Dehumidifier Size Calculator

Introduction: why dehumidifier capacity is a sizing problem, not a shopping preference

When indoor air stays damp for weeks at a time, the consequences show up on the walls: black speckling in corners, a musty smell that never quite airs out, paint that lifts, and wood trim that swells. A dehumidifier pulls water vapour out of the air so that relative humidity settles into a band where mould spores and dust mites struggle to establish themselves. The catch is capacity. Buy a unit that is too small and it runs around the clock without ever pulling the room down to target; buy one far larger than the space needs and you have paid for compressor and energy capacity you will never use, and the machine short-cycles instead of running at its efficient steady state.

Capacity is quoted in pints per day: the mass of water a unit condenses out of the air in 24 hours under a standardised laboratory condition. That last phrase carries more weight than most buyers realise, because the standard changed. Until 2019 portable dehumidifiers in the United States were rated at 80 °F and 60% relative humidity. From June 2019 the Department of Energy test procedure moved the rating point to 65 °F, much closer to the cool basement where most of these machines actually live. Less water condenses on a colder coil, so the same hardware now wears a smaller number on the box. ENERGY STAR publishes the mapping: an old 50-pint portable is roughly a 30-pint portable today, and an old 70-pint unit is about 40 to 45 pints. Every sizing chart printed before 2019 — including the classic AHAM table this calculator is built on — is written in the old units, which is exactly why so much online sizing advice now overshoots.

This page keeps both scales visible. It reads the AHAM guide in its original pre-2019 pints, converts to the 2020 scale, applies a ceiling-height and space-condition correction, and then rounds up to a size you can actually buy. It also keeps the older area-plus-humidity rule of thumb on screen as a sanity check, and takes whichever of the two is larger as the design figure, because under-sizing is the expensive mistake.

How to use this dehumidifier size calculator

1. Measure the space. Stretch a tape across the longest and widest parts of the room and enter both in feet. For an L-shaped or open basement, split the footprint into rectangles, add the areas, and enter a length and width that reproduce that total.
2. Enter the ceiling height. Leave it at 8 feet for an ordinary room. A vaulted, two-storey or converted-warehouse space holds far more air over the same floor, and the calculator scales the area it looks up accordingly.
3. Read the humidity. Leave a digital hygrometer in the room for 15 to 30 minutes at about chest height, away from a sunny window or a draught, then enter the figure. Relative humidity is how much water vapour the air holds compared with the most it could hold at that temperature.
4. Pick the space conditions. A cool below-grade room, a laundry or bathroom with a steam load, and a room that opens onto a stairwell each get a documented margin on top of the table figure.
5. Press Calculate. Read the dampness class first — it tells you which row of the AHAM table you landed on — then the recommended purchase size, and finally the warnings.

No meter to hand? Fog on the window glass, damp-smelling storage boxes and air that feels heavy all suggest you are on the high side, but a two-dollar digital gauge will make the estimate far more trustworthy than a hunch. Everything runs in your browser; nothing you type is sent anywhere.

Formula: the AHAM selection guide as a straight line

The AHAM Dehumidification Selection Guide is a five-column grid: four descriptions of how damp a space feels, crossed with floor areas of 500 to 2,500 square feet. Each row of that grid turns out to be perfectly linear in area, which means the whole table collapses into one equation per dampness class. Start from floor area:

Published tables assume an ordinary 8-foot residential ceiling, so the calculator first converts floor area into an equivalent 8-foot-ceiling area before it reads the table:

The table row for the space's dampness class then gives capacity on the pre-2019 (80 °F) scale as an intercept plus a slope times that effective area:

with and for a moderately damp space, 7 and 0.010 for very damp, 8 and 0.012 for wet, and 9 and 0.014 for extremely wet. Those four pairs reproduce every published cell of the AHAM grid exactly. Converting to the 2020 DOE scale and applying the space-condition margin gives the nameplate capacity to shop for:

The 0.62 factor is a least-squares fit to the ENERGY STAR old-to-new capacity mapping (30 → 20, 40 → 25, 50 → 30, 60 → 35, 70 → 40–45, 90 → 55 pints per day). Alongside all this the calculator still evaluates the older area-and-humidity rule of thumb that circulates on appliance forums, which is quoted on the pre-2019 scale:

Here P is pints of water removed per day, A is floor area in square feet and H is the room's current relative humidity. It never returns a negative figure: a room already at or below the 50% set point contributes nothing to the second term. The rule of thumb runs conservative — it over-predicts against the AHAM table for large rooms — so the calculator takes the larger of the two as the design figure and tells you which one governed.

Reading the dampness class and the AHAM table

The four AHAM rows are descriptions of a space, not humidity readings, and that is deliberate: the guide was written for people standing in a cellar without a meter. The relative-humidity bands below are the commonly used proxies, and they are what this calculator applies to your entered figure.

Two things are worth noticing. First, the slope matters more than the intercept once a space gets large: moving from moderately damp to extremely wet adds 4 pints at 500 square feet but 18 pints at 2,500 square feet, because a bigger volume of wetter air simply holds more water. Second, every one of those numbers is on the old scale. Divide by roughly 1.6 to read them in the pints printed on a machine bought today.

