation you enter. Real site conditions vary with weather, humidity and system operating state, and the standard atmosphere is a model rather than a measurement. Where a result is sensitive to density — high altitude, hot extract, cold outdoor intake — treat it as indicative and confirm against project design conditions.

Gauge, weight and construction output

Sheet metal gauge selection shown on this site is indicative. Compliant duct construction under the SMACNA HVAC Duct Construction Standards is a combined decision about sheet thickness, reinforcement type and spacing, and transverse joint class, and several different combinations satisfy the same duty. Weight and surface area figures are estimating quantities for straight duct with a seam allowance; they exclude fittings, supports, access doors and site waste. Do not fabricate, procure or price contractually from these numbers without checking them against the construction standard and the project specification.

Velocity bands and acoustics

The recommended velocity bands are conventional design guidance, not an acoustic calculation. Actual sound levels in a space depend on fitting geometry, damper position, duct lining, breakout through duct walls, terminal selection and the room itself. Where a project specifies a noise criterion, that criterion governs, and it needs a proper acoustic analysis rather than a velocity check.

Codes, standards and professional responsibility

Nothing on this site is a substitute for a design produced, checked and signed by a qualified engineer working from the actual system layout and the codes applicable in the project's jurisdiction. Fire and smoke duct requirements, ventilation rates, energy code limits on fan power, and local authority requirements are outside the scope of these tools entirely. Reference to a standard on this site is a statement of the method used, not a claim of compliance or certification.

Errors

Despite verification against worked examples, mistakes are possible. If a result looks wrong, please report it through the contact page — reports of calculation errors are corrected as the site's highest priority.