Utility · Mobile-first

HVAC duct sizing estimator

Enter CFM and a target face velocity — see a theoretical round duct ID and the next common inch size. Velocity-only sketch, not a full duct design.

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HVAC duct sizing estimator

Not Manual J. This is a round-duct velocity sizing sketch for conversation on site. Duct design needs friction loss, fittings, length, equipment external static, codes and a licensed designer. Never use this output as the only basis for installs.

Supply branches often land ~700–900 FPM in residential rules-of-thumb — your office standards win.

Theoretical round ID (exact math)15.64 in
Next common stock size (≥)16"

Key takeaways

  • Shows both exact math diameter and next common stock size.
  • Velocity presets for fast what-if on the hood of a truck.
  • Big red banner states non-replacement for Manual J.
  • No cloud upload of job details.
  • Pairs with your friction chart and fittings allowance.

Who built this

ContractorHQ is edited by Bernard Guido — 25+ years in trade operations and software. These utilities follow the same quality bar as our reviews: fast pages, honest disclosure, and tools that stay useful without selling your data.

Frequently asked questions

Why is the answer in inches ID?

U.S. sheet-metal ducts are sold by inside diameter for round stock in many catalogs. Always confirm with your supplier.

What about rectangular ducts?

Not modeled here. Round is the simplest teaching geometry; rectangular needs aspect ratio and friction tables.

Return air runs?

Often sized at lower velocities than supplies. Drop the FPM target in the field if that matches your office standard.

Can I screenshot this for permits?

No. Permits need professional calcs. This page is for quick truck conversations only.