Air Changes Per Hour Calculator

This air changes per hour calculator works out how many times the entire volume of air in a room is replaced with fresh air every hour, the key ventilation figure known as ACH. Enter the room length, width and height in metres so the calculator can work out the room volume, then enter the airflow of your fan, extractor, ventilation system or air conditioner and pick whether that figure is in cubic metres per hour or litres per second. The calculator divides the airflow by the room volume to give the air changes per hour, and it also shows the room volume it used, the airflow converted to cubic metres per hour, and how many minutes it takes to change the air once. ACH is used to size extractor fans and ventilation systems, to check that a bathroom, kitchen, workshop, office or grow space gets enough fresh air, and to compare a space against the change rates recommended for its use. A bathroom might aim for six to eight changes an hour while a living area needs far fewer. Enter your own room and fan figures to see where your space sits, or use the worked example below to confirm the tool before you rely on it.

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m
m
m
3.1 ACH
air changes per hour
Room volume48 m³
Airflow150 m³/h
Minutes per air change19.2 min

At 150 m³/h into a 48 m³ room, the air is fully replaced 3.1 times an hour, once every 19.2 minutes.

ACH is airflow divided by room volume. Recommended rates vary with room type and occupancy, so treat the result as a planning estimate.

How it works

The room volume is length times width times height. The airflow is put into cubic metres per hour, so litres per second is multiplied by 3.6 first. Air changes per hour is then the airflow in cubic metres per hour divided by the room volume in cubic metres. The minutes per air change is 60 divided by the ACH, which is how long the fan takes to move a volume of air equal to the whole room.

Worked example

A room 5 m long, 4 m wide and 2.4 m high has a volume of 5 x 4 x 2.4, which is 48 cubic metres. A fan rated at 150 cubic metres per hour gives 150 / 48, which is about 3.1 air changes per hour. That means the air is replaced roughly once every 60 / 3.1, or 19.2, minutes. If the same fan were rated in litres per second at, say, 40 L/s, you would first convert to 40 x 3.6, which is 144 cubic metres per hour, before dividing by the volume.

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Frequently asked questions

How do you calculate air changes per hour?

Divide the airflow into the room by the room volume, using the same time and volume units. With airflow in cubic metres per hour and volume in cubic metres, ACH equals airflow divided by volume. For example, 150 cubic metres per hour into a 48 cubic metre room gives about 3.1 air changes per hour.

How do I convert litres per second to cubic metres per hour?

Multiply litres per second by 3.6 to get cubic metres per hour, because one litre per second is 3,600 litres, or 3.6 cubic metres, every hour. So a 40 litre per second fan moves 144 cubic metres per hour.

How many air changes per hour do I need?

It depends on the use of the space. Living areas often aim for around one to two changes an hour, bathrooms and kitchens for roughly six to eight, and workshops or spaces with fumes for more. Check the guideline for your room type, because required rates vary with occupancy and activity.