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Duct Airflow (CFM) Calculator

Work out duct airflow (cfm) instantly with clear inputs, formula shown and shareable results.

About Duct Airflow (CFM) Calculator

Airflow follows directly from the sensible heat a duct must carry: CFM = BTU/h / (1.08 x ΔT). Once airflow is known, dividing by the design velocity gives the cross-sectional area, and the equivalent round diameter follows from the area of a circle.

Formulas

Airflow

CFM = sensible BTU/h / (1.08 x ΔT)

1.08
Air density x specific heat x 60 min/h at standard conditions

Duct size

Area ft² = CFM / velocity fpm; diameter in = sqrt(4 x area / pi) x 12

Worked example

With the default values below, Duct Airflow (CFM) Calculator returns Required airflow of 1,111. Change any input above and every figure here is recalculated in your browser.

Inputs used

Sensible heat loadminimum 1 BTU/h
24,000 BTU/h
Supply-to-room temperature differenceminimum 1 °F
20 °F
Design duct velocityminimum 100 fpm
900 fpm

Results

Required airflow
1,111 CFM
Equivalent capacity
2.00 tons
Duct cross-section needed
1.235 ft²
Equivalent round diameter
15.0 in

Step by step

  1. 1Values used
  2. 2Airflow
  3. 3Duct size
  4. 4Required airflow= 1,111 CFM
  5. 5Equivalent capacity= 2.00 tons
  6. 6Duct cross-section needed= 1.235 ft²
  7. 7Equivalent round diameter= 15.0 in

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Duct Airflow (CFM) figure worked out?

CFM = sensible BTU/h / (1.08 x ΔT). Airflow follows directly from the sensible heat a duct must carry: CFM = BTU/h / (1.08 x ΔT).

What velocity should I design to?

Residential trunks are typically 700-900 fpm and branches 600 fpm to keep noise down. Commercial mains run 1200-2000 fpm where acoustic treatment allows.

What values do I need to enter?

This calculator takes 3 inputs: Sensible heat load, Supply-to-room temperature difference, Design duct velocity. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

Related Calculators

Duct Airflow (CFM) Calculator – Calculate Required airflow