Calculator
Bathroom and WC fan output calculator
Enter whether you are ventilating a bathroom or WC, the area, ceiling height and length of the pipe. The calculator will show how much air the ventilator should extract, how many times per hour it will exchange the air and what type of ventilator to choose. A bathroom needs at least 50 m³/h, recommended 90 m³/h, a separate WC 25 to 50 m³/h.
Enter the data
Result
Short pipe: an axial ventilator with a diameter of 100 mm is sufficient. The catalogue flow applies without the pipe – choose a ventilator with a reserve.
Pipe longer than 3 m: choose a radial or ducted ventilator and pipe with a diameter of 120 to 150 mm. Verify the actual flow in the manufacturer's performance curve.
The ventilator will only extract air if there is new air flowing in – leave a gap under the door or install a grille in the door.
Bathroom ventilators at vkupelni.sk ↗
Air extraction according to the ventilation requirements table for flats (ČSN EN 15665/Z1). The electrician connects the ventilator according to the zones in the bathroom.
How much air should the fan extract
The calculator is based on the table of ventilation requirements for flats according to the Czech national supplement to the EN 15665 standard. The same values are used in the article Ventilator for bathroom:
| Room | At least | Recommended |
|---|---|---|
| bathroom | 50 m³/h | 90 m³/h |
| separate WC | 25 m³/h | 50 m³/h |
| bathroom with WC (sum) | 75 m³/h | 140 m³/h |
The German standard DIN 18017-3 states for bathrooms 40 to 60 m³/h and for WC 20 to 30 m³/h. The calculator uses higher recommended values to ensure humidity after showering is removed quickly.
Air exchange: what the number means
Air exchange indicates how many times per hour the ventilator replaces all the air in the room. A bathroom in a prefab block of flats 2 × 1.6 m with a height of 2.5 m has 8 m³. A ventilator with a performance of 90 m³/h will exchange the air in it approximately 11 times per hour, that is once every 5 to 6 minutes. Thus, humid air after showering is removed in a few minutes.
Be cautious of the data in the catalogue: manufacturers state the maximum flow when the ventilator is blowing freely without pipes. Pipes, elbows, backdraft dampers and grilles create resistance and the actual flow is lower. Therefore, choose a ventilator with a reserve and for longer pipes follow the manufacturer's performance curve.
What ventilator according to the length of the pipe
| Pipe | Type of ventilator | Diameter |
|---|---|---|
| short, directly through the wall or into the shaft | axial | 100 mm |
| longer with elbows, through the roof | radial (centrifugal) | 120 to 150 mm |
| in the ceiling or attic, multiple rooms | duct fan | according to the manufacturer |
The calculator recommends a radial or duct fan for ducts longer than 3 m. For example, the axial fan CATA E-100 GTH has a maximum duct length of 3 m according to the manufacturer. A backdraft damper should be installed in the duct behind the fan to prevent cold air or odours from flowing back.
Timer, hygrostat, and air supply
- Timer – the fan continues to run for a few minutes after the light is turned off. Ideal for bathrooms without windows.
- Hygrostat – the fan turns on automatically when humidity rises and turns off when it drops.
- Air supply – the fan will only extract air if there is a source for new air to flow in. Leave a gap under the door or install a ventilation grille in the door.
When humidity lingers in the bathroom for too long, mould can appear on cold walls and in grout joints. How to prevent it is explained in the article Mould in the bathroom.
Electricity and installation
The electrician connects the fan according to the zones in the bathroom – it must have appropriate protection above the shower and bath. When passing through an external wall or roof, consult a professional, and if connecting to a shared duct in a prefab block of flats, consult the building manager in advance.
Where to choose a fan
You can find fans with timers and hygrostats on the e-shop vkupelni.sk in the category Bathroom fans – each model lists the flow rate and noise level. You can also see heating and ventilation solutions for bathrooms in person at the store in Senec at Bratislavská 71.
Related: Bathroom fan · Mould in the bathroom · Heating and ventilation · All calculators
Sources: table Requirements for Ventilation of Residential Spaces (tzb-info.cz, according to ČSN EN 15665/Z1: bathroom 50 / 90 m³/h, WC 25 / 50 m³/h), DIN 18017-3 (bathrooms 40 to 60 m³/h, WC 20 to 30 m³/h, according to tzb-info.cz), technical data for CATA fans on vkupelni.sk (flow rate, duct length, November 2026), STN 33 2000-7-701 (zones, simplified).
Terms in the dictionary: Air exchange · Hygrostat
Frequently Asked Questions
What power should a bathroom fan have?
At least 50 m³/h, recommended 90 m³/h. For a separate WC, 25 to 50 m³/h is sufficient, for a bathroom with WC recommended around 140 m³/h.
How many times per hour should the ventilator exchange the air?
It depends on the size of the bathroom. A ventilator with a performance of 90 m³/h will exchange the air in a bathroom with a volume of 8 m³ approximately 11 times per hour, that is once every 5 to 6 minutes.
Why does the ventilator extract less than stated in the catalogue?
The catalogue flow applies without the pipe. Pipes, elbows, dampers and grilles create resistance, so choose a ventilator with a reserve and follow the performance curve for longer pipes.
Axial or radial ventilator?
For short pipes directly through the wall or into the shaft, use axial; for longer pipes with elbows and through the roof, use radial or ducted.
What diameter of pipe?
For a small bathroom and short pipe, 100 mm; for longer pipes or larger bathrooms, 120 to 150 mm. The diameter of the ventilator must match the pipe.
Why isn't the fan extracting air?
Most often, there is no new air source. A gap under the door or a grille in the door can help. Also, check for blocked ducts and the backdraft damper.
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