What Size Dehumidifier Do I Need?
Your square footage and how damp the space actually is give you the pints-per-day capacity to buy — on today's post-2019 rating scale, with the old-to-new conversion so you don't buy by the number on your old unit.
How this tool works
The matrix is not ours — it is EPA's. The starting point here is the sizing table ENERGY STAR publishes on its own dehumidifier buying-guidance page. It crosses three named dampness conditions, each with a relative-humidity band, against two room sizes:
- Slightly to moderately damp (50–75% RH; "space feels damp and has musty odor that may be intermittent") → 20–30 pints/day under 2,000 sq ft, 30+ above it.
- Very damp (75–90% RH; "space consistently feels and smells damp, damp spots appear on walls and floors") → 25–40 pints/day, 40+ above 2,000 sq ft.
- Wet (90–100% RH; "walls or floor sweat, or seepage is present") → 30–50 pints/day, 50+ above 2,000 sq ft.
Notice how coarse the real standard is. ENERGY STAR has exactly two square-footage columns. The tidy 500/1,000/1,500/2,000 sq ft grids you see on retail sites are the retailer's invention, not a published standard, so we do not pretend to have one: we place you inside ENERGY STAR's published range by square footage and tell you on the page when a number is an interpolation rather than a quoted figure.
The 2019 rating change is the thing that trips everyone up. DOE finalised new dehumidifier standards effective June 2019 built on a new metric, Integrated Energy Factor, and — crucially — a new test procedure that runs portable units at 65°F instead of 80°F, because that is closer to a real basement. Cool air holds less water, so the same physical machine now declares a smaller capacity. ENERGY STAR publishes the rough correlation: an old 70-pint unit is about 40–45 pints today, an old 50-pint is about 30. Every number on this page is on the current scale. If you are replacing an old dehumidifier, do not match the label on the old one — that is how people end up with twice the machine they need, and it is also why a "30-pint" unit today is not the weakling it sounds like.
What we add, and we are flagging it as ours. Three adjustments sit on top of the published matrix and they are our engineering judgement, not part of the ENERGY STAR table: a basement multiplier (cooler air, damp soil contact), a crawl-space multiplier (bare earth keeps feeding moisture in and you cannot reach a bucket), and a climate multiplier (humid outdoor air reloads the space every time a door opens; genuinely dry climates need less). We then round up to a capacity you can actually buy, because ENERGY STAR's own advice is blunt about it: "Remember, it's better to oversize than undersize!" A unit sized exactly at the edge of the range runs continuously and never reaches setpoint.
Whole-home is a different product. If you pick whole home we give you the portable-equivalent load and then stop, because ENERGY STAR says a ducted whole-home unit "is sized and installed correctly by a professional" and it is rated on its own scale (70–170 pints on the new scale). Use the number here to sanity-check an HVAC quote, not to place an order.
Two operating limits worth knowing before you buy. Below about 65°F frost forms on the coils and a standard compressor unit cycles without removing water, so a cold basement or an unheated crawl space needs a model specified for low temperatures, an anti-frost sensor, or a desiccant type. And a bucket is a trap in a space you rarely visit: most portables take a hose to a floor drain or sump, but gravity drainage needs the unit sitting above the drain — otherwise you want a built-in condensate pump.
Sizing guidance, not a diagnosis. A dehumidifier manages humidity; it does not stop water getting in. If you have seepage, sweating walls or standing water, the grading, gutters, downspouts, sump or a plumbing leak come first. EPA's own guidance is to get professional help for mould over roughly 10 square feet.
The published ENERGY STAR dehumidifier sizing matrix, plus DOE's old-to-new capacity conversion
| Condition | Relative humidity | What it looks like | Under 2,000 sq ft | Over 2,000 sq ft |
|---|---|---|---|---|
| Slightly to moderately damp | 50-75% relative humidity | space feels damp and has musty odor that may be intermittent | 20–30 pints/day | 30+ pints/day |
| Very damp | 75-90% relative humidity | space consistently feels and smells damp; damp spots appear on walls and floors | 25–40 pints/day | 40+ pints/day |
| Wet | 90-100% relative humidity | walls or floor sweat, or seepage is present; high-load conditions such as laundry drying may be present | 30–50 pints/day | 50+ pints/day |
| Replacing an old 30-pint portable | pre-2020 label | Same machine, new test at 65°F instead of 80°F | buy 20 pints/day today | — |
| Replacing an old 40-pint portable | pre-2020 label | Same machine, new test at 65°F instead of 80°F | buy 25 pints/day today | — |
| Replacing an old 50-pint portable | pre-2020 label | Same machine, new test at 65°F instead of 80°F | buy 30 pints/day today | — |
| Replacing an old 55-pint portable | pre-2020 label | Same machine, new test at 65°F instead of 80°F | buy 30-35 pints/day today | — |
| Replacing an old 60-pint portable | pre-2020 label | Same machine, new test at 65°F instead of 80°F | buy 35 pints/day today | — |
| Replacing an old 70-pint portable | pre-2020 label | Same machine, new test at 65°F instead of 80°F | buy 40-45 pints/day today | — |
| Replacing an old 90-pint portable | pre-2020 label | Same machine, new test at 65°F instead of 80°F | buy 55 pints/day today | — |
FAQ
What size dehumidifier do I need for a 1,000 sq ft basement?
