What size dehumidifier do I need?
Answer a few questions and this works out how many litres a day your dehumidifier should be rated for, whether a cold room needs a desiccant model, and what it'll cost per hour on today's electricity price. For a typical 2 or 3 bedroom home with steamy windows in the morning, it's usually somewhere between 10 and 20 litres a day.
Ask for a local specialist Free, no obligation, one specialist
The sums happen in your browser. Nothing you enter leaves this page.
Reading your result
Dehumidifiers are sold by extraction rate: the litres of water they can take out of the air in a day, usually measured in warm, humid test conditions. That's the figure on the box. Your machine won't actually collect that much every day. Once the room is dry enough it slows down or cuts out.
The size bands are taken from buying guides by Meaco, a British dehumidifier company. They were the only UK guides we found that give sizes in litres and square metres with a date on them. Meaco sells dehumidifiers, so read them with that in mind. We've drawn the line between 3 and 4 bedrooms at 150 m² ourselves, because Meaco doesn't give one.
The calculator goes up a size if you can see mould, if you tick two or more signs of damp, or if more people live there than the home is sized for. That follows Meaco's own examples: 12 litres rather than 10 for a small flat with more than two people in it, and 20 rather than 12 for a 3 bedroom house with six.
Compressor or desiccant?
Most dehumidifiers sold in England are compressor models. They work like a small fridge: air passes over a cold coil and the water drips into the tank. They're cheaper to run in a heated home but lose a lot of their effect as the room gets colder. Meaco's advice is desiccant only below 10°C, and either a desiccant or a 20 to 25 litre compressor model between 10°C and 15°C.
A desiccant machine draws the water out with an absorbent material that's then dried by a heater. It keeps working in a cold room and warms the air slightly, which helps in a garage, a conservatory or a bedroom you don't heat. It uses more power to do it.
Running costs
Meaco gives typical power draw as 150 to 260 watts for a compressor model, depending on its size, and about 330 watts for a desiccant one. At the current price cap rate of 26.32p per kWh, that works out at:
- a compressor model: 4p to 7p an hour, so 32p to 55p if it runs overnight for eight hours
- a desiccant model: roughly 9p an hour, which comes to 69p over the same eight hours
The 26.32p is Ofgem's average unit rate for electricity on a standard variable tariff paid by Direct Debit, for 1 October to 31 December 2026. The government has taken VAT off electricity bills until 31 March 2027, so there's none to add. It doesn't include the daily standing charge. Your own rate is on your bill. For a particular model, divide its wattage by 1,000, then multiply by the hours you'll run it and your price per kWh.
When a dehumidifier isn't the answer
A dehumidifier dries the air. It can't stop water getting into the building, so it won't sort out:
- rising damp, a band of damp and salts along the bottom of a ground floor wall
- penetrating damp, a patch that spreads after heavy rain because of a blocked gutter, worn pointing, cracked render or a roof problem
- a leaking pipe, a failed shower seal or water behind tiles
It won't remove mould that's already growing either. That needs cleaning off properly, or treating if it's spread. If you're not sure what you're dealing with, start with damp or condensation?
Keeps coming back?
Give it a few weeks. If the dehumidifier is filling its tank every day and the mould is still spreading, or a patch on one wall never dries out, the moisture is probably getting in from somewhere. That's when it's worth having someone look. Send us the details and we'll pass them to one independent damp and mould specialist who covers your postcode. It costs you nothing, and they'll quote you directly.
If you rent, keeping damp at bay shouldn't fall to you and a dehumidifier forever. The tenants' guide explains how to report it to your landlord and what they're expected to do.
Sources
Sizes, temperatures and power use come from Meaco's buying guides: flats (updated 2 January 2026), 2 bedroom houses (updated 27 November 2024), 3, 4 and 5 bedroom houses (updated 13 November 2023), desiccant or compressor (updated 20 October 2025) and setting the humidity (updated 27 February 2024). The electricity price is from Ofgem's price cap announcement for 1 October to 31 December 2026 (published 26 August 2026). We don't recommend particular products or earn from them.
Facts, dates and prices on this page were last checked on 2 November 2026.
Common questions
Is the litres figure how much the tank holds?
No. It's the most water the machine can pull out of the air in 24 hours. Tanks are much smaller, often two or three litres, so in a damp house in winter you may be emptying it every day. Many models take a hose if there's a drain nearby.
Should I just buy the biggest one I can afford?
Not necessarily. A bigger machine dries the air quicker, which helps on washing days, but it costs more and is usually noisier. Most switch themselves off once the room reaches the humidity you've set, so a size up doesn't mean it runs flat out all day.
What humidity should I aim for?
Meaco suggests setting it to 55% relative humidity for most homes, and says mould starts to grow at 68%. Set it at 55% and let it cycle on and off.
Will running a dehumidifier get rid of mould?
It helps stop condensation mould coming back by keeping the air drier. It won't clean off mould that's already growing, and it does nothing about rising damp, rain coming through a wall or a leaking pipe.
Does the calculator save what I type?
No. It works out the answer in your browser. Nothing is sent to us or stored on your device.
Ask for a local damp and mould specialist
Free to use, no obligation, and your details go to one local specialist, not a list.
We're a matching service, not the firm that does the work. The specialist contacts you, looks at the problem and quotes you directly. Whether you go ahead is up to you.