Ventilation Rate and Air Changes per Hour (ACH) Calculator

How is ventilation rate calculated?

The air a space needs is found in one of three ways. Which one applies depends on the space type and the standard in force.

By air changes: the room volume is replaced a set number of times per hour. Q = n × V. Common for stores, plant rooms, toilets and workshops.

By people and area: the fresh air requirement is the sum of two components: Q = N × q_p + A × q_a. The people component deals with contaminants from occupants and the area component with those from materials and furnishings. Used for densely occupied spaces such as offices, classrooms and meeting rooms.

By a CO₂ limit: in steady state the flow needed to hold the indoor concentration at a limit is Q = N × G / (C_in − C_out).

What is air changes per hour (ACH)?

Air changes per hour is the volume of air supplied in one hour divided by the room volume: n = Q / V. Six air changes per hour means the air is replaced every 10 minutes on average. It does not mean all contaminant is gone in 10 minutes: in a fully mixed room one air change removes about 63 percent of it.

Example

An office of 100 m² with a 3 m ceiling and 10 people. At 2.5 L/s per person and 0.3 L/s per square metre the fresh air requirement is 25 + 30 = 55 L/s, or 198 m³/h. That flow corresponds to 0.66 air changes per hour.

Checking with CO₂

A seated adult produces about 18 litres of CO₂ per hour. Holding the room 600 ppm above outdoor air takes 0.018 / 0.0006 = 30 m³/h per person, which is where the familiar figure of 30 cubic metres per person comes from.

Limits

The tool calculates the flow, it does not decide which value applies. For design values see the relevant standard, code and project specification.