FreeHVACCalculators

Air Changes per Hour Calculator

Convert between airflow and air changes per hour for a room or building.

Method reviewed October 22, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Result

Air changes per hour
5

200 CFM changes the air 5 times per hour.

Minutes per air change
12min
Volume
2,400ft³
Show the working
  1. Volume
    20 × 15 × 8 = 2,400 ft³
  2. ACH = CFM × 60 ÷ volume
    200 × 60 / 2,400 = 5 ACH

The relationship

ACH = CFM × 60 ÷ volume, and CFM = ACH × volume ÷ 60. One air change means a volume of air equal to the room has passed through it. With perfectly mixed air, one air change removes about 63 % of an airborne contaminant, not all of it — so several changes are needed to clear a room.

ACH is not ACH50

A blower-door test reports ACH50: leakage with the house depressurised to 50 pascals. It is a measure of airtightness, not of ventilation, and natural air changes in normal weather are a small fraction of it.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Calculate
ACH from airflow
Length
20 ft
Width
15 ft
Height
8 ft
Airflow
200 CFM

Result

Air changes per hour
5
Minutes per air change
12 min
Volume
2,400 ft³

200 CFM changes the air 5 times per hour.

Assumptions

  • Assumes perfectly mixed air; real rooms with poor mixing have zones that are ventilated less than the average.

Limitations

  • ACH from a blower-door test (ACH50) measures leakage at 50 Pa and is not the same as natural or mechanical air changes.

Frequently asked questions

How many air changes per hour do I need?

It depends on the space and the applicable code or guideline. Residential whole-house ventilation is usually specified in CFM (for example by ASHRAE 62.2) rather than ACH; bathrooms, kitchens, workshops and healthcare spaces each have their own requirements.

Does an air purifier count toward ACH?

For particle removal, people often quote “equivalent air changes” using the purifier’s clean air delivery rate (CADR) in place of CFM. It is not outdoor-air ventilation and does not dilute gases such as CO₂.

Sources

  1. Air Change Rates in Rooms — The Engineering ToolBox. ACH = CFM × 60 / room volume.

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