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Calculation Machine

ACH to CFM Calculator

Fan size from the air changes you want.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Fan size to buy
343 CFM
Theoretical requirement
299 CFM
In m³/h
584
In L/s
162.1

Working

  1. 12,240 cu ft x 8 / 60 = 299 CFM, +15% = 343 CFM

Quick answer

Required CFM equals room volume times ACH divided by 60.

The formula

CFM = volume x ACH / 60

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Room volume2240 cu ft
  • Target air changes8 ACH
  • Duct loss allowance15 %

Result

  • Fan size to buy343 CFM
  • Theoretical requirement299 CFM
  • In m³/h584
  • In L/s162.1

How to use the ACH to CFM Calculator

  1. 1Work out the room volume in cubic feet.
  2. 2Choose the ACH target for that room type.
  3. 3Add a duct loss allowance — 10% for short runs, 25% with filters and long flex.

Frequently asked questions

Should I buy exactly the calculated CFM?

Buy the next size up and use a speed controller; a fan running slower is quieter than a small fan at full tilt.

Do carbon filters change it?

Yes — a carbon filter can cut delivered airflow by 20–25%, so include it in the loss allowance.

About this calculator