Sizing calculator
Size a crawl space dehumidifier from the air volume and how damp the space is, adjusted for the cool crawl temperature that makes most units underperform — plus where the condensate should drain.
A dehumidifier controls humidity after water sources are fixed. Standing water or leaks are a drainage job first.
Length, width, and average height set the air volume the unit has to dry — height matters, not just floor area.
Size chart
AHAM pints per day to shop for, by crawl space size and how damp it runs. Held at a 3 ft crawl with a floor liner and vents closed, at an average ~62°F crawl temperature — the same model the calculator above uses.
| Crawl space size | Light (just above target) | Damp (musty, 60-70% RH) | Wet (condensation, 70%+) | Common unit to buy |
|---|---|---|---|---|
| 500 sq ft | 20 ppd | 20 ppd | 25 ppd | 50-pint |
| 800 sq ft | 30 ppd | 30 ppd | 40 ppd | 50-pint |
| 1,000 sq ft | 35 ppd | 40 ppd | 50 ppd | 50-pint |
| 1,200 sq ft | 40 ppd | 45 ppd | 60 ppd | 50-pint |
| 1,500 sq ft | 50 ppd | 55 ppd | 75 ppd | 70-pint |
| 2,000 sq ft | 65 ppd | 75 ppd | 95 ppd | 90-pint |
| 2,500 sq ft | 85 ppd | 90 ppd | 120 ppd | 90-pint |
These are AHAM ratings to shop for, not the moisture load itself — the rating is already derated for the cool crawl. A taller crawl, a bare-soil floor, or open vents all push the figure up; enter your own numbers above to size for the space you actually have.
How to read it
The rating on a dehumidifier box is measured in warm