Quick facts
- Bedroom (about 30 m³): 1–2 litres a day through the heating season
- Open-plan room (about 90 m³): 3–5 litres a day
- Rule of thumb: roughly 1 litre a day per 25 m³ of room
- Tank: 6 litres at 300 ml an hour runs about 20 hours
- Target: 40–60% relative humidity, never above
Output is the spec that sizes the room; the tank only sets the refill interval
Two numbers are printed on every humidifier box, and buyers routinely read the wrong one.
Tank capacity tells you how long the machine runs between refills.
Output — millilitres an hour, or litres a day — tells you whether it can lift the room at all.
A 6-litre tank behind a feeble mist head is just a large bucket that empties slowly.
Size on output. Then check the tank is big enough that you are not refilling it at three in the morning.
The room leaks, and the leak sets the number
A humidifier is not filling a sealed box. It is topping up a room that keeps swapping its air with the outside.
An ordinary British house exchanges something between half and one full change of air an hour, through trickle vents, gaps and extractor fans.
Every one of those changes carries the moisture you just added straight back out.
So the output you need is set by the leak rate, not by how dry the air felt when you walked in.
Air at 20 °C holds about 17 grams of water per cubic metre when fully saturated.
Lifting a room from 30% to 45% relative humidity therefore means adding roughly 2.6 grams to every cubic metre.
A 3m by 4m bedroom is about 30 m³, so that first lift costs around 78 grams of water.
Then you pay it again at every air change. At 0.7 changes an hour that is about 55 grams an hour, or 1.3 litres across a day.
Run the same two sums for a 40 m² open-plan room and you land near 4 litres a day.
You are not really humidifying a room. You are replacing, hour after hour, the moisture the house keeps posting outside.
Cold outdoor air is why a wet country has dry houses
This is the part that baffles people living in one of the damper corners of Europe.
Outdoor air at 5 °C holds at most about 6.8 grams of water per cubic metre, even at 100% humidity.
Draw that air indoors, heat it to 20 °C, and the same water now sits in air capable of holding 17 grams.
Relative humidity collapses to somewhere near 30%, and nothing has actually dried out.
Which is why the dry-air season begins when the heating does and ends when it goes off.
It also means your required output climbs as the weather turns colder, not as the forecast turns drier.
Oversizing shows up on the windows before it shows up anywhere else
Bottom line: Push a room past 60% relative humidity and the moisture will find the coldest surface in it — usually a window reveal or an outside wall — and condense there. That is how mould starts.
Older British stock supplies no shortage of cold surfaces: single glazing, solid walls, unheated back bedrooms.
A humidistat that shuts the machine off at a set figure is the real safeguard, and it is worth more than an extra litre of capacity.
Buy a hygrometer as well, and keep it well away from the mist, or it will report the cloud rather than the room.
Read the official guidance on damp and mouldHealth risks and causes, GOV.UK →
If the arithmetic points above 3 litres a day, you are into large-tank territory — the class covered in our Levoit LV600S review.
For one bedroom, a 4-litre tank running quietly overnight covers everything the sums ask for.
And if the windows stream in the morning, the answer is not a bigger humidifier.
