Vent Size Calculator
Enter the room length, width and height and the type of room. The calculator works out the airflow the extractor fan needs from air changes an hour, checks it meets common minimums and suggests the duct size.
- Extractor fan CFM
- Litres a second
- Duct size
Kitchens are usually covered by the cooker hood, sized for the hob as well.
Extractor fans are sized by how many times an hour they change the air. A bathroom needs about 8 air changes an hour. A 2.5 × 2 m bathroom with a 2.4 m ceiling is 12 m³, so it needs about 56 CFM (27 L/s), a 60 CFM fan on a 4 in (100 mm) duct. Kitchens need about twice the air changes.
Bathroom fan chart
8 ft ceiling, 8 air changes an hour. Search for a size.
| Bathroom (sq ft) | CFM | L/s | Duct (in) |
|---|---|---|---|
| 30 | 32 | 15 | 3 |
| 40 | 43 | 20 | 3 |
| 50 | 53 | 25 | 4 |
| 60 | 64 | 30 | 4 |
| 80 | 85 | 40 | 4 |
| 100 | 107 | 50 | 4 |
| 120 | 128 | 60 | 5 |
| 150 | 160 | 76 | 5 |
At least 15 L/s.
Bathroom size and fan
8 ft ceiling. Slide the floor area.
The drawing shows the fan and duct with the airflow.
How to size an extractor fan
Room to CFM. Step through it.
Measure
L × W × H.
Run the fan for 15 to 20 minutes after a shower to clear the moisture.
Bathroom, kitchen or toilet
Room types. Compare them.
Steam.
Steam and smells.
Smells.
Inline fans in the loft are quieter and can push air through longer ducts.
Before you fit
Vent checks. Tick each item.
Tick each item as you go.
Fans in bathroom zones near the bath or shower must have the right IP rating and often a low-voltage supply.
What you get
Vent sizing. Each tile explains one result.
Airflow.
Metric.
Also metric.
Size.
Room.
Every bend in a duct can cut airflow by about 10%.
Frequently Asked Questions
No question matches that search.
What size extractor fan do I need for a bathroom?
About one cubic foot a minute for every square foot of floor, with fifty CFM as a minimum.
How do I calculate CFM for a bathroom?
Multiply the room volume in cubic feet by the air changes an hour, then divide by sixty.
What is the UK minimum for a bathroom fan?
Fifteen litres a second for an intermittent fan.
What duct size does a bathroom fan need?
Usually four inches, or one hundred millimetres.
How long should the fan run after a shower?
About fifteen to twenty minutes.
How do I convert CFM to litres a second?
Multiply by zero point four seven two.
Does a longer duct need a stronger fan?
Yes. Long ducts and bends reduce the airflow.
Fresh, dry air
Size the fan for the room and keep the duct short and straight, and steam and smells clear fast.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.