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.

How do I convert CFM to m³/h?

1 CFM is about 1.699 m³/h, so multiply a CFM figure by 1.699 to get m³/h, or divide an m³/h figure by 1.699 to get CFM.

Does UK Building Regulations set a minimum extract rate?

Approved Document F sets minimum intermittent extract rates for wet rooms -- commonly cited figures are around 30 l/s for a kitchen extractor near the hob (or 60 l/s elsewhere in the kitchen) and around 15 l/s for a bathroom -- worth converting to CFM or m³/h to compare against this calculator's output.

CFM Calculator — Ventilation Airflow (m³/h)

How much airflow (CFM) does a room need?

CFM -- cubic feet per minute -- is the standard rating for extractor fans, ventilation ducting and HVAC equipment. This calculator works out how much airflow a room actually needs, in CFM or the metric m³/h, from its volume and a target air changes per hour (ACH): how many times per hour the room's full volume of air should be replaced. Enter the room's length, width and ceiling height, pick a room-type ACH preset (or type your own), and read off the required fan or duct capacity.

How to use the CFM calculator

Enter the room length, width and ceiling height, in metres or feet.
Choose a room-type preset -- bedroom, bathroom, kitchen, office and more -- to auto-fill a recommended air changes per hour (ACH), or enter your own target.
Read off the required airflow in CFM or m³/h -- the minimum capacity to look for in an extractor fan or ducted ventilation system.

The CFM formula

Required airflow = room volume x target ACH / 60, when volume is in cubic feet and airflow is in CFM (cubic feet per minute). In metric units the /60 disappears: required airflow (m³/h) = room volume (m³) x target ACH, because m³/h is already a per-hour rate.

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. At a typical bedroom target of 5.5 ACH, required airflow = 28.8 x 5.5 = 158.4 m³/h -- about 93 CFM (1 CFM ≈ 1.699 m³/h).

Typical ACH targets

As a typical UK example, having a ventilation engineer assess a room and fit an extractor fan or mechanical ventilation system sized to the calculated airflow typically costs around £120-£480, depending on room access and duct run length.

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.
Round the calculated CFM up to the next fan or duct size a supplier actually stocks -- undersized ventilation is a common cause of lingering odours and condensation.