Dehumidifier size calculator

There is no proprietary model here and no secret sauce. The starting point is the capacity table the Association of Home Appliance Manufacturers published for residential dehumidifiers: a 500 square foot space that is merely damp needs 10 pints a day removed, rising to 16 pints a day if it is extremely wet. Every additional 500 square feet adds 4 pints a day on top.

Those are the numbers most sizing articles quote, and they are the numbers we start from too. What almost none of those articles say is that the AHAM table is written in pre-2019 pints. It predates the change in how a pint is measured. So we take AHAM’s figure, keep it visible as the old scale answer, and multiply by 0.71 to get the label a unit sold today must carry to be the same machine. Both come out of one function, in one file, so no two pages on this site can disagree about a size.

The dampness bands are doing more work than they look like they are. Moving from “damp” to “extremely wet” at a fixed floor area is a 60% increase in the base figure — which is why an honest calculator asks what the room looks like rather than only how big it is. A dry 2,000 square foot basement and a wet one are not the same problem, and a chart that only takes square feet is answering a question you did not ask.

In 2019 the US Department of Energy changed the conditions a dehumidifier is tested under. The old test ran at 80°F and 60% relative humidity. The new one runs at 65°F and the same 60% RH. Nothing about the machines changed. The thermometer in the test chamber changed.

Cold air holds less water, so there is less water arriving at the coil to condense, so the same appliance pulls a smaller number out of a colder room. The result is that a unit which was honestly sold as a 70 pint dehumidifier in 2018 is honestly sold as a 50 pint dehumidifier now. Same box, same compressor, same coil, different badge.

This matters for sizing because every sizing chart in circulation is downstream of the AHAM table, and the AHAM table is on the old scale. If you read 24 pints off a chart and match it to a modern 24 pint machine, you have bought roughly 41% more dehumidifier than the chart intended — not dangerous, but expensive and prone to short-cycling. Run the same mistake in the other direction, by trusting the badge on a pre-2019 machine already sitting in your basement, and you are about 29% short of what you think you have.

Which direction bites you depends entirely on which scale your number came from, which is why this tool refuses to give you only one. The full explanation of the 2019 change is here, including how to tell which scale a chart or an old unit is written on.

The step per 500 square feet is flat — 4 old-scale pints, every time — which means capacity requirements grow much more slowly than floor area does. Doubling a basement from 1,000 to 2,000 square feet does not double the machine you need. It is worth seeing that laid out, because the instinct to buy proportionally larger is what sells a lot of unnecessary capacity.

A crawl space is not a small basement. It is unconditioned, it runs colder than the house above it, and its moisture does not come from people showering and cooking — it comes up out of the ground, continuously, all year, whether anyone is home or not. Bare soil under a house is an unlimited moisture source in a way that a finished room never is.

The temperature part compounds the problem in a way that connects directly back to the 2019 rating change. A dehumidifier is a refrigeration machine, and its output falls as the air it is fed gets colder — the exact effect the DOE test change measured. A unit rated at 65°F, working in a 55°F crawl space in shoulder season, delivers less than its badge. That is not a defect. It is the same physics, one step further down the thermometer.

So practitioners size crawl spaces up. We apply +40%, which takes a 2,000 square foot very damp space from 17 to 24 modern pints per day. We want to be straight about the status of that number: it is common practice among people who install these things, not a published standard. There is no AHAM crawl space table to convert. If someone quotes you a crawl-space multiplier as though it were a specification, they are dressing up the same judgement call we just showed you the size of.

The other crawl-space specific answer the calculator gives you is a preference for a unit with a built-in condensate pump. Crawl spaces rarely have a floor drain sitting below the machine, and water does not run uphill. A unit without a pump in a crawl space usually means a bucket nobody can reach, which is the same as no dehumidifier at all by about week three.

You will notice this tool never tells you a unit “covers 5,000 square feet”, and never ranks anything on that number. That is deliberate, and it is the second thing we found that most of this category gets wrong.

Coverage claims in square feet are not comparable between brands, because there is no test behind them. We can show you that from our own corpus rather than asserting it: of the 30 units we track with a trustworthy tested capacity, 28 publish a coverage claim, and the claimed area per rated pint runs from about 11 sq ft per pint to 100. Two manufacturers can therefore describe near-identical machines as covering wildly different areas and neither of them is lying, because the phrase means nothing in particular.

Pints per day is different. It is a measured quantity, produced by a defined test at a defined temperature, and it is the thing the DOE changed in 2019 precisely because it is the number that carries meaning. So we size on pints, quote coverage claims only with the brand’s name attached, and never rank on them.

A sizing calculation is a starting capacity, not a guarantee of a dry building. Four things sit outside it, and all four can beat a correctly sized machine.

An open moisture source. A crawl space with bare soil and no vapour barrier, a downspout discharging against the foundation, a failed sump. A dehumidifier can hold the line against ground moisture indefinitely, but it is paying an electricity bill to do a job that a sheet of plastic or a length of downpipe extension would do for good. Fix the source first; size the machine for what is left.

Air you did not account for. The floor area you enter assumes a bounded space. An open stairwell to a humid ground floor, a vented crawl space in a humid climate, a door left open — each of these quietly enlarges the volume the unit is working on, and no number of pints fixes an unbounded room.

Temperature. Every capacity figure discussed here is a rating at a specific temperature. Below roughly 60°F, output falls and many compressor units start cycling into defrost; well below that, a desiccant machine is the right tool and a refrigerant one is not. If your space is cold rather than merely damp, the pint figure is the wrong end of the problem.

Where the water goes. A unit sized correctly and drained badly is a unit that runs until the bucket fills and then stops. Continuous drainage — gravity hose to a drain, or a condensate pump if the drain is above the outlet — is what turns a correct size into a dry space.

We have never run any of these units in a crawl space, and we are not going to tell you that we have. What we do instead is publish the arithmetic, on both rating scales, so you can check it. The AHAM base figures, the 4-pint step, the 0.71 conversion factor and the crawl-space uplift all live in one file with the reasoning written beside them.

The product capacities the calculator matches against are the manufacturers’ published DOE figures, taken from live listings and re-checked before publication. Where a listing’s own title contradicts its specification table, that unit is excluded from every calculation on this site rather than quietly averaged in. Our corpus currently runs from 50 to 160 tested pints per day, which is why a very small room or a very large commercial load will get an honest “nothing we track fits this” rather than a recommendation stretched to cover the gap.

If the calculator returns a number that disagrees with a chart you have in front of you, the first question worth asking is not which of you is wrong. It is which scale the chart is on — and if it does not say, that is the answer.