What size dehumidifier do I need?

Quick size table

Start here, then adjust for the factors in the next sections. Sizes are for compressor models rated at the common 30 °C and 80 % RH test condition.

Read the litres a day figure carefully

Litres a day is the most water the machine extracts in 24 hours at a set test condition. Many compressor models in the UK are rated at 30 °C and 80 % RH. Some makers also give a figure at 27 °C and 60 % RH, which is lower and closer to a home in summer. In a heated UK room in winter at around 20 °C and 60 % RH, real extraction is lower again.

That does not mean the rating is useless. It is a fair way to compare machines that were tested the same way. Just check which condition each maker quotes before comparing, and do not expect to fill the tank at the rated speed. Our explainer on dehumidifier extraction ratings covers this in depth.

When to size up

A bigger machine reaches the target humidity faster, then switches off. In practice it often runs fewer hours than a small machine straining to keep up. Size up if any of these apply.

• You dry washing indoors several times a week
• The home has single glazing or solid walls that run cold
• You see condensation on most windows every morning
• The house is large or open plan
• You want it to work quietly on a low fan setting

Compare 20 litre dehumidifiers →

Cold rooms change the maths

Compressor ratings fall sharply below about 15 °C. A 20 litre compressor model in a 10 °C garage might extract only a fraction of its rating, and spend time defrosting. Desiccant dehumidifiers are usually rated at lower figures, often 6 to 10 litres a day, but their output stays much steadier in the cold.

So for a garage, cellar, caravan or unheated room, a 7 or 8 litre desiccant model can outperform a much larger compressor unit. See the best desiccant dehumidifiers and the best dehumidifiers for garages and cellars.

Other things to check besides litres

Tank size decides how often you empty it. A 2 litre tank fills fast during laundry. A continuous drain option avoids emptying altogether if you can run a hose to a drain. Noise matters in bedrooms and living rooms, so compare the stated dB on low fan. Weight matters if you plan to carry it between rooms.

Power draw matters for running costs. Larger compressor models usually draw more watts, but they may also run for fewer hours. Our dehumidifier running costs article explains how to work it out in kWh, and the best low energy dehumidifiers guide compares stated watts.

One large unit or two small ones?

In a single open space, one well sized unit is usually simplest. In a home with several damp rooms on different floors, moving one machine around works, but two smaller machines may suit better if you need both rooms dry at once. A dehumidifier mainly treats the room it is in, plus nearby rooms with open doors.

If you cannot decide, the 20 litre class is a common choice for family houses because it copes with laundry and winter condensation. Our guide to the best dehumidifiers for UK homes covers both sizes.

Frequently asked questions

Is a 10 litre dehumidifier big enough? For a flat or a small house with mild condensation, often yes. For laundry, larger homes or damp problems, a 12 or 20 litre model copes better.

What does 20L dehumidifier mean? It can extract up to 20 litres a day under the maker's test conditions, usually 30 °C and 80 % RH. Real extraction in a UK home is lower.

Can a dehumidifier be too big? Not really for performance, as it switches off at the target. A bigger unit costs more to buy, weighs more and may draw more watts while running.

What size dehumidifier for a garage? Size by a desiccant model's rating rather than a compressor's, because cold garages cut compressor output. A 7 to 10 litre desiccant model is a common choice.