Room Ventilation Calculator
Find the exhaust fan airflow in CFM (and m³/h) a room needs from its size and a target air-change rate.
Calculator
Free, runs entirely in your browser, and your numbers never leave your device. Results are estimates for planning only.
What this calculator does
This ventilation calculator works out the exhaust fan airflow a room needs from its volume and a target number of air changes per hour. It returns the required airflow in CFM along with the metric m³/h equivalent, so you can match fans rated in either unit, and it flags the common minimum used for small bathrooms.
How to use it
- Enter the room's length, width, and ceiling height.
- Enter the air changes per hour you want — bathrooms are often planned around 8 ACH.
- Read the required airflow in CFM and its m³/h equivalent.
- Choose a fan rated at or above that figure, allowing a little margin for ductwork.
The formula
Room volume = length × width × height. Airflow (CFM) = volume × ACH ÷ 60. Metric: m³/h = room volume in cubic meters × ACH (equivalently, CFM × 1.699).
Example calculation
An 8 ft × 6 ft bathroom with an 8 ft ceiling, ventilated at 8 air changes per hour:
- Volume: 8 × 6 × 8 = 384 cu ft
- Airflow: 384 × 8 ÷ 60 = 51.2 CFM → round up to 52 CFM
- Metric equivalent: 52 × 1.699 ≈ 87 m³/h
Result: About 52 CFM (≈ 87 m³/h) — comfortably above the common 50 CFM minimum for a small bathroom.
Buying and planning tips
- Size up a little: duct length, bends, and grilles cut real airflow below the fan's rated CFM.
- Match the fan to the room — an oversized fan is noisy and wastes conditioned air.
- Look for the fan's CFM measured at realistic duct conditions, not just the headline number.
Common mistakes to avoid
- Sizing a small bathroom below the common 50 CFM minimum because the raw math suggests less.
- Ignoring duct resistance, so the installed fan moves less air than its rating.
- Confusing CFM with m³/h when comparing fans sold in different markets.
Assumptions and limits
- ACH (air changes per hour) is how many times the room's full air volume is replaced each hour.
- Bathrooms are often planned around roughly 8 ACH as a rule of thumb, not a universal rule.
- Real airflow is reduced by ductwork, so the result is a starting point, not a guarantee.
Frequently asked questions
What size exhaust fan do I need for a bathroom?
For small bathrooms, a common rule of thumb is at least 50 CFM, or roughly 1 CFM per square foot. This calculator sizes by air changes per hour, which scales more sensibly to larger rooms.
What does CFM mean?
CFM is cubic feet per minute — the volume of air a fan moves each minute. A higher CFM clears moisture and odors from the room faster.
What are air changes per hour (ACH)?
ACH is how many times the room's entire air volume is replaced in an hour. Bathrooms are often planned around 8 ACH as a rule of thumb; steamier or busier rooms may want more.
How do I convert CFM to m³/h?
Multiply CFM by about 1.699 to get cubic meters per hour. The calculator shows both figures so you can compare fans rated in either unit.
Should I size the fan larger than the calculated CFM?
A little margin helps, because duct length, bends, and grilles reduce real airflow below the fan's rating. Round up, and check the rated CFM at realistic duct conditions.
Does a bigger fan always ventilate better?
Not always. An oversized fan can be noisy and pull out conditioned air. Match the airflow to the room and aim for quiet operation that you will actually leave running.
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