What size dehumidifier do I need?
Sized by how many pints of water it can pull out of the air in a day. Enter your details and the answer appears with every step of the arithmetic shown, so you can check it against any quote you have been given.
This is an estimate from the standard industry rule of thumb, not an engineering calculation. For anything you are about to spend money on, have the figure confirmed by a load calculation.
How dehumidifier capacity is worked out
Capacity depends on both the floor area and how damp the space actually is. A space that merely smells musty needs far less capacity than one with standing water or visible wet patches on the walls.
The calculation is shown step by step under the result rather than hidden behind the answer. That is deliberate: most sizing guides online are lead capture forms that give you a number and a phone call, and a number you cannot check is a number you cannot argue with when a contractor disagrees. Every adjustment is listed, so you can see what changed the answer and by how much.
Nothing you type is sent anywhere. The arithmetic runs in your browser, there is no form to fill in, and no one calls you afterwards. That is the whole reason this page exists — you should be able to what size dehumidifier do i need without trading your phone number for it.
What going too big or too small actually costs
Overcapacity is not harmful in the way it is for air conditioning — the unit simply reaches the target humidity and cycles off. It costs more to buy and takes more space, but it does not damage anything.
An undersized dehumidifier runs constantly and never reaches the target, which shows up as a high electricity bill alongside a damp basement. This is the failure mode people actually hit.
Both directions cost money, which is why it is worth ten minutes to what size dehumidifier do i need before you take a quote at face value. The asymmetry is worth knowing though: for most equipment on this site, being slightly under is a comfort problem you notice on a handful of extreme days a year, while being significantly over is a permanent efficiency and equipment-life problem you pay for every month.
Before you buy
If the dampness is coming from a specific source — a leaking foundation, a failed gutter, an unvented dryer — no amount of dehumidifier capacity fixes it. Find the source first.
Where a rule of thumb stops being enough
Every number on this page comes from the standard industry rule of thumb, and a rule of thumb is not an engineering calculation. The real method is a Manual J load calculation, which models your wall and attic insulation, your window area and orientation, how leaky the building envelope is, and how much your ducts lose. It routinely lands a size away from any square-footage estimate — in either direction, which is the part people forget.
So treat this as a sanity check rather than a specification. Ten minutes to size it properly tells you whether a quote is in the right neighbourhood, which is genuinely useful — it is enough to catch a contractor who has quoted you two sizes up because that is what was on the truck. It is not enough to order equipment from.
Capacity ratings changed in 2019. The DOE test conditions were revised to a cooler, more realistic temperature, so a unit rated at 50 pints under the current standard is roughly equivalent to a 70 pint unit under the old one. Comparing an old listing with a new one without knowing this leads to buying far more capacity than needed.