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Free HVAC DUCT SIZE Calculator

HVAC Duct Size Calculator

A free HVAC duct size calculator using the velocity method. Enter the CFM the duct needs to carry and your target air velocity to get the recommended round duct diameter, actual velocity, friction rate, and a rectangular equivalent.

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Calculate Your HVAC Duct Size

Enter the airflow the duct needs to carry and your target velocity. The calculator updates the recommended round duct size, actual velocity, friction rate, and rectangular equivalent as you work.

Airflow & velocity

The CFM the duct must carry and your target air speed.

Airflow (CFM)

Target velocity (FPM)

Rectangular option

For a rectangular equivalent, set one side.

Duct height (in)

Round duct size

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Exact diameter

Cross-section

Actual velocity

Airflow

Target velocity

Friction / 100 ft

Rect equivalent

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Estimates use your inputs — adjust to match your real costs. Benchmarks are starting points, not advice.

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THE VELOCITY METHOD — SIZE IT QUIET, NOT JUST BIG ENOUGH

Duct size affects both airflow and noise. A duct that’s too small increases air velocity, static pressure, and noise. A duct that’s too large slows the air and can add unnecessary material and space requirements.

The velocity method starts with the airflow the duct needs to carry and the target air velocity:

Cross-sectional area = CFM ÷ velocity

The required area is then converted into a round duct diameter.

Typical residential targets are:

Supply trunk: 700–900 FPM
Supply branch: around 600–700 FPM
Return: 500–600 FPM

Velocities above roughly 900 FPM can increase noise risk in residential applications, although acceptable velocity depends on the duct location, material, fittings, equipment, and system design.

HOW TO USE THE HVAC DUCT SIZE CALCULATOR

Use the calculator to estimate a duct size from the airflow the run needs to carry and your target velocity. The result gives you a starting point for selecting a duct size and checking whether the resulting airflow speed is likely to be acceptable for a residential application.

Enter the CFM, choose a target velocity based on the duct’s role, and review the recommended round diameter, actual velocity, friction rate, and rectangular equivalent. Use the results as a sizing and sanity-check tool rather than as a substitute for a complete duct design.

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WHAT GOES IN

Use the calculator above, or run it yourself.

1. Enter the CFM. Enter the airflow the duct run needs to carry.
2. Set a target velocity. Use 700–900 FPM for supply trunks, 600–700 FPM for supply branches, and 500–600 FPM for return ducts as residential starting points.
3. Read the round size. The calculator gives the recommended diameter, actual velocity, and a quiet/acceptable/noisy indication.
4. Set the duct height. Enter one side of the rectangular duct to get a matching rectangular equivalent.
5. Check the friction rate. A friction rate of roughly 0.08–0.10 in. w.g. per 100 ft can serve as a preliminary reference, but the actual design friction rate should be calculated from available static pressure and total effective length.

Frequently Asked Questions

It depends on the target air velocity. At 700 FPM, rough examples are 100 CFM → 5–6 in., 400 CFM → 10 in., 800 CFM → 14 in., and 1,200 CFM → 17–18 in. round. The calculator performs the actual area and diameter calculation, rounds to a standard size, and shows the resulting velocity so you can evaluate whether sizing up or down makes sense.

For a preliminary residential estimate, common starting ranges are 700–900 FPM for supply trunks, around 600–700 FPM for supply branches, and 500–600 FPM for return ducts. Lower velocity generally means quieter airflow but requires a larger duct. The calculator flags velocities above roughly 900 FPM in the residential range as potentially noisy.

Round duct generally moves air efficiently for a given cross-sectional area and works well for trunks and longer runs. Rectangular duct can be useful where ceiling or wall space limits the available height. The calculator provides a rectangular equivalent so you can compare the two approaches for the same airflow.

Flex duct generally creates more resistance than smooth metal, particularly when it is compressed, kinked, or poorly supported. Keep flex runs short, pull the inner liner tight, and support the duct properly. Depending on the design, you may need to account for the additional resistance when selecting the duct size.

No. This calculator provides a velocity-based estimate for an individual duct run. A complete residential duct design requires additional information, including the heating and cooling loads, equipment performance, available static pressure, total effective length, fittings, duct material, and other design factors addressed by ACCA Manual D. Use this calculator for preliminary sizing and sanity-checking, not as a substitute for a complete duct design.

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