What the pint number actually measures
A dehumidifier's capacity is quoted in pints of water removed per 24 hours, and that figure is the result of a laboratory test at a defined temperature and relative humidity. It is a comparison figure, in the same way a car's fuel economy rating is a comparison figure. It is not what the machine will do in your basement.
This matters more than usual right now because the test condition changed. For decades the ratings people are used to were measured at 80°F and 60 percent relative humidity, which is why 30, 50 and 70 pint units were the familiar sizes. The current US test condition is 65°F and 60 percent, which is cooler and therefore harder, so the same hardware earns a smaller number. A machine that would have been sold as 70 pints is now labelled somewhere around 50. Nothing about the machine changed.
The practical consequence is that a sizing table you find online may be on either scale, usually without saying which. This page shows both: the requirement on the older scale, because that is what the long-standing guidance table was built on, and an approximate translation to the current scale so you can shop with it. The translation is a rough factor, not a conversion — it varies by machine.
The sizing table, and its limits
The figures underneath this calculator come from the long-established guidance table that pairs floor area with a description of how damp the space is. On the older 80°F scale it runs like this:
| Condition | 500 sq ft | 1,000 | 1,500 | 2,000 | 2,500 |
|---|---|---|---|---|---|
| Moderately damp — musty in humid weather | 10 | 14 | 18 | 22 | 26 |
| Very damp — damp patches, constant smell | 12 | 17 | 22 | 27 | 32 |
| Wet — seepage, wet spots after rain | 14 | 20 | 26 | 32 | 38 |
| Extremely wet — standing water | 16 | 23 | 30 | 37 | 44 |
The usual adders on top are five pints each for several regular occupants, for a door or opening to outside or to an unconditioned space, and for a washer or dryer in the room. The calculator interpolates between the columns rather than making you round to the nearest one.
What the table cannot see is the moisture source, and the moisture source is what actually sets the load. A basement whose walls are damp because of hydrostatic pressure from a high water table has a completely different load from one that is damp because humid summer air is condensing on cool walls, even at the same square footage and the same subjective description. Floor area is a proxy for the volume of air to be treated, not a measure of how much water is arriving.
Temperature, and why the cold basement disappoints
A refrigerant dehumidifier works by making a coil colder than the dew point of the air, condensing water onto it, and draining it away. The colder the room, the colder that coil has to be, and the closer it gets to freezing. In a cool space the coil ices up, and the machine has to stop dehumidifying and run a defrost cycle to clear it. The colder the room, the greater the share of its running time goes into defrost rather than into removing water.
The effect is not marginal. A unit rated in a warm laboratory can deliver a fraction of that in a 55°F crawlspace, and many will not run at all below about 41°F. Some units include an active defrost that helps; some are specifically designed and rated for low temperature operation and say so. Desiccant dehumidifiers use an entirely different mechanism — a moisture-absorbing wheel rather than a cold coil — and hold their capacity down into temperatures where a refrigerant unit has given up. They use more electricity per pint and put noticeable heat into the room, which in an unheated garage is often a feature rather than a fault.
The allowance this calculator applies for a cool or cold space is deliberately rough. It nudges you toward a larger machine rather than pretending to model the physics.
Before you buy the machine at all
There is a category of damp problem where the correct dehumidifier is no dehumidifier. If water is arriving faster than a machine can remove it, running one is an expensive way to lose slowly, and the electricity bill for a unit running continuously through a humid season is not trivial. Go outside and look at where the water comes from first: ground sloping back toward the foundation, downspouts discharging at the wall instead of away from it, blocked gutters, window wells without drainage, a failed sump. Inside, look for an uninsulated cold water line dripping onto the floor, a dryer venting into the space instead of outside, and whether a crawlspace has a ground vapour barrier — bare soil in a crawlspace evaporates a startling quantity of water into the building every day, and a sheet of poly over it is the cheapest dehumidifier ever sold. Deal with those and the machine you need afterwards is smaller, runs less, and costs less to own. A dehumidifier is the right answer for the residual humidity that remains once the actual water is under control, and the wrong answer for the water itself.
Questions people ask
What size dehumidifier do I need for a 1,000 square foot basement?
For a moderately damp basement — the musty smell in humid weather, no visible water — the long-standing guidance gives about 14 pints a day on the older 80 degree rating scale, which is roughly 10 to 11 pints on the current 65 degree scale. In practice nobody sells a machine that small for a basement, so you would be shopping at the bottom of the range, around a 20 to 22 pint current-scale unit. Add five pints for each of: regular occupants, an opening to outside, or a washer and dryer in the space. And if the basement runs cool, size up, because a refrigerant unit loses capacity as the room gets colder. If the basement is actually wet rather than damp, the answer changes and so does the problem.
Why did dehumidifier pint ratings change?
The US test procedure was revised. Ratings were historically measured at 80 degrees Fahrenheit and 60 percent relative humidity; the current test condition is 65 degrees and 60 percent. Cooler air holds less moisture and is harder to dry, so the same machine removes fewer pints under the newer test and gets a smaller number on the label. A unit that would have been marketed as 70 pints is now typically labelled around 50. This causes real confusion when comparing an older machine, or an older sizing chart, with what is on the shelf now. When comparing two units, check they were rated the same way, and be suspicious of any listing that quotes a large pint figure without naming the condition.
Will a dehumidifier work in a cold garage or crawlspace?
A standard refrigerant unit will work poorly and may not run at all. It condenses water on a coil colder than the surrounding air, so in a cold room that coil frosts, and the machine spends progressively more of its time defrosting instead of drying. Many shut off below roughly 41 degrees. Two options work: a refrigerant unit specifically rated for low temperature operation, which will say so on the specification rather than in the marketing, or a desiccant dehumidifier, which uses a moisture-absorbing wheel instead of a cold coil and keeps working at temperatures where refrigerant units stop. Desiccant units cost more to run per pint and add heat to the space. For a crawlspace, also check the ground is covered with a vapour barrier before deciding a machine is the answer.
Is it better to get a bigger dehumidifier than I need?
Moderately oversizing is much less harmful here than it is with an air conditioner, and it is often the right call. A larger unit reaches the target humidity faster and then cycles or throttles, and because you are controlling humidity rather than temperature there is no equivalent of the short-cycling comfort problem. A larger machine also tends to be more efficient per pint removed and quieter at a given output. What you pay is more up front, more space, and more electricity if it is genuinely far too big for the load. The stronger argument for going a size up is the temperature one: if the space is cool, the rated capacity is not the capacity you will get, and buying larger is how you cover that. Whatever the size, arrange a gravity drain — a big unit in a damp space fills a bucket faster than anyone will empty it.