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Dehumidifier size calculator: what size do I need?

Six quick questions about your home and your damp problem give you a sensible starting size and type — and what to read next. It takes under a minute.

A cutaway of a two-storey house showing the bedroom, bathroom, living room and kitchen, with rain outside

Quick answer

As a starting point: 10–12L for a room or 1–2 bed flat, 12–20L for a 3-bed house, and 20–25L for larger homes or daily indoor laundry — choosing a desiccant model if rooms are often below about 15°C. The calculator below adjusts that for your temperature, laundry, household and how bad the problem is, then estimates the running cost and where to put it.

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1. What do you need to dry?
2. How warm is it in winter?
3. What’s the main problem?
4. How bad is it?
5. How much washing do you dry indoors?
6. How many people live there?
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Why use the calculator rather than a size chart?

A chart sizes by home type alone. In practice, two homes the same size can need very different dehumidifiers: a cold flat drying washing every day needs a different type and more capacity than a warm one with occasional condensation. The calculator weighs those factors together, switches to desiccant when your home is cold, warns you when the problem sounds like a leak or rising damp rather than condensation, and estimates what the recommended class would cost to run each day.

How the calculator works

This is practical guidance, not an engineering calculation. It weighs up the size of the space, how serious the moisture problem is, how much washing you dry indoors and how many people live there, then picks a capacity class. Temperature decides the type: below about 15°C, compressor dehumidifiers lose much of their performance, so the calculator switches to desiccant models.

Litre ratings describe how much water a unit can remove in 24 hours in test conditions — usually far warmer and wetter than a British home in winter. That’s why we size slightly generously for cool homes. Once it’s running, a hygrometer reading of around 50% tells you it’s doing the job.

Dehumidifier size chart for UK homes

SpaceHeated home (compressor)Cold home or outbuilding (desiccant)
Single room or bedroom10–12L7–8L
Studio or 1-bed flat10–12L7–8L
2-bed flat or small house10–12L (12–20L with daily laundry)7–10L
3-bed house12–20L10–12L, or two units
4+ bed house, serious damp or heavy laundry20–25LTwo units, or heat the space a little and use a compressor
Garage, conservatory, caravan or cellarNot ideal when cold7–10L, with continuous drainage

Size up if you dry washing indoors most days, if condensation appears in several rooms, or if the house is older with solid walls. For the differences between capacity classes, see 10L vs 12L vs 20L dehumidifiers.

What size dehumidifier for a bedroom?

A 10–12L compressor model with a proper sleep mode suits most bedrooms; a 7–8L desiccant is better if the room is cold overnight. Noise and lights matter more than capacity here — see the best dehumidifier for a bedroom.

What size dehumidifier for a 2-bedroom flat?

10–12L is the usual answer, moving to 12–20L if you dry washing indoors most days or have condensation in more than one room. Flats often have no tumble dryer and limited ventilation, so laundry is frequently the deciding factor — our guide to dehumidifiers for flats covers noise, space and carrying units up stairs.

What size dehumidifier for a 3-bedroom house?

12–20L. A 20L unit placed on the landing or in a hallway can serve a typical three-bed house, especially with doors open between rooms. If one room is the real problem, a smaller unit in that room with the door closed often works better than a big one in the hall. Our guide to using a dehumidifier explains placement.

Garages, conservatories, caravans and cellars

These spaces are often well below 15°C, so a desiccant dehumidifier is the usual choice. Continuous drainage through a hose is worth having so the unit doesn’t stop when the tank fills. See desiccant vs compressor for why.

Frequently asked questions

What size dehumidifier do I need for a 2 bedroom flat?

A 10–12 litre compressor dehumidifier suits most 1–2 bedroom flats. Choose 12 litres, or consider 20, if you dry washing indoors most days or there is condensation in several rooms. If the flat is cold — often below about 15°C — a 7–10 litre desiccant model is a better fit.

What size dehumidifier do I need for a 3 bedroom house?

A 12–20 litre compressor dehumidifier suits most three-bedroom houses. Go for 20 litres with daily indoor laundry, mould in more than one room or an older house with solid walls. A single unit placed centrally (a hallway or landing) can serve the house, or you can move it to the worst room.

What size dehumidifier do I need for a bedroom?

A 10–12 litre compressor model is plenty for a typical bedroom; choose one with a proper sleep mode. In a cold bedroom, a 7–8 litre desiccant model works better. Bigger units work too, but they are louder and heavier than one room needs.

Is it better to get a bigger dehumidifier?

Slightly oversizing is usually fine — a bigger unit reaches your target humidity faster and then cycles off. The downsides are a higher price, more weight and often more noise. Undersizing is the more common mistake, especially in cool UK homes where compressor units work below their rated capacity.

How accurate is this calculator?

It applies practical rules of thumb, broadly in line with UK manufacturer sizing guidance. It is not an engineering calculation and cannot account for every home. Use it as a starting point, then check how quickly a hygrometer reading falls once the dehumidifier is running.

Sources and further reading