Fan CFM – Airflow Requirement

Size required airflow using Ventilation (ACH) or Heat Removal (BTU/hr). Live results, unit support, and Show Work.

How to Use

  1. Pick a mode: Ventilation (ACH) or Heat Removal.
  2. Enter the required inputs (dimensions + ACH, or BTU/hr + ΔT).
  3. Read the required airflow in CFM (and optional metric conversions).
  4. Open Show Work to see the formulas and steps.
Airflow Sizing
Use ACH (ventilation) or heat removal (sensible load).
Required
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Metric
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Inputs OK
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Mode
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Guidance: —
Airflow Bar
Scales with required CFM (visual only).
Tip: Real fan performance depends on static pressure, restrictions, filters, ducting, and noise limits.
Inputs & Settings
Choose a mode above, then fill in the matching inputs.
Room length.
Room width.
Ceiling height.
Ventilation sizing uses: CFM = (Volume × ACH) ÷ 60.
Show Work (step-by-step)
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Work shows the exact formula path used (ACH or heat removal). Conversions are shown where helpful.

Formulas

  • Room volume (ft³): Volume = L × W × H
  • Ventilation airflow (CFM): CFM = (Volume × ACH) ÷ 60
  • Heat removal (sensible) airflow (CFM): CFM = BTU/hr ÷ (1.08 × ΔT°F)
Notes: The 1.08 factor is a common HVAC rule-of-thumb near sea level. Real systems vary with altitude and humidity.

FAQ

What is ACH?

ACH means Air Changes per Hour: how many times the room’s air volume is replaced per hour. Higher ACH means more airflow.

Why does my fan’s rated CFM not match real life?

Fan ratings are usually at free air (near zero static pressure). Filters, grilles, ducts, bends, and restrictions reduce delivered airflow.

Can I use this for PC cases or electronics enclosures?

Yes—ACH mode is a quick ventilation estimate. Heat mode is better if you can estimate heat load and allowed temperature rise. For tight enclosures, static pressure matters a lot.

Tool Info

Last updated:

Updates may include unit support, calculation edge-case handling, and UI improvements.