Last Updated on October 24, 2026 by Umar Farooq
How many dehumidifiers do I need? One for each separate damp zone, which in most houses means one or two, not one per room. A zone is a stretch of air that the machine can actually reach, and that is a smaller idea than “the house” and a bigger one than “the room”.
The rest is three checks you can do for the price of a hygrometer, and none of them involves a spreadsheet.
How many dehumidifiers do I need if the internal doors stay open?
Often just one. A single machine on a landing or hallway with the internal doors open treats every room that opens onto it, because moisture drifts towards the drier air the machine creates. I covered the mechanism in where to put a dehumidifier, so I will not repeat it here.
The catch is the phrase “every room that opens onto it”. Close a door and you have drawn a line the machine cannot cross, and each closed-off room becomes its own zone. That is why two people with identical houses can need one machine or three: one of them leaves the doors open and the other has a draught excluder the length of the hall.
Check 1: count the air spaces, not the rooms
Walk round the house and sort the rooms into spaces that share air:
- Open-plan or open-door areas on one floor count as one zone.
- Each floor is usually its own zone, because the air has to climb or descend a stairwell to reach the next one. That is my reasoning rather than a measured figure, and it is the reason I treat floors separately until a hygrometer says otherwise.
- A basement or cellar is its own zone almost always. It is colder, it is in contact with the ground, and it tends to have a door at the top of the stairs.
Most houses come out as two or three zones on paper. That is the ceiling, not the number to buy.
Check 2: put a hygrometer on every level
Before buying a second machine, buy a cheap hygrometer for each zone. The EPA suggests keeping indoor humidity below 60 percent, ideally 30 to 50 percent, and it says humidity can be measured with a moisture or humidity meter, which it describes as low cost. That gives you the threshold, and the hygrometer gives you the answer.
Leave one in the middle of each zone for a week, away from the kitchen and the bathroom door, and read it morning and evening. You want what the room sits at, not the spike after a shower. A cheap meter that reads high will happily sell you a second machine, so put two in the same spot for an hour first and see whether they agree.
Then apply a simple rule, which is mine and not a standard: a zone that stays at 60 percent or more across the week earns a machine, and one that sits below it does not. A house that reads 52 percent downstairs and 65 percent in the cellar needs one dehumidifier, in the cellar. It does not need two “to be safe”.
Check 3: size each zone separately, and label the arithmetic
Capacity is the part where people reach for a total. Do not. The numbers are per space, and they do not add up across a closed door.
For portable units, ENERGY STAR gives these ranges for spaces under 2,000 square feet: 20 to 30 pints a day for slightly to moderately damp, 25 to 40 for very damp, and 30 to 50 for wet. Above 2,000 square feet it is 30+, 40+ and 50+. It also says it is better to oversize than undersize. The full room-size chart is in what size dehumidifier do I need, so I will not copy it.
What I will add is the arithmetic of the multi-zone case, which is mine and not ENERGY STAR’s:
- A very damp basement reads as a 25 to 40 pint job.
- A moderately damp first floor reads as a 20 to 30 pint job.
- Those are two jobs, so two machines in those ranges. They are not one 45 to 70 pint machine, because one big machine in the basement does nothing for the bedroom.
Pint labels also changed in 2019, when the test temperature dropped to 65°F from 80°F, so an older label overstates what a unit does in a cool space. ENERGY STAR’s testing page explains the change.
A damp basement plus living space is usually two jobs
This is the commonest case where one machine is not enough, and also the one most often bought wrongly. People put the biggest unit on the ground floor, hope the basement catches up, and conclude that dehumidifiers do not work.
Treat the basement as a space apart. It gets a machine placed centrally and off the floor, sized from the wet bands above, and running whenever its own hygrometer says so. The living space gets a separate reading, and its own machine only if it fails the 60 percent test. The basement guide goes through the buying decision for the cellar half.
Flats, bungalows and open-plan homes: one machine often does it
With one level and few walls there is only one zone, and a single correctly sized machine in the middle is usually the whole answer. A flat is the clearest case, and the apartment humidity guide covers the habits that matter more than a second unit.
When the answer is a whole-house unit
If you have counted three zones and all three read above 60 percent, you are in whole-house territory. ENERGY STAR says whole-home dehumidifiers are installed to use the home’s air ducts to dehumidify one or more rooms, and that they suit relatively well-sealed homes with central air where humidity is consistently above 55 percent. That is an HVAC job for a qualified installer, which is outside what this site covers, so the sensible next step is to ask a professional rather than follow a guide from me.
Buy one, read the meter for a fortnight, then decide on a second
The answer to how many dehumidifiers you need is the number of levels that stay above 60 percent after you have fixed the water, which is nearly always one or two. A third machine needs a reading that justifies it, and a fourth is usually a sign a leak or a missing extractor fan is being treated as a humidity problem.
So buy one, put it where the worst reading is, keep the other hygrometers running, and look again after two weeks. If another zone still fails, buy the second machine with a clear conscience. And there is no honest per-square-foot rule: how wet a zone is and whether the air reaches the machine are not in the floor plan.
FAQ
How many dehumidifiers do I need for a 3-bedroom house?
Usually one or two. One if the floor is open and the hygrometer stays under 60 percent elsewhere, two if there is a damp basement or a closed-off room that fails the test. Three bedrooms tell you the size of the house, not the number of damp zones.
Can one dehumidifier dry a whole house?
It can hold a single floor with the internal doors open, but it is a poor bet for several floors. Each level should be read separately, and a damp cellar needs its own machine. For a sealed home with ducted air, a whole-house unit installed by an HVAC professional is the other route.
Is it worth having a dehumidifier in every room?
No. A machine in every room is the expensive way of ignoring the doors. Put machines in zones that stay damp, and leave dry ones alone, because a dry room with its own machine only wastes electricity.
How do I know if I need a second dehumidifier?
If a different zone stays at 60 percent or more across a week of readings after your first machine has run for a fortnight, add one there. If it only spikes after showers or cooking, a fan and a closed door cost less.
Can you have too many dehumidifiers?
Yes, because machines in a space that is already dry are just heaters with a tank. The EPA range runs 30 to 50 percent ideally, and below that you are paying to over-dry the air. Stop adding units once the hygrometers are below 60 percent.
Do I need a bigger dehumidifier or two small ones?
Two small ones only when there are two zones. In a single open space, one correctly sized machine beats two small ones in the same room. Across a closed door, two are the better choice.
