Size ducts
right
the first time.
Instant round and rectangular duct sizing from ASHRAE friction-rate charts — free, offline, no account. Pro unlocks flex & flat oval, reverse mode, fittings and pressure loss.
Why DuctSizer
Built for the field, not the office
Instant duct sizing
Round and rectangular sizing from airflow and friction rate or velocity — free, offline, in seconds.
Reverse mode
Enter a duct size and get its max CFM at your target friction rate or velocity — for checking existing ductwork.
Flat oval & flex
Pro adds flat oval and flexible duct sizing alongside round and rectangular.
Fittings & friction rate
Manual D friction rate (FR = ASP×100/TEL) and fitting losses, so total pressure drop is accounted for — not just straight runs.
Saved jobs & export
Keep a history of sizing jobs per project, and export them to CSV or PDF.
100% offline & private
No account, no analytics, no internet connection required. Everything runs and stays on your device.
The math
ASHRAE-based, not a guess
Sizing runs on published ASHRAE friction-rate chart fits and standard duct fluid-flow equations — the same physics behind the printed charts, computed to more decimal places.
ASHRAE duct-friction-chart data, curve-fit for round duct across the working range.
Darcy–Weisbach with the Haaland approximation, solved for turbulent flow in duct.
Round-to-rectangular equivalent diameter for equal friction loss — the rectangular and flat-oval math.
On device
The real app, not a mockup
Sizing
Size detail
Reverse mode
Saved jobs
Questions, answered
Is DuctSizer free to use?+
Yes. Forward sizing for round and rectangular ducts — from airflow and friction rate or velocity — is free, with no account and no time limit.
What does Pro unlock?+
A one-time purchase unlocks flex duct and flat oval sizing, reverse mode (enter a duct size to get its max CFM), fittings and pressure loss (Manual D friction rate = ASP×100/TEL), altitude and temperature density correction, and saved jobs with CSV/PDF export.
Is Pro a subscription?+
No. Pro is a one-time purchase, no subscription.
Does DuctSizer work offline?+
Yes. Everything runs on your device — no account, no internet connection needed, nothing leaves your device.
What units does it support?+
Both IP + SI units, switchable anytime.
How accurate is the sizing math?+
Sizing runs on ASHRAE friction-rate chart fits and the Darcy–Weisbach/Haaland equations, with Huebscher equivalents for round-to-rectangular conversion — the same math behind the classic cardboard duct wheel, just faster and more precise.
Does it work on iPad?+
Yes — DuctSizer runs on iPhone & iPad, iOS 17+.
What happens to my data?+
Nothing leaves your device. DuctSizer has no account system and no analytics — your jobs and settings stay local, 100% offline · no account · no analytics.
Can I save and export my sizing jobs?+
Yes, with Pro — save jobs and export them as CSV or PDF for your records or to share.
Is DuctSizer affiliated with ASHRAE or ACCA?+
No. Independent HVAC duct sizing tool. Not affiliated with or endorsed by ASHRAE, ACCA, or Trane.