HVAC Duct Calculator
Size your ductwork the right way. Enter airflow (CFM) and friction rate to get the required round duct diameter, equivalent rectangular size, and air velocity using the proven equal-friction method. Free and instant, by Online Tools.
Recommended Duct Size
Round Diameter
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Equivalent Rectangular
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Air Velocity
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Duct Cross-Section Area
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Velocity Rating
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Friction Rate Used
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About This Tool
The HVAC Duct Calculator helps you size supply and return ductwork correctly using the industry-standard equal-friction method. Enter the required airflow in CFM (cubic feet per minute) and your target friction rate, and the tool returns the proper round duct diameter, an equivalent rectangular duct size, the resulting air velocity, and a velocity rating that tells you whether the run will be quiet and efficient. It is built for US HVAC contractors, installers, and serious DIY homeowners who need fast, accurate duct sizing without flipping through a ductulator wheel.
Undersized ducts cause high static pressure, noisy airflow, and overworked equipment, while oversized ducts waste material and money and can hurt air distribution. Getting duct size right is one of the most important steps in any heating and cooling system, and this calculator turns the standard ASHRAE and ACCA Manual D math into instant results.
How the HVAC Duct Calculator Works
The calculator uses the equal-friction method, the same approach professionals use for residential and light commercial duct design:
- Airflow (CFM): the volume of air the duct must carry, often based on room load or 400 CFM per ton of cooling
- Friction Rate: the pressure loss per 100 feet of duct, typically 0.08 to 0.10 in. w.g. for residential systems
- Duct Type: choose round, or rectangular if you have a height constraint and need to solve for width
From these inputs the tool solves for the round duct diameter that delivers the target friction rate, converts it to an equivalent rectangular size, and calculates the air velocity (CFM ÷ duct area). It then rates the velocity so you know if the run will be too noisy, just right, or too slow.
Recommended Air Velocity Ranges
Air velocity is a key check. Too fast means noise and high static pressure; too slow means poor throw and possible stratification. Here are typical US residential and commercial targets:
| Duct Application | Recommended Velocity (FPM) | Notes |
|---|---|---|
| Main supply trunk (residential) | 700 – 900 | Quiet, efficient airflow |
| Branch ducts (residential) | 600 – 700 | Low noise at registers |
| Return air ducts | 500 – 700 | Keep low to reduce return noise |
| Commercial main duct | 1,000 – 1,500 | Higher velocity acceptable |
| Over 1,500 FPM (residential) | Too high | Expect noise and pressure problems |
Common Duct Sizing Rules of Thumb
- 400 CFM per ton: most US residential systems move about 400 CFM of air per ton of cooling capacity
- 0.08 to 0.10 friction rate: the standard residential design friction rate per 100 feet
- Round is most efficient: round duct moves air with the least friction for a given area
- Keep velocity reasonable: aim for 700 to 900 FPM on trunks to stay quiet
Benefits of Using the HVAC Duct Calculator
- Size ducts correctly: get the right diameter the first time, no guessing
- Convert round to rectangular: instantly find the equivalent rectangular size for tight spaces
- Check air velocity: avoid noisy, high-pressure runs before you build
- Save material and money: stop oversizing and wasting sheet metal or flex
- Faster than a ductulator: get professional results in seconds on any device
- Free and private: runs in your browser, no signup, nothing stored