What size dehumidifier do I need?

Why not just use a square-footage table

Because the square footage on a dehumidifier listing is not a measurement. In our registry of 21 current units, machines with identical certified capacity claim wildly different coverage:

It also ignores the two variables that actually determine how much water a machine has to remove: how wet the space is, and how cold it is. A dry 2,000 sq ft basement is a smaller job than a wet 800 sq ft one.

The method

1. 1Measure, for a week. A hygrometer costs about ten dollars. Put it in the middle of the room, away from a wall, and read it morning and evening. One reading tells you nothing; a week tells you the range.
2. 2Note the temperature at the same time. If the space sits below about 60F for much of the year, a compressor unit is going to struggle and you should read desiccant versus compressor before choosing a capacity at all.
3. 3Look for liquid water, not just humidity. Damp to the touch on the floor or walls, a white bloom on block, staining, or anything actively growing means the moisture is arriving as water rather than vapor. That changes the answer and, more importantly, means a dehumidifier is managing a symptom.
4. 4Then pick the class. Using the table below, which is built on humidity and floor area together rather than area alone.

The three cases that override the table

1. A cold space

Below roughly 60F a compressor dehumidifier's output falls sharply, and near 41F it ices and effectively stops. In an unheated garage, an outbuilding or a winter crawl space, the correct capacity is whatever a desiccant unit gives you, because a nominally larger compressor unit will deliver less. Ivation's own figures put a desiccant rotor at 11.8 pints a day at 41F against a compressor's 2.9.

2. Nowhere to drain

If the machine will run on its bucket, capacity is limited by how often you will empty it rather than by the room. A 50-pint unit in a damp basement fills a 1.7 gallon bucket in about six hours. That is not a schedule anyone keeps, so either arrange drainage or accept that the unit will spend most of its life switched off with a full tank. Drainage.

3. Liquid water

If water is entering the space - through the slab, through the walls, from a leak - a dehumidifier is running against an unlimited supply. It will work continuously, cost you the full running figure every month, and never reach its target. Gutters, grading, a working sump and a plumber come first. The machine is for the humidity that remains afterwards.

What each class costs, so the decision has a price

The 30-32 pint class at 318 W is about $21 a month on the same twelve-hour schedule, against the 50-pint class's $32. That $11 gap is the actual cost of going a size up - and it disappears entirely if the smaller machine is undersized, because an undersized unit never idles and runs at full draw all day. Calculate yours.