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.

What ventilation standard do US contractors follow?

Residential ventilation in the US is commonly sized against ASHRAE Standard 62.2, which sets minimum whole-house and local (kitchen/bathroom) exhaust rates in CFM -- this calculator's ACH-based method is a simpler room-by-room approach useful for sizing a single fan or duct run.

CFM Calculator — Required Ventilation Airflow

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 US example, having an HVAC contractor assess a room and size an exhaust fan or ventilation system typically costs around $150-$600 for the assessment and install callout.

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.