On ENERGY STAR's published matrix a space under 2,000 sq ft that is slightly to moderately damp — musty smell, feels damp — calls for 20–30 pints per day, and one that is consistently damp with damp spots calls for 25–40. A 1,000 sq ft basement sits mid-range, so 30 pints is the usual answer for a musty basement and 40 for a genuinely wet one, both on the current post-2019 scale. Add capacity if the basement is cold or you are drying laundry down there, which ENERGY STAR specifically lists as a high-load condition.
Why do dehumidifiers have smaller pint ratings now?
Because DOE changed the test, not the machines. New standards took effect in June 2019 with a new efficiency metric (Integrated Energy Factor, which unlike the old Energy Factor counts the energy used while cycled off) and a new test procedure that runs portable units at 65°F instead of 80°F, to reflect a real basement. There is less water to remove in cooler air, so the declared capacity dropped. ENERGY STAR publishes the rough correlation: old 30 → new 20, old 50 → new 30, old 70 → new 40–45, old 90 → new 55. So a modern 50-pint unit is roughly what used to be sold as a 90.
Is it better to oversize or undersize a dehumidifier?
Oversize — that is ENERGY STAR's own wording: "Remember, it's better to oversize than undersize!" An undersized unit runs continuously, never reaches your setpoint, and wears out sooner. An oversized one hits the target humidity and cycles off, which is exactly what you want, and on the new Integrated Energy Factor metric the idle energy is counted, so the efficiency comparison you see on the label already accounts for cycling. The one caveat is space and noise: a 50-pint unit is a bigger, louder object than a 20.
Will a dehumidifier work in a cold basement or crawl space?
Not reliably below about 65°F with a standard compressor unit. ENERGY STAR is explicit: frost forms on the condensing coils below that temperature and the compressor cycles on and off without removing moisture from the air. Look for a model specified for low-temperature use, or one with an anti-frost sensor that shuts down and defrosts automatically. A desiccant-type dehumidifier is the other answer — it works at lower temperatures than a refrigerant unit, at the cost of running warmer and usually using more electricity.
Do I need a dehumidifier with a pump or just a hose?
It depends on geometry, not on the machine. Almost every portable will take a garden hose on the bucket fitting and drain continuously, but that is gravity drainage — it only works if the dehumidifier sits above the drain. If the only drain is a floor drain at the same level, or the water has to go up into a sink or out a window, you need a model with a built-in condensate pump or an external pump. In a crawl space or an unfinished basement you rarely visit, treat continuous drainage as mandatory: a bucket that fills and shuts the unit off has stopped working, and you will not know.
How many pints per day does a whole-home dehumidifier need?
That is a different product on a different scale, and ENERGY STAR says it should be sized and installed by a professional. Ducted whole-home units are rated 70–170 pints per day on the current scale (the same DOE re-rating applies: old 100 → new 85, old 200 → new 170). Use the portable-equivalent figure this tool gives you as a sanity check on an HVAC quote, not as a spec to order from — duct static pressure, where the unit ties in, and how the return is arranged all change the answer.
What relative humidity should I aim for in a basement?
EPA's guidance for the home generally is to keep indoor relative humidity below 60%, ideally between 30% and 50%. For a basement, 50% is a sensible target: low enough to stop mould and musty smells, high enough that the machine is not running pointlessly. Buy a cheap hygrometer and put it where the problem is rather than trusting the dehumidifier's own display, which reads the air right at the intake. If the number will not come down with a correctly sized unit running continuously, you have a liquid-water problem, not a humidity problem.
Does a dehumidifier fix a musty smell?
It fixes the cause of a new musty smell, which is airborne moisture feeding mould and bacteria, and ENERGY STAR uses exactly that odour as the marker for its "slightly to moderately damp" band. What it will not do is remove mould that has already grown, or stop water coming through a wall. If the smell persists with humidity held at 50%, look for the source — a damp carpet pad, a leaking supply line, standing water in a sump, or soil that slopes toward the house. EPA advises getting professional help for mould over roughly 10 square feet.