What is a good ACH for a bathroom?

Most guidance puts bathrooms at around 6-7 air changes per hour, a little higher than a bedroom or living room, because of the moisture generated by showers and baths.

What's the difference between ACH and CFM?

CFM (cubic feet per minute) is a raw airflow rate; ACH expresses that same airflow relative to a room's volume, as room-volumes per hour. This calculator finds the ACH a known airflow rate delivers -- the cfm calculator does the reverse, finding the airflow needed to hit a target ACH.

Is CFM commonly used in Malta, or m³/h?

Maltese retailers commonly list extractor fans in m³/h, following the wider EU convention, though CFM ratings appear on some imported and specialist units -- use this calculator's unit toggle to compare both.

Air Changes Per Hour (ACH) Calculator

How do you calculate air changes per hour (ACH)?

Air changes per hour (ACH) measures how many times a room's entire volume of air is replaced in one hour -- the key number for checking whether a room's ventilation is actually adequate. This calculator works out the ACH a room is getting from its dimensions and a known airflow rate (from an extractor fan, vent or duct spec sheet) -- the inverse of the cfm calculator, which starts from a target ACH and works out the airflow needed. Enter the room's length, width and ceiling height plus the airflow rate and unit, and optionally pick a room-type target ACH to see whether the room meets it.

How to use the ACH calculator

Enter the room's length, width and ceiling height, in metres or feet.
Enter the known airflow rate -- from an extractor fan, vent or duct spec sheet -- and its unit (CFM, m³/h, L/s and more).
Optionally pick a room-type preset (or your own target ACH) to see whether the calculated ACH meets or falls short of it.

The ACH formula

ACH = (airflow x 60) / room volume, when airflow is in CFM (cubic feet per minute) and volume is in cubic feet. In metric units, ACH = airflow (m³/h) / room volume (m³), because m³/h is already an hourly rate -- it's the exact inverse of the cfm calculator's formula.

Worked example: a 4 m x 3 m bedroom with a 2.4 m ceiling has a volume of 4 x 3 x 2.4 = 28.8 m³. A fan rated at 100 CFM (about 170 m³/h) gives 170 / 28.8 ≈ 5.9 ACH -- above the typical bedroom target of 5.5 ACH, so the room is adequately ventilated.

Typical ACH targets

As a typical Maltese example, having a technician assess a room and fit an extractor fan or mechanical ventilation system sized to the calculated ACH typically costs around €85-€330.

Bedrooms and living rooms usually target 5-8 ACH; bathrooms and kitchens need more (6-8 ACH and 7-8 ACH) because of moisture and cooking fumes, and commercial kitchens can require 14-60+ ACH.
If the calculated ACH falls short of the target, the fan or duct is undersized for the room -- look at a higher-capacity unit or a shorter/wider duct run before assuming the room just needs a window opened.

Air Changes Per Hour Calculator