For most apartments and flats, start with a 10L to 12L dehumidifier, then adjust up or down based on your actual humidity level, layout, and daily moisture sources. A 10L model is usually enough for a small one- or two-bedroom flat with light condensation. Choose 12L if you dry laundry indoors, cook often, have more than two people at home, or see damp returning every day.
Quick Answer
For a typical apartment, choose a 10L dehumidifier for mild condensation in a small one- or two-bedroom flat, a 12L model for regular laundry drying or persistent damp, and a 20L unit for larger open-plan spaces or humidity that stays above 60% after ventilation.
Key Takeaways
- Measure humidity first with a hygrometer; aim for roughly 30% to 50% relative humidity and avoid letting damp rooms sit above 60%.
- A 10L unit suits light, seasonal condensation; a 12L unit is better for indoor laundry, higher occupancy, or damp that returns daily.
- Capacity ratings are daily moisture-removal ratings under test conditions, so real-world results depend on temperature, airflow, and how wet the flat is.
- A dehumidifier helps control air moisture, but it does not fix leaks, failed ventilation, rising damp, or mould caused by a water source.
At a Glance
| Time Required | 10 minutes to check room size and features; 24 hours to monitor humidity properly |
| Difficulty | Easy |
| Tools Needed | Hygrometer, tape measure or floor-plan estimate, socket near the damp area, optional drain hose |
| Cost | Usually a compact 10L or 12L unit; running cost depends on wattage and your electricity tariff |
What Size Dehumidifier Does Your Apartment Need?
To choose the right dehumidifier size for your apartment, match capacity to three things: floor area, moisture level, and how much moisture you create each day. Cooking, showers, indoor clothes drying, poor ventilation, and cold external walls all raise the moisture load.
In the UK, compact units are usually described in litres per day. In the US, capacity is commonly listed in pints per 24 hours. ENERGY STAR explains that capacity depends on both the size of the space and the dampness conditions, not floor area alone.
Note: A 10L dehumidifier is roughly equal to 21 US pints per day, while a 12L unit is roughly equal to 25 US pints per day. Do not confuse a 10L model with a 10-pint model; they are not the same size.
| Apartment condition | Best starting size | Good fit for |
| Light condensation, no musty smell, humidity usually below 60% | 10L per day | Studio, small one-bedroom, or mild seasonal damp |
| Regular window condensation, indoor laundry, two or more occupants, or humidity often above 60% | 12L per day | Most one- to two-bedroom flats with daily moisture |
| Open-plan layout, larger flat, heavy laundry drying, persistent musty smell, or damp patches | 15L to 20L per day | Larger apartments or spaces with a higher moisture load |
| Wet walls, leaks, water staining, or mould spreading despite ventilation | Do not size by appliance alone | Find and fix the water source first |
If you are buying a US-rated model, check the pint-per-day rating and remember that modern pint ratings are lower than older ratings for some comparable units. ENERGY STAR notes that DOE test changes from 2019 onward use cooler test conditions, so an older “30-pint” unit may compare roughly with a newer “20-pint” unit.
How Damp Is Your Apartment?
Before you buy, check how damp your apartment actually is. Use a hygrometer in the dampest room for at least 24 hours. Place it away from direct steam, radiators, and open windows so the reading reflects normal room air.
The EPA recommends keeping indoor relative humidity below 60% and ideally between 30% and 50% where possible. In real apartment use, a practical target is often around 45% to 50% in damp rooms, as long as the air does not become uncomfortably dry.
- 30% to 50% RH: Good control for most homes. Avoid pushing much below 30%, especially in winter.
- 50% to 60% RH: Watch closely. Use ventilation, extractor fans, and a dehumidifier if condensation appears.
- Above 60% RH: Damp risk is higher, especially on cold walls, window frames, bathroom ceilings, and behind furniture.
- Musty smell, damp patches, or visible mould: Treat this as a moisture problem, not just a comfort issue.
A hygrometer is the cheapest way to avoid guessing. If your flat sits above 60% RH after cooking, showering, and overnight window closure, choose more capacity or improve ventilation.
Also look at your routine. A single person in a well-ventilated studio may only need 10L. A couple drying clothes indoors in a one-bedroom flat may be better served by 12L. A family in a larger flat with daily laundry may need 15L to 20L or more.
Should You Buy a 10L or 12L Dehumidifier?
