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Air Changes Per Hour Calculator

Work out air changes per hour instantly with clear inputs, formula shown and shareable results.

About Air Changes Per Hour Calculator

Air changes per hour compares the volume of air delivered in an hour with the volume of the room itself: ACH = CFM x 60 / room volume. It is the standard yardstick for ventilation effectiveness because it is independent of room size, so a small bathroom and a large workshop can be judged on the same scale.

Formulas

Air changes

ACH = CFM x 60 / volume ft³

Turnover time

Minutes per change = 60 / ACH

Worked example

With the default values below, Air Changes Per Hour Calculator returns Air changes per hour of 10.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Supply airflowminimum 1 CFM
450 CFM
Room lengthminimum 1 ft
20 ft
Room widthminimum 1 ft
15 ft
Ceiling heightminimum 1 ft
9 ft

Results

Air changes per hour
10.00 ACH
Room volume
2,700 ft³
Minutes per air change
6.0 min
Airflow intensity
1.500 CFM/ft²

Step by step

  1. 1Values used
  2. 2Air changes
  3. 3Turnover time
  4. 4Air changes per hour= 10.00 ACH
  5. 5Room volume= 2,700 ft³
  6. 6Minutes per air change= 6.0 min
  7. 7Airflow intensity= 1.500 CFM/ft²

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges 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 Air Changes Per Hour figure worked out?

ACH = CFM x 60 / volume ft³. Air changes per hour compares the volume of air delivered in an hour with the volume of the room itself: ACH = CFM x 60 / room volume.

What ACH should a space have?

Typical guidance is 6-8 ACH for kitchens, 8-10 for bathrooms, 4-6 for offices and 10 or more for workshops handling fumes. Local codes take precedence.

What values do I need to enter?

This calculator takes 4 inputs: Supply airflow, Room length, Room width, Ceiling height. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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