Dehumidifier Size Calculator
The rating collapses in a cold room.
Work out Dehumidifier Size. The rating collapses in a cold room. Names the misconception directly.
From a proper heat loss calculation rather than a rule of thumb
Dehumidifier output recommended
0.86 kW
2,948 BTU/h · Undersized for this load
Dehumidifiers are rated by litres of water removed per day at a stated temperature and humidity, and that rating falls sharply in a cold room — a unit rated at 20 °C can manage a fraction of it in an unheated garage. Refrigerant models struggle below about 15 °C; desiccant models work down to near freezing and use more energy. Sizing from room area alone ignores where the moisture is actually coming from, which is usually the more productive question.
How the Dehumidifier Size Calculator works
Dehumidifier capacity for a space, with an oversizing check. Ratings are quoted at a stated temperature and humidity, and they fall sharply in a cold room — which is usually exactly where one is needed.
Also known as: what size dehumidifier do i need · dehumidifier litres per day sizing · dehumidifier for a damp room · dehumidifier capacity calculator
Ratings are measured somewhere warmer than your garage
Dehumidifier capacity is quoted in litres per day at a stated temperature and humidity, commonly 30 °C and 80% relative humidity. Those are laboratory conditions, and they are considerably warmer and wetter than the room most units end up in.
In a 12 °C garage a refrigerant unit manages a fraction of its rating, because less moisture condenses on a coil that is closer to the ambient temperature. The rating is not dishonest; it is simply measured somewhere else.
Desiccant units absorb moisture chemically rather than condensing it, so they keep working near freezing. They use more energy per litre removed, which is the trade — and in an unheated space they are often the only thing that works at all.
Find the source before sizing the machine
Occupants generate several litres of moisture a day through breathing, cooking, showering and drying laundry indoors. That is normal and it is what ventilation is for. Rising damp, penetrating damp and leaks are not, and no dehumidifier fixes them.
Extract ventilation at the source — kitchen and bathroom — removes moisture before it spreads through the house, which a dehumidifier standing in a hallway cannot. It also costs almost nothing to run.
Running a machine indefinitely to manage a cause that could be fixed is the expensive path. The diagnostic question is whether the moisture is being generated inside or arriving from outside, and the answers are very different.
Mould, corners and the right humidity
Mould grows in corners because corners are colder than the middle of a wall — more surface loses heat to less internal volume. Relative humidity is therefore highest there even when the room average is fine, which makes it a thermal bridge problem as much as a moisture one.
The comfortable band is roughly 45 to 55% relative humidity. Above about 65% supports mould growth; below 40% causes timber shrinkage, static and dry airways. Running a dehumidifier past its target wastes energy and dries furnishings.
Essentially all the electricity a dehumidifier uses ends up as heat in the room, plus the latent heat released as water condenses. In winter that is a partial benefit rather than a pure cost, which is worth knowing when comparing it against ventilation.
Where to go next
The Dehumidifier Size question rarely arrives on its own. These are the ones that usually come with it:
- HVAC BTU Calculator — Oversized is worse than slightly undersized.
- Attic Ventilation Calculator — Intake must exceed exhaust, not the reverse.
- Heat Loss Calculator — Floor area alone cannot tell you the answer.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
What size dehumidifier do I need?
It depends on the space and on how much moisture is entering it, which is why the load per square metre is an input. A damp cellar and a room drying laundry are very different problems.
Why do ratings not match real performance?
Because they are measured at a specific temperature and humidity, commonly 30 °C and 80% relative humidity. In a 12 °C garage a refrigerant unit manages a fraction of its rating.
What is the difference between refrigerant and desiccant?
Refrigerant units condense moisture on a cold coil and work well in warm rooms. Desiccant units absorb it chemically and keep working near freezing, at the cost of more energy per litre removed.
Should I find the source of the damp first?
Almost always. A dehumidifier manages symptoms; rising damp, a leak or inadequate ventilation are causes, and running a machine indefinitely is more expensive than fixing them.
Does a dehumidifier heat the room?
Yes — essentially all the electricity it uses ends up as heat, plus the latent heat released as water condenses. In winter that is a partial benefit rather than a pure cost.
What humidity should I aim for?
Around 45 to 55% relative humidity is comfortable and discourages mould. Below about 40% causes shrinkage in timber and discomfort; above 65% supports mould growth.
Where does the moisture in my house come from?
Occupants breathing, cooking, showering and drying laundry indoors — several litres a day for a family. Rising damp, penetrating damp and leaks are additional and need fixing rather than dehumidifying.
Is ventilation better than a dehumidifier?
In many cases yes, and it costs nothing to run. Extract ventilation at the source — kitchen and bathroom — removes moisture before it spreads, which a dehumidifier cannot.
Why does mould grow in corners?
Because corners are colder than the middle of a wall, through the geometry of heat loss, so relative humidity is highest there. It is a thermal bridge problem as much as a moisture one.
Should I run a dehumidifier continuously?
Better to run it to a humidistat setpoint. Continuous running past the target wastes energy and can dry timber and furnishings below a comfortable level.
Do dehumidifiers help drying laundry indoors?
Substantially, and it is one of the clearest uses. Drying a load indoors releases a couple of litres of water into the house, which has to go somewhere.
What humidity is too low?
Below about 40% causes shrinkage in timber, static, and dry airways. The comfortable band is roughly 45 to 55%, and mould risk rises above 65%.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/dehumidifier-size-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Dehumidifier Size Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/dehumidifier-size-calculator" style="color:#64748b">Dehumidifier Size Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.
Related calculators
HVAC BTU Calculator
Oversized is worse than slightly undersized.
OpenAttic Ventilation Calculator
Intake must exceed exhaust, not the reverse.
OpenHeat Loss Calculator
Floor area alone cannot tell you the answer.
OpenRing Size Calculator
Finger measurement to US, UK, EU and Japanese sizes.
Open