MasterTool

Air Changes per Hour Calculator

Calculate the air changes per hour a given airflow delivers in a room, and the CFM needed to hit a target ACH.

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What this tool does

Calculate the air changes per hour a given airflow delivers in a room, and the CFM needed to hit a target ACH. Room is treated as a rectangular box. Enter outdoor airflow if you want fresh-air changes rather than recirculation.

How to use the Air Changes per Hour Calculator

  1. Enter or select airflow (cfm).
  2. Enter or select room length in ft.
  3. Enter or select room width in ft.
  4. Enter or select room height in ft.
  5. Enter or select target air changes per hour.
  6. Read the calculated result; change any measurement to compare alternatives.

Formula

ACH = CFM × 60 ÷ (length × width × height); required CFM = target ACH × volume ÷ 60
cfm
Airflow (CFM)
length
Room length (ft, before metric conversion)
width
Room width (ft, before metric conversion)
height
Room height (ft, before metric conversion)
targetAch
Target air changes per hour

Room is treated as a rectangular box. Enter outdoor airflow if you want fresh-air changes rather than recirculation.

Worked example

For air changes per hour calculator, the following measurements illustrate the exact method: Airflow (CFM): 500; Room length: 20; Room width: 15; Room height: 8; Target air changes per hour: 6.

Inputs

  • Airflow (CFM)500
  • Room length20 ft
  • Room width15 ft
  • Room height8 ft
  • Target air changes per hour6

Result

  • Air changes per hour (ACH)12.5
  • Room volume (cu ft)2,400
  • CFM needed for target ACH240

Results explained

Air changes per hour (ACH)
Air changes per hour (ACH) from the formula above. Room is treated as a rectangular box. Enter outdoor airflow if you want fresh-air changes rather than recirculation.
Room volume (cu ft)
Room volume (cu ft) from the formula above. Room is treated as a rectangular box. Enter outdoor airflow if you want fresh-air changes rather than recirculation.
CFM needed for target ACH
CFM needed for target ACH from the formula above. Room is treated as a rectangular box. Enter outdoor airflow if you want fresh-air changes rather than recirculation.

Frequently asked questions

ACH = airflow (CFM) × 60 ÷ room volume (ft³). A 500 CFM fan in a 20 × 15 × 8 ft room (2,400 ft³) gives 500 × 60 ÷ 2,400 = 12.5 air changes per hour.

Common design targets: bedrooms and offices about 4–6, kitchens and bathrooms higher with local exhaust, and healthcare or lab spaces set by code. Treat these as orientation — local ventilation codes govern.

They are the same equation rearranged. The CFM tool starts from a target ACH and returns airflow; this one starts from an airflow and returns the ACH it achieves, plus the CFM a target ACH would need.

Only if the airflow is outdoor air. Recirculated air through a filter changes air movement but not fresh-air ACH; enter the outdoor-air portion of the flow for ventilation ACH.

The room's air volume: length × width × ceiling height. Sloped ceilings and large fixed furniture make this an approximation.