Duct Calculator: HVAC Sizing

Size HVAC ductwork or check an existing duct, then save the calculation for the job.

Enter airflow and a design friction rate to get round and rectangular sizes that meet both the friction limit and the residential velocity guide. Save the inputs and results on your device. You can reopen them on site or share a text report.

Ductulator requires a subscription to use its tools. Eligible new subscribers can start with a free trial: 7 days on the yearly plan or 3 days on the monthly plan. The subscription renews automatically unless canceled in Apple Account settings.

Your subscription includes

Size a new duct
- Recommended round and rectangular sizes that meet your design friction rate and the velocity guide
- A separate label for the round diameter based on friction alone
- Set one rectangular side and calculate the other
- Air velocity and actual friction at the recommended sizes
- Supply and return settings, with a design friction rate you can set yourself

Fit available space
- Enter maximum inside height and width
- Compare rectangular options that meet your friction and velocity limits
- Search a 2-inch size grid from 4 to 60 inches; confirm fabrication availability
- Send an option to Check and save it with your job
- Allow for insulation and supports outside the entered dimensions

Check an existing duct
- Enter a round diameter or rectangular width and height
- Calculate velocity, friction rate and capacity at the entered conditions
- See whether the duct is adequate for the requested airflow
- Use an undersized duct's inputs to find a suitable size, or send a recommended round size to Check

Work out your friction rate
- Calculate friction rate from available static pressure and total effective length
- Apply the result directly to the calculator
- Keep your chosen design rate when switching between supply and return
- Help with what to enter for pressure and effective length

Save calculations by job
- Restore the last valid calculation when you reopen the app
- Browse the last 20 completed calculations
- Save named calculations with a project and notes
- Reopen the inputs while keeping the original saved report unchanged
- Share a saved report as text

Materials and calculation method
- Galvanized sheet metal, duct board and fully extended flexible duct
- Darcy-Weisbach with the Colebrook friction factor
- Huebscher equivalent round diameter for rectangular ducts
- Standard air at 70 degrees F and sea level
- Residential velocity guide: 900 fpm supply and 700 fpm return

Units are CFM, inches, feet, fpm and inches of water per 100 feet. Results describe the conditions you enter; the app does not measure airflow. You supply the pressure and fitting-length data for the friction worksheet. It does not produce a complete Manual D design or correct for flex compression or altitude.

Calculations and saved jobs stay on your device unless you choose to share a report. You do not need an account. Deleting the app removes its local data.

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