# Duct Calc — HVAC Duct Sizer — Round, rect & flex duct sizing

> Round, rect & flex duct sizing (iPhone/iPad app by Gun woo Kim.)

- Source: https://appshunter.io/ios/app/duct-calc-hvac-duct-sizer/id6792500757 (this page in markdown: same URL + `.md`)
- Developer: [Gun woo Kim](https://appshunter.io/developer/1700263119)
- Category: Productivity, Utilities
- Price: Free with in-app purchases
- Age rating: 4+
- Requires: iOS 13.0 · 29 MB
- Languages: American English
- Released: 2026-07-29
- Data updated: 2026-08-03
- User reviews in markdown: https://appshunter.io/ios/app/duct-calc-hvac-duct-sizer/id6792500757/reviews.md

## What is Duct Calc?

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.

## Pricing and in-app purchases

Base price: Free.

**One-time purchases**

| Purchase | Price | Type |
| --- | --- | --- |
| Duct Calc Pro | $9.99 | one-time |



## Version history (last 2 releases)

### 1.0.1 — 2026-08-03

Stability and reliability improvements.

### 1.0 — 2026-07-29

No release notes.

## More apps by Gun woo Kim

- [picswap - picture swap](https://appshunter.io/ios/app/picswap-picture-swap/id6456039579)
- [mubo - dance formation editor](https://appshunter.io/ios/app/mubo-dance-formation-editor/id6753949227)
- [LinePrompter](https://appshunter.io/ios/app/lineprompter/id6784582453)
- [Conduit Bender: Bend Calc](https://appshunter.io/ios/app/conduit-bender-bend-calc/id6786548483)
- [Tape Calc: Feet Inch Fraction](https://appshunter.io/ios/app/tape-calc-feet-inch-fraction/id6786612707)
- [Pipe Calc: Pipefitter Offsets](https://appshunter.io/ios/app/pipe-calc-pipefitter-offsets/id6786981763)

All apps by Gun woo Kim: https://appshunter.io/developer/1700263119

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*Data collected daily from the US App Store and indexed by [AppsHunter](https://appshunter.io/). User reviews are verbatim App Store reviews. Ratings, prices and chart positions refresh continuously; this snapshot is from 2026-08-03.*