Why square metres are not enough
Two bedrooms with the same floor area can require different treatment. One may be warm with a short overnight humidity rise; the other may have cold external walls, two occupants and laundry drying nearby. Ceiling height, open doors and air leakage also change the volume being treated.
Retail charts can help screen products, but they are not a moisture calculation. Ask what enters the air, how often, at what temperature and how quickly you need it removed.
Start with four measurements
Calculate approximate room volume by multiplying length, width and height. Log temperature and RH across the coldest representative week. Note every major moisture event and whether the room is closed or connected to the rest of the home.
Then define the outcome: preventing morning window condensation is different from drying a 5 kg wet laundry load before bedtime. Capacity only has meaning against a target and deadline.
- Room volume and connected air volume
- Minimum and typical operating temperature
- Moisture source, frequency and approximate wet load
- Target RH or completion deadline
Translate the problem into a size class
For one heated room with moderate background moisture, begin by comparing compact compressor models. For connected rooms or frequent family laundry, compare the next capacity class with higher airflow and a larger tank. For spaces persistently below about 10°C, compare desiccant performance as well as compressor models.
This is a shortlist method, not a guarantee. A machine serving open rooms is affected by the whole air path. Closing internal doors can reduce the task, but only when doing so is compatible with safe ventilation and the actual moisture source.
| Observed task | First category to compare | Evidence to verify |
|---|---|---|
| One heated room; modest persistent RH | Compact compressor | Low-speed noise and extraction near 20°C / 60% RH |
| Frequent family laundry | Higher-airflow compressor | kWh and time per complete load |
| Several connected heated rooms | Larger compressor class | Air path, runtime and tank/drainage |
| Cold garage or utility room | Desiccant plus compressor comparison | Extraction and power around 10°C |
| Leak or rain penetration | Repair, not appliance sizing | Building investigation |
Read litres per day correctly
Headline capacity is normally measured in warm, humid air. Meaco's own data illustrates the gap: its Arete Two 10L publishes 4.19 L/day at 20°C and 60% RH, while the 20L publishes 8.50 L/day. At 10°C and 60% RH those figures fall to 1.38 and 3.35 L/day.
That condition-specific table is more useful than pretending that either machine will collect its model number every day. Prefer manufacturers that disclose test conditions and multiple points.
Avoid both under-sizing and reflexive over-sizing
An undersized machine may run continuously, fill frequently and miss the required deadline. An unnecessarily large machine costs more, takes more space and can be louder, although its compressor may cycle once the target is reached.
If two adjacent sizes share the same controls, compare total kWh for the task and return conditions. Choose enough headroom for the recurring worst normal load—not a one-off flood and not an invented whole-house promise.
Stop sizing when the problem is structural
A dehumidifier manages airborne moisture. It cannot repair a roof, failed seal, plumbing leak, bridged damp-proof course or severe thermal bridge. Damp that worsens after rain, local staining or standing water should change the investigation before it changes the appliance size.
Extensive mould or health concerns need appropriate professional advice. Keep readings and photographs, especially in rented housing.
Common questions
Questions this guide should answer
Is a 10L dehumidifier enough for a bedroom?
Often it is a sensible category to compare for one heated bedroom, but occupants, temperature, connected rooms and moisture source matter more than the label alone.
Do I need a 20L dehumidifier?
It is easier to justify for frequent family laundry, larger connected spaces or a measured task that a compact model cannot complete efficiently.
Can one dehumidifier treat a whole house?
Only if air can move effectively to the machine and the moisture load is within its real capacity. Closed doors and distant cold rooms often make whole-house claims unrealistic.
Should I oversize so it runs less?
Some headroom can reduce completion time, but larger is not automatically more efficient. Compare condition-specific extraction, airflow, power and total kWh for your task.
Sources and review trail
We favour primary public guidance and link to the exact source so you can inspect it yourself.