Air changes per hour (ACH) describe how quickly a ventilation system, exhaust fan or air purifier replaces the air in a space. Homes typically need 0.35 ACH of fresh air, while kitchens, bathrooms, workshops and classrooms need much more. Air purifiers are often rated so they deliver 4–5 air changes per hour in their rated room size.
How to use the Air Changes per Hour Calculator
- Room length
- Room width
- Ceiling height
- Airflow
- Every result field is editable: type into Air changes per hour and Airflow plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the room length, width and ceiling height and the airflow of the fan or system. The calculator finds the volume and divides the hourly airflow by it. Type a target ACH to find the airflow needed.
Typical air change rates
| Space | ACH |
|---|---|
| Whole house (fresh air) | 0.35 minimum |
| Bedroom / living room (purifier) | 4–6 |
| Bathroom | 6–8 |
| Kitchen | 7–8 |
| Classroom | 4–6 |
| Workshop / garage | 10–12 |
| Laboratory | 6–12 |
Tips for an accurate estimate
- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer’s coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.
Common mistakes to avoid
- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.
Inputs explained
| Field | What to enter |
|---|---|
| Room length | Units: ft, m, in, yd, cm |
| Room width | Units: ft, m, in, yd, cm |
| Ceiling height | Units: ft, m, in, yd, cm |
| Airflow | Units: CFM, m³/h, L/s, m³/s |
What each result means
- Room volume = [Room length]×[Room width]×[Ceiling height]
- Air changes per hour = [Airflow]×3600÷[Room volume]
- Minutes per air change = 60÷[Air changes per hour]
Formula
ACH = airflow × 60 ÷ room volume (with airflow in ft³/min and volume in ft³)Time per air change = 60 ÷ ACH minutesWorked example
Inputs — Room length: 15 ft; Room width: 12 ft; Ceiling height: 8 ft; Airflow: 100 CFM.
Results — Room volume = 1,440 ft³; Air changes per hour = 4.17; Minutes per air change = 14.4.
Frequently asked questions
What is a good number of air changes per hour?
About 0.35 fresh-air ACH for living areas, 4–6 for air purifiers in bedrooms, 6–8 for bathrooms and kitchens and 10–12 for workshops.
How do I convert ACH to CFM?
CFM = ACH × room volume (ft³) ÷ 60.
Does ACH include recirculated air?
It depends on context – fresh-air ACH counts only outside air; purifier ACH counts filtered recirculated air.
What formula does the Air Changes per Hour Calculator use?
ACH = airflow × 60 ÷ room volume (with airflow in ft³/min and volume in ft³).