Choose a 10L model if your apartment has mild condensation, the humidity usually stays below 60%, and you mainly want to keep windows clear in colder months. It is the better choice when you want a compact unit that is easy to move and does not take over a small room.
Choose a 12L model if damp comes back every day, you dry washing indoors, your bathroom has poor extraction, or more than two people live in the flat. The extra capacity gives the unit more headroom, so it can reach the target humidity faster and cycle off sooner when conditions improve.
- 10L: Best for light, seasonal condensation and smaller moisture loads.
- 12L: Better for higher humidity, regular laundry drying, and persistent dampness.
- 15L to 20L: Better for larger open-plan apartments, heavy laundry use, or flats that stay damp even after ventilation.
Pro Tip: If you are stuck between two sizes, choose the larger unit if it has a humidistat. A slightly larger dehumidifier can remove moisture faster, then switch off once it reaches the set humidity.
Do not buy only by the biggest number on the box. A powerful unit can be useful, but it may also be heavier, louder, and harder to place in a small flat. The best choice is the smallest unit that can consistently hold your target humidity during normal daily use.
Where Should You Place It in a Flat?
Place your dehumidifier where moisture builds up most. In many flats, that is near the bathroom door after showers, in the kitchen after cooking, beside an indoor clothes airer, or in a hallway that connects damp rooms.
For whole-flat control, put it in a central spot and keep internal doors open. For laundry drying, use the smallest safe room you can, close external windows and doors, and leave space around the clothes rack so air can move through the fabric.
- Keep air intakes and outlets clear. Follow the model’s manual; many units need roughly 20–30 cm of clearance unless designed for wall placement.
- Close outside windows and doors while the unit is running, otherwise it will keep pulling in damp outdoor air.
- Use extractor fans during and after showers or cooking so the dehumidifier is not doing all the work.
- If using continuous drainage, keep the hose short, sloped, secure, and away from walkways.
- Move furniture slightly away from cold external walls so air can circulate behind it.
Warning: Do not place a standard plug-in dehumidifier where it can be splashed by a bath or shower unless the manufacturer specifically says it is safe for that location. Keep plugs, extension leads, and drainage water away from each other.
If the machine runs constantly but the humidity barely drops, try a more central position, clean the filter, close external windows, and check whether a leak or ventilation fault is adding moisture faster than the unit can remove it.
What Features Matter Most in Apartments?
In an apartment, the most useful dehumidifier features are the ones that save space, reduce noise, and cut daily hassle. Capacity matters, but so does whether you can live comfortably with the unit running.
- Humidistat: Lets you set a target humidity so the unit can cycle on and off automatically.
- Quiet operation: Look for a low dB rating if you will use it in a bedroom, office, or open-plan living space.
- Laundry mode: Useful if you dry clothes indoors, because it runs the fan and compressor more aggressively for a set period.
- Continuous drainage: Helpful if you run the unit for long periods or place it near a suitable drain.
- Auto shut-off: Stops the unit when the tank is full.
- Washable filter: Keeps airflow strong and helps the machine work efficiently.
- Carry handle and manageable weight: Important if you need to move it between rooms.
- Low-temperature operation: Important for cooler rooms, utility areas, or unheated spaces.
For cooler rooms, check the operating temperature. ENERGY STAR advises choosing a model specified for lower-temperature use if the space often falls below 65°F. Compressor units usually perform best in warmer rooms, while desiccant models can be more useful in cooler spaces, though they may use more energy.
How Much Will It Cost to Run?
Do not compare dehumidifiers by “pence per kWh” alone. Electricity is sold by the kilowatt-hour; the appliance cost depends on how many watts the unit uses and how long it runs.
Use this simple formula:
Running cost per hour = wattage ÷ 1,000 × your electricity unit rate
For example, if electricity costs 25p per kWh:
- A 165W unit costs about 4.1p per hour while actively running.
- A 250W unit costs about 6.3p per hour while actively running.
- A 300W unit costs about 7.5p per hour while actively running.
Actual cost may be lower if the unit has a humidistat and cycles off after reaching the set humidity. UK tariffs also change, so check your bill or the latest Ofgem price cap unit rates if you are on a default tariff in Great Britain.
For the best balance, choose an energy-efficient model with a humidistat, use it with windows closed, clean the filter, and tackle moisture at the source. A cheap unit can become expensive if it runs all day because it is undersized or badly placed.
