Foster Tools
HVAC Ductulator app icon
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Duct size calculator

Enter the airflow and get the round or rectangular duct size, the standard size to buy, and the friction rate. Free, right here, no signup. Take it to the job site with the iPhone app.

Download on theApp Store

Size a duct from CFM

Part of Foster Tools: 7 pro calculators in one appOpen in app →
Calculator
Air velocity
Duct shape
Duct material

Round duct diameter

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Enter an airflow to size the duct

Friction (in.w.c./100 ft)
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Area (sq ft)
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Velocity (FPM)
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Other sizes that carry the same airflow
OptionSizeFriction

Uses the same equal-friction math as the HVAC Ductulator app (standard air at 70°F). Pro modes are part of Foster Tools Pro, or get them in the iPhone app.

Estimate from standard equal-friction sizing for galvanized duct and standard air. It isn't a Manual J or Manual D design. Check code-required installs against your local mechanical code and a qualified HVAC professional. Terms

HVAC Ductulator round duct sizing with CFM, velocity, and friction rate
HVAC Ductulator reverse mode showing airflow capacity for a known duct size
HVAC Ductulator Saved tab with sizings grouped by job
HVAC Ductulator rectangular duct sizing and equivalent diameter

Built for the job site.

Every feature is designed for HVAC contractors and DIY installs who need a real number before they cut metal.

Round & Rectangular

Size from CFM and target velocity, both shapes, fully free. Friction rate included with every result.

Reverse Mode

Already have a duct size on hand? See exactly what airflow it can carry — built for service calls and retrofits.

Branch Calculator

Size a trunk and every branch run at once, with a warning if the numbers don't add up.

Every Duct Material

Galvanized, flex duct, PVC, and fiberboard — each with its own accurate friction rate.

Save by Job

Every sizing saved and grouped by job, ready to reopen or hand off to the next visit.

Fully Private

No account. No login. Your sizings and job data stay on your device — not on a server.

PRO

HVAC Ductulator Pro

Unlock reverse mode, the branch calculator, every duct material, and unlimited saved sizings. Start with a 3-day free trial on the weekly plan — no charge until the trial ends.

  • Reverse mode — know what an existing duct can move
  • Branch calculator for full trunk-and-branch runs
  • Flex duct, PVC, and fiberboard, in addition to galvanized
  • Unlimited saved sizings and PDF export, organized by job

How it’s calculated

Duct area = CFM ÷ velocity (FPM); round diameter = √(4 × area ÷ π). Rectangular sizes use the equivalent-diameter formula 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. Friction rate uses the Darcy-Weisbach equation with the Altshul-Tsal friction factor for standard air at 70°F.

Assumptions

  • Velocity targets of 500, 700 and 900 FPM for quiet, standard and high-capacity runs are common residential rules of thumb, not a Manual D design.
  • Duct roughness: 0.0003 ft for galvanized steel; flex, PVC and fiberboard use published approximations. Real flex duct loses more when it's compressed or sagging.
  • Standard round sizes are the common off-the-shelf diameters from 3" to 48".

Sources

Same code as our iPhone apps, tested against worked examples. Spot a mistake? Tell us and we’ll fix it. How we calculate

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Duct sizing questions

How do you calculate duct size from CFM?

Divide the airflow by the target air velocity to get the duct's cross-sectional area: area (sq ft) = CFM ÷ FPM. For round duct, the diameter in inches is √(4 × area in sq in ÷ π). For example, 400 CFM at 700 FPM needs 0.57 sq ft, which is a 10.2" round duct, so you'd use 12" round.

What air velocity should I size ducts for?

Lower velocity is quieter but needs bigger duct. Around 500 FPM suits returns and rooms where noise matters, around 700 FPM is a common residential supply target, and around 900 FPM is a practical upper limit for trunks before noise becomes a problem.

How do I convert a round duct size to rectangular?

Pick one side of the rectangular duct, then find the other side that gives the same equivalent diameter as the round size, using the Huebscher equation: De = 1.30 × (a × b)0.625 ÷ (a + b)0.25. Choose Rectangular in the calculator, enter the side you're limited to, and it solves the other side for you.

What is a good friction rate for duct design?

Residential systems are commonly designed around 0.08 to 0.10 inches of water column per 100 feet of duct. The calculator shows the friction rate of the size it gives you so you can compare it with the rate your system design calls for.

Does the calculator work for flex duct?

The free web calculator uses galvanized steel. Flex duct is much rougher inside, so it has a higher friction rate at the same size. The HVAC Ductulator app's Pro mode includes flex duct, PVC and fiberboard.

Guides

Free to download.

Get the numbers you need before you cut metal.

Download on theApp Store

Support

Email Support

Email support@fosterapps.com and include:

  • A short description of what you were doing.
  • Steps to reproduce the issue, if you know them.
  • What you expected to happen vs what actually happened.
  • Your device model and iOS version.
  • Any screenshots that might help.

Privacy policy

Read the full policy at the HVAC Ductulator privacy page.

Terms of use

Read the full terms at the HVAC Ductulator terms page.