Duct Calc — HVAC Duct Sizer

Round, rect & flex duct sizing

Free · In‑App Purchases · Designed for iPad. Not verified for macOS.

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.

Ratings & Reviews

  • This app hasn’t received enough ratings or reviews to display an overview.

Stability and reliability improvements.

Version 1.0.1

The developer, Gun woo Kim, indicated that the app’s privacy practices may include handling of data as described below. For more information, see the developer’s privacy policy .

  • Data Not Linked to You

    The following data may be collected but it is not linked to your identity:

    • Usage Data
    • Diagnostics

Privacy practices may vary, for example, based on the features you use or your age. Learn More

Accessibility

The developer has not yet indicated which accessibility features this app supports. Learn More

Information

Seller
Gun woo Kim
Size
30.5 MB
Category
Productivity
Compatibility
Requires iOS 13.0 or later.
  • iPhone
    Requires iOS 13.0 or later.
  • iPad
    Requires iPadOS 13.0 or later.
  • iPod touch
    Requires iOS 13.0 or later.
  • Mac
    Requires macOS 11.0 or later and a Mac with Apple M1 chip or later.
  • Apple Vision
    Requires visionOS 1.0 or later.
Languages
English
Age Rating
4+
In-App Purchases
Yes
Copyright
© 2026 WittyApps
  • Privacy Policy

    • More by Gun woo Kim

    You Might Also Like