Air Changes Calculator Formula

Understand the math behind the air changes calculator. Each variable explained with a worked example.

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Formulas Used

Room Volume

room_volume = volume

Required Airflow

airflow_m3h = volume * ach

Required Airflow

airflow_lps = volume * ach / 3.6

Variables

Variable	Description	Default
`room_length`	Room Length(m)	10
`room_width`	Room Width(m)	6
`room_height`	Room Height(m)	3
`ach`	Air Changes per Hour(ACH)	6
`volume`	Derived value`= room_length * room_width * room_height`	calculated

How It Works

Air Changes per Hour (ACH)

ACH defines how many times the entire room volume of air is replaced per hour.

Formula

Airflow = Room Volume x ACH

Required ACH depends on the room function. Adequate ventilation controls temperature, humidity, odours, and airborne contaminants.

Worked Example

A 10m x 6m x 3m room requiring 6 air changes per hour.

room_length = 10room_width = 6room_height = 3ach = 6

01Volume = 10 x 6 x 3 = 180 m^3
02Airflow = 180 x 6 = 1080 m^3/h
03Airflow = 1080 / 3.6 = 300 L/s

Frequently Asked Questions

What ACH is required for different spaces?

Office: 4-6 ACH. Classroom: 6-8. Kitchen (commercial): 15-30. Workshop: 8-12. Hospital operating room: 15-25. Toilet/bathroom: 8-15. Warehouse: 2-4.

Is higher ACH always better?

Not necessarily. Excessive air changes waste energy by conditioning (heating/cooling) more air than needed. Balance indoor air quality needs against energy efficiency.

How does this relate to CO2 levels?

Ventilation removes CO2 exhaled by occupants. ASHRAE recommends keeping CO2 below 1000 ppm. The required outdoor air per person (typically 8-10 L/s/person) may give a different answer than ACH-based calculations.

Ready to run the numbers?

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