When a Dehumidifier Is Not Enough
A dehumidifier removes moisture from the air, but it does not repair the cause of damp. If you have a leak, blocked extractor fan, missing ventilation, failed window seals, penetrating damp, or water coming through a wall, fix that problem first.
Call your landlord, building manager, or a qualified tradesperson if you notice:
- Water stains that grow after rain
- Wet plaster, bubbling paint, or crumbling skirting boards
- Mould spreading across walls or ceilings
- A musty smell that remains even when humidity is controlled
- Condensation inside double-glazing panes
- Extractor fans that do not pull steam out of bathrooms or kitchens
Note: If you rent, report leaks, failed fans, damp patches, and recurring mould in writing. A dehumidifier can reduce symptoms of excess moisture, but it should not become a substitute for building repairs.
Troubleshooting Common Dehumidifier Problems
If your apartment still feels damp after buying the right size, check the setup before replacing the unit.
- Humidity is not dropping: Close external windows, clean the filter, move the unit closer to the damp source, and check for leaks or constant moisture input.
- Tank fills very quickly: The flat may be wetter than you realised. Keep running the unit, but investigate leaks, wet laundry, poor extraction, or cold-wall condensation.
- Unit is noisy: Make sure it sits level, keep it away from loose furniture, and use quiet mode overnight if available.
- Coils frost or performance drops: The room may be too cold for that model. Check the operating-temperature range.
- Air smells musty: Empty and clean the tank, wash or replace the filter, and inspect nearby walls, carpets, and furniture for mould.
- Clothes dry too slowly: Use a smaller room, space garments apart, close the door, and direct airflow near the drying rack without blocking the unit.
Frequently Asked Questions
How big of a dehumidifier do I need for my apartment?
Most apartments need a 10L to 12L dehumidifier. Choose 10L for mild condensation in a small flat, 12L for regular laundry drying or daily damp, and 15L to 20L for larger flats, open-plan layouts, or humidity that stays above 60%.
Should you use a dehumidifier if you have COPD?
A dehumidifier may help if high humidity, mould, or dust mites make your indoor air feel worse, but it is not a COPD treatment. If you have COPD, asthma, or another lung condition, keep humidity in a healthy range, avoid mould, and follow your clinician’s action plan.
Will a dehumidifier help dry out plaster?
Yes, a dehumidifier can help remove moisture from the air around drying plaster, but do not force fresh plaster to dry too fast with high heat or extreme airflow. Use gentle ventilation, moderate room temperature, and check that the plaster has properly set before decorating.
Can a dehumidifier help with dust mites?
Yes. Dust mites prefer humid conditions, and the American Lung Association recommends keeping home humidity below 50% to help reduce them. A dehumidifier helps, but you should also wash bedding, reduce dust traps, and use allergen covers if dust mites are a problem.
Is a 10L dehumidifier enough for drying laundry indoors?
A 10L unit can help with occasional laundry in a small room, but a 12L or larger unit is better if you dry clothes indoors often. Use a small room, close the door and windows, space clothes apart, and choose a model with laundry mode if possible.
Should I leave a dehumidifier running all day?
You can run it for long periods if the manual allows it and the unit has auto shut-off or continuous drainage. For daily use, set a target humidity instead of running it blindly. Once the room holds around 45% to 50% RH, the unit should not need to work constantly.
Conclusion
In most apartments, the right dehumidifier size is 10L for light condensation, 12L for regular damp or indoor laundry, and 15L to 20L when the flat is larger or the humidity stays high. Check your indoor humidity with a hygrometer, aim for controlled air rather than bone-dry air, and choose a model with a humidistat, laundry mode, washable filter, and safe drainage options. If damp keeps returning despite the right unit, look for the source: leaks, poor extraction, cold walls, or building defects need repair, not just more dehumidification.
Sources
- US EPA Mold Course: Humidity — supports the 30% to 50% RH target and the below-60% mould prevention guidance.
- ENERGY STAR Dehumidifiers Buying Guidance — supports capacity, placement, drainage, operating temperature, and energy-efficiency guidance.
- ENERGY STAR Dehumidifier Testing and Capacity — supports the DOE capacity-rating update and pint/day rating explanation.
- Ofgem Energy Price Cap Unit Rates — supports the running-cost formula context for UK electricity tariffs.
- American Lung Association: Dust Mites — supports keeping humidity below 50% to help reduce dust mites.
- American Lung Association: Extreme Heat and Lung Disease — supports cautious wording for humidity, COPD, asthma, and chronic lung disease.