Worked example: a 20 by 15 foot damp cellar

Take a cellar measuring 20 ft by 15 ft with a standard 8-foot ceiling, reading 70% on the hygrometer. Floor area is 20 × 15 = 300 square feet, and because the ceiling is exactly 8 feet the effective area is also 300 square feet. A reading of 70% lands on the wet row, so the table line is 8 + 0.012 × 300 = 11.6 pints per day on the pre-2019 scale. The old rule of thumb gives 300 × 0.02 + (70 − 50) × 0.5 = 6 + 10 = 16 pints per day, which is larger, so 16 becomes the design figure. Converting to the modern scale gives 0.62 × 16 = 9.9 pints per day, and the cool-basement margin of 1.20 raises that to 11.9. Because portable units on the 2020 scale effectively start at 20 pints, the recommendation is a 20-pint model — comfortably above the requirement, which is the right side to err on in a cellar.

Change one input and watch the answer move. Raise the ceiling to 12 feet and the effective area becomes 450 square feet, pushing the AHAM figure to 13.4 pints and leaving the rule of thumb unchanged at 16, so the recommendation holds at 20 pints. Now take a 2,000 square foot finished basement at 78% instead: AHAM gives 8 + 0.012 × 2000 = 32 pints old-scale against a rule-of-thumb 54, so the design figure is 54, the modern equivalent is 33.5, the basement margin lifts it to 40.2, and the recommendation is a 50-pint unit on today's labels — a machine that would have been badged 80 pints before 2019.

Turning the pint number into a purchase

Treat the pint figure as a floor, not a specification. Nudge up one size when the space genuinely misbehaves: a below-grade room, a corner that keeps growing mildew, or air that smells damp even after a dry week. Think a season ahead too — if you are about to finish the basement, add a washer and dryer, or open the area to another room, buying the next size up now is cheaper than replacing the unit later. Check the operating temperature range before you buy for an unheated garage or crawl space: a compressor unit's output collapses below about 60 °F and its coil can frost, which is where desiccant models earn their keep despite using more electricity per pint.

The sizing job is only half the work; the machine works far less hard when the building cooperates. Repair roof and plumbing leaks and route gutters and downspouts well away from the foundation so bulk water never gets a foothold. Run bathroom and kitchen extract fans during and after showers and cooking to vent steam at its source. Leave interior doors ajar where you can, and keep vents and returns unblocked so air actually moves past the unit. In a basement or crawl space, a correctly installed vapour barrier and insulation cut how much moisture arrives from the ground and walls in the first place. And keep the appliance healthy: clean or replace the filter on schedule, keep the intake and exhaust clear of walls and furniture by several inches, and make sure any drain hose runs continuously downhill without a kink.

Limitations and assumptions behind the pint figure

This is a practical estimate, not a site-specific engineering design, and a handful of assumptions ride underneath it. The AHAM guide describes a space qualitatively; mapping "very damp" onto 60%–70% relative humidity is a widely used convention rather than a published equivalence, so a borderline reading of 69% or 71% can jump you a whole row. The ceiling-height correction scales area linearly with volume, which is a first-order approximation: it ignores stratification, and in a two-storey stairwell the warm upper air genuinely behaves differently from the cool floor slab.

The calculation assumes you are drying airborne moisture rather than fighting a leak. Active flooding, a dripping supply line, a failed gutter or groundwater seeping through a foundation wall are drainage and plumbing problems; no dehumidifier will keep up until the source is fixed, and buying a larger unit only raises your electricity bill. The maths is most honest when one machine serves a single room or a genuinely open set of connected rooms, not a scatter of closed-off bedrooms across two floors. Rated pints assume the laboratory test point, so a cold, draughty or frequently opened room will pull down more slowly than the number implies, and a unit rated at 65 °F will still underperform at 45 °F. The 0.62 old-to-new scale factor is a regression fit across ENERGY STAR's published pairs, so individual models vary by a few pints either way. Finally, an eyeballed humidity reading only produces an eyeballed result, which is why a cheap hygrometer pays for itself within one purchase decision. For all these reasons, rounding up to the next standard size is the safe default in basements, laundry rooms and anywhere that stays damp year-round.

Questions people ask before buying a dehumidifier

Sources for the capacity table and the humidity target

Capacity figures come from Table 1, the AHAM Dehumidification Selection Guide, reproduced in the Cornell University Department of Design and Environmental Analysis / Cornell Cooperative Extension housing fact sheet Selecting a Dehumidifier, which cites the Association of Home Appliance Manufacturers Directory of Certified Dehumidifiers; that source also documents the 80 °F / 60% RH rating point behind the pre-2019 pint scale. The old-to-new capacity mapping and the June 2019 move of the portable-dehumidifier test point from 80 °F to 65 °F are published by ENERGY STAR, Dehumidifier Testing and Capacity. The 30%–50% indoor relative-humidity target that sets the 50% set point follows US EPA guidance on mould and health and the ENERGY STAR dehumidifier buying guidance. The area-and-humidity rule of thumb retained as a cross-check is a consumer heuristic, not a manufacturer engineering chart, and is reported here only alongside the AHAM figure.