Duct Calc: HVAC Duct Sizer

Size ductwork in seconds — round, rectangular, and yes, flex.

Duct Calc is an offline HVAC duct calculator built for techs and designers who need answers on the job, not another subscription.

FREE, FULLY WORKING
• Duct sizing (equal friction, round) — enter CFM and friction rate (in. w.c./100 ft), get the exact diameter, the stock size to grab, and the velocity in FPM
• Unit converter — airflow, velocity, and duct area. CFM · L/s · m³/h at one tap

PRO (ONE-TIME PURCHASE)
• Flex duct sizing — friction based on published research data (fully stretched to 30% compressed). Compressed flex loses far more pressure than the charts admit; Duct Calc corrects for it
• Rectangular conversion — equivalent diameter to W×H (1-inch increments), with aspect-ratio warning
• Airflow back-calc — know the duct, get the CFM: from round diameter or rectangular W×H, by velocity or friction rate
• Register / diffuser selection — neck size, free area, and throw estimate from CFM and neck velocity
• Static pressure — straight-run loss plus fittings (elbows, tees, transitions), totaled in in. w.c.

BUILT FOR THE FIELD
• 100% offline — works in the attic, the crawlspace, the mechanical room
• Big touch targets, screen stays awake, no account, no sign-in
• Your inputs never leave the device

ONE-TIME PURCHASE
The free version is fully functional — round sizing and conversion are never crippled. One purchase unlocks everything else, forever.

No subscription. No ads. No features taken away later.

Calculations follow standard practice (Darcy–Weisbach / Colebrook friction, ASHRAE equivalent-diameter equation, published flex-duct compression data). Reference only — verify final designs against applicable codes and manufacturer data.