What Size Dehumidifier Do I Need?
Enter the room dimensions and how damp the space feels, and the calculator returns the capacity to buy in pints and litres per day. It runs entirely in your browser, asks for nothing, and accounts for the 2019 rating change that makes most online dehumidifier sizing charts read wrong today.
How dehumidifier sizing actually works
Dehumidifiers are rated by how many pints of water they can pull from the air in 24 hours, so “what size dehumidifier do I need” resolves to a number of pints per day before it becomes a product choice. That number depends on two things: the volume of the space, and how much moisture its air is holding. Floor area stands in for volume on most sizing charts, which quietly assumes an 8 foot ceiling; this calculator scales for taller rooms, because a 12 foot ceiling adds half again as much air to dry.
Dampness matters more than size. A slightly musty 1,500 square foot living area needs less extraction than a wet 600 square foot basement, which is why the calculator asks how the space feels and smells rather than only how big it is. The four dampness levels follow the bands used in AHAM guidance, from moderately damp, where the space smells musty only in humid weather, up to extremely wet, where water stands or laundry dries indoors every day.
Moisture sources add load on top of the room itself. A load of laundry releases around 5 pints of water as it dries. People add roughly a pint each per day through breathing and washing. Ground moisture works through basement and crawl space floors continuously. The checkboxes add these to the total rather than leaving you to guess a margin.
The pint rating changed in 2019, and most charts did not
Until 2019, US capacity ratings were measured at 80 degrees Fahrenheit and 60 percent relative humidity. The Department of Energy then moved the test to 65 degrees at the same humidity, which is cooler air holding less water, so every machine collects fewer pints during the test without changing at all. The 70 pint unit of 2018 and the 50 pint unit of today are broadly the same machine.
This is the single most common way people buy the wrong size. An older sizing chart says a damp basement needs 70 pints, the shop sells nothing bigger than 50, and the buyer either overspends on a whole-house system or assumes portable units cannot cope. Once you have the number, our guide to the best dehumidifiers for mold covers which units hold up in a cold basement, which need a pump, and which one not to buy. The calculator above works in current-standard pints and shows the old equivalent alongside, so its numbers match what is actually on shelves. When you compare models, check which standard the listing uses; litres per day, common on UK and EU listings, has no such split, and the calculator gives litres for that reason.
Dehumidifier size by room
Basement
Basements combine the three loads that drive capacity: ground moisture through the slab and walls, cooler air that reaches saturation sooner, and poor air exchange with the rest of the house. Treat any basement as at least one dampness level worse than it looks when choosing a dehumidifier size for a basement. A typical 800 to 1,200 square foot space lands at 20 to 30 pints per day, and the basement checkbox in the calculator adds the ground moisture load. Position the unit away from walls, keep the drain hose falling continuously if you use one, and expect the first week to run hard as stored moisture leaves the structure and its contents.
Bedroom
A standard bedroom of 120 to 200 square feet needs far less capacity than the smallest common units provide, so a 20 pint machine covers nearly every bedroom with a wide margin, and quietness becomes the deciding factor instead. Sleeping adds around a pint of moisture per person per night, which is why bedroom windows stream on winter mornings while the rest of the house stays clear. Condensation on the inside of bedroom windows is a humidity signal worth acting on, since the same moisture is reaching colder wall corners you cannot see behind furniture.
Bathroom and laundry area
Bathrooms produce moisture in bursts rather than steadily, and the right tool for a burst is extraction at the source: a fan sized to change the room's air at least eight times an hour, run through the shower and for 20 to 30 minutes after. A dehumidifier earns its place where laundry dries indoors, which releases about 5 pints per load into whichever room it hangs in. If growth keeps appearing overhead despite the fan, the guide to mold on a bathroom ceiling works through whether condensation or a leak is putting it there.
Whole house and crawl space
Above roughly 2,500 square feet, or wherever the damp is spread across several rooms and floors, single portable units stop being practical and the choice moves to a ducted whole-house dehumidifier or one unit per problem area. Crawl spaces are their own case: sizing follows the footprint, but the lasting fix is usually a ground vapour barrier with a compact crawl space unit, because bare earth releases water continuously no matter how much air you dry.
Getting the most from the machine
- Fix leaks and water entry first; the dehumidifier holds the line after the source is dealt with
- Set the humidistat to 30 to 50 percent and let the unit cycle, rather than running it flat out
- Close windows and doors in the space while it runs, so it dries your room and not the street
- Use the drain hose for continuous running; a tank that fills overnight stops the machine at the worst time
- Check the reading against a separate cheap hygrometer, since built-in sensors drift
- Clean the filter monthly; a clogged filter cuts extraction well below the rated capacity
- Buying from a pre-2019 sizing chart without checking which standard the shop listing uses
- Running a dehumidifier against an active leak, which loses to the water every time
- Expecting it to remove mold already on walls; existing growth needs cleaning or removal separately
- Drying the air below 30 percent, which irritates airways and cracks wooden furniture for no gain
- Placing the unit hard against a wall where its airflow has nowhere to go
- Treating a musty smell as solved because the tank is filling; the smell has a source worth finding
Why the 30 to 50 percent target matters
Mold does not need a flood. Sustained relative humidity above 60 percent lets colonies establish with no leak at all, and some species manage it at 70 percent with no liquid water anywhere. Between 30 and 50 percent, the air is too dry for growth and still comfortable to live in, which is why that range appears throughout our guides and why a correctly sized dehumidifier, cycling on its humidistat, is the cheapest standing defence a damp-prone home can buy.
A persistent musty smell with no visible growth usually means moisture is feeding something out of sight, and drying the air will suppress the smell without settling where it comes from. The guide to finding the source of a musty smell gives the search order. Whether the damp has already reached the air you breathe is a separate question: condensation forming on windows and cold corners is the visible end of it, and our dew point calculator shows the humidity your surfaces can tolerate at their temperature. If a damp home has been paired with fatigue, brain fog or congestion that improves when you are away, the free 15-minute screening test checks for one measurable effect of biotoxin exposure, and the symptom reference covers the pattern in full.
Dehumidifier sizing questions
What size dehumidifier do I need for a 1,000 square foot basement?
A very damp 1,000 square foot basement needs around 20 pints per day of extraction under the 2019 DOE standard, so a 20 to 22 pint unit fits, and a 30 pint unit adds margin for laundry or a humid summer. If the basement is wet rather than damp, with sweating walls or seepage after rain, move up to a 30 or 35 pint unit and deal with the water entry as well.
Is a bigger dehumidifier always better?
Extra capacity within reason is useful because the unit reaches the target humidity faster and cycles less, which also saves energy. Buying far beyond your need wastes money up front, and a unit without a humidistat that keeps running past 50 percent dries the air below the comfortable range. Size from the space and the dampness level, then buy one step up if you are between sizes.
Why does the same dehumidifier show two different pint ratings?
The US test standard changed in 2019. Capacity used to be measured at 80 degrees Fahrenheit and 60 percent humidity; it is now measured at 65 degrees and 60 percent, which is cooler and closer to real basement conditions, so the same machine collects less water during the test. A unit sold as 50 pints under the current standard is roughly the machine that was sold as 70 pints before, and retailers still mix the two figures.
Will a dehumidifier get rid of mold?
It removes the moisture that new growth needs, and holding a space at 30 to 50 percent relative humidity stops most species establishing. It does nothing to colonies already on a surface, which have to be cleaned or removed separately, and it cannot keep up with an active leak. Dry the air, fix the water, and treat the existing growth as three separate jobs.
What humidity should I set my dehumidifier to?
Set the humidistat between 30 and 50 percent relative humidity. Below 30 the air gets uncomfortably dry for skin, eyes and wooden furniture. Above 60 percent, sustained, mold can establish with no leak at all. In winter, the lower half of the range also keeps windows clearer, because cold glass condenses moisture that warmer surfaces hold as vapour.
Damp Home, Unexplained Symptoms?
Drying the air protects the building. If the damp has been around a while and you feel better away from home, the free Visual Contrast Sensitivity test screens for a measurable effect of biotoxin exposure in about 15 minutes.