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ENGINEERING TOOL

Air Changes per Hour Calculator

Calculate air changes per hour from airflow in CFM and room dimensions in feet.

Air Changes per Hour Calculator

FREE ONLINE TOOL

Result (ACH)

5

Formula and assumptions

ACH = CFM × 60 / (length × width × height)

Assumes a rectangular room and constant flow. Supply airflow alone does not establish outdoor-air ventilation.

STEP BY STEP

How to use air changes per hour calculator

  1. Read the result and units.
  2. Check the formula and assumptions.

IN PRACTICE

Example

200 CFM × 60 / (20 × 15 × 8) = 5 ACH.

GOOD TO KNOW

What assumptions does this calculation use?

Assumes a rectangular room and constant flow. Supply airflow alone does not establish outdoor-air ventilation.

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What an ACH result means

ACH compares hourly airflow with room volume. A result of 5 ACH means a volume equal to five room volumes passes through the system each hour. It does not mean every air molecule is replaced five times: mixing, obstructions and short-circuiting affect actual air distribution.

Choose the right airflow measurement

Use airflow measured for the room, not a fan’s free-air rating. Duct resistance and filters can reduce installed flow. Enter dimensions in feet and airflow in CFM. For a room measuring 10 × 12 × 8 ft and a flow of 160 CFM, the calculation is 160 × 60 ÷ 960 = 10 ACH.

Compare changes without changing the room

For a fixed room volume, doubling measured airflow doubles calculated ACH. Outdoor airflow, recirculated airflow and filtered clean-air delivery are different quantities; label the source you use and do not treat this calculation as a ventilation compliance check.