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HVAC Duct CFM Calculator — Airflow from Duct Size

CFM = duct area (sq ft) × velocity (ft/min).

Compute CFM through round duct from diameter and velocity. Planning estimates only — verify measurements and local codes on site.

Inputs

424
4002000

Smart analysis

Recommendations

Tailored to your live inputs and result—guidance only, not professional advice.

  • Interpret

    Treat this as a planning estimate

    Primary result: Airflow CFM = 314. Re-run with optimistic and pessimistic inputs to understand the range—not just a single point.

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Plain-English guide

Terms & how it works

Confused by a field? Read the short definitions here while you use the tool.

  • Duct diameter (in)

    The “Duct diameter (in)” value used in this tool’s formula. Adjust it to see results update live.

  • Air velocity (FPM)

    The “Air velocity (FPM)” value used in this tool’s formula. Adjust it to see results update live.

How this calculator works

Compute CFM through round duct from diameter and velocity. Planning estimates only — verify measurements and local codes on site. CFM = duct area (sq ft) × velocity (ft/min). Example: 8 in round duct at 900 FPM → ~314 CFM.

CFM = duct area (sq ft) × velocity (ft/min).

Example: 8 in round duct at 900 FPM → ~314 CFM.

When to use this Ductwork CFM

Common use cases

  • Compute CFM through round duct from diameter and velocity.
  • Sanity-check with this case: 8 in round duct at 900 FPM → ~314 CFM.
  • The relationship is: CFM = duct area (sq ft) × velocity (ft/min).

Localized examples

  • HVAC & Climate Control: the example on this page uses the same formula as the widget.

What you get

  • Fields: Duct diameter (in), Air velocity (FPM)
  • CFM = duct area (sq ft) × velocity (ft/min).
  • Example on the page: 8 in round duct at 900 FPM → ~314 CFM.

How to Calculate Ductwork CFM Step-by-Step

  1. 1Enter Duct diameter (in), Air velocity (FPM). Use the values you have, not a catalog average, unless that is all you know.
  2. 2CFM = duct area (sq ft) × velocity (ft/min).
  3. 3Worked case: 8 in round duct at 900 FPM → ~314 CFM.
  4. 4Buy materials after you walk the job. This is a first-pass quantity, not a purchase order.

Governing formula

CFM = duct area (sq ft) × velocity (ft/min).

8 in round duct at 900 FPM → ~314 CFM.

Example: set Duct diameter (in) = 8, Air velocity (FPM) = 900, then read the result.

Variable definitions

  • ductDiameterIn

    Duct diameter (in)

    The “Duct diameter (in)” field. Default 8; typical range 4–24.

  • velocityFpm

    Air velocity (FPM)

    The “Air velocity (FPM)” field. Default 900; typical range 400–2000.

  • The takeoff (CFM = duct area (sq ft) × velocity (ft/min).) ignores waste, code, and what you find when you open the wall. Measure twice on site.
  • Nothing here is uploaded for the calculation itself.

Frequently Asked Questions

CFM = duct area (sq ft) × velocity (ft/min).

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Community

Reviews & Ratings

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4.8

out of 5.0

4 reviews

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Your rating

Aisha K.

Mar 12, 2026

Clean layout and the results update instantly. Exactly what I needed for a quick planning check.

Daniel R.

Feb 28, 2026

Very usable on mobile. Would love a save/export option later, but the math feels solid.

Priya S.

Feb 3, 2026

The FAQ section answered my questions before I even had to search. Smooth dark mode too.

Marcus L.

Jan 19, 2026

Simple inputs, clear outputs. This replaced three bookmarks I used to juggle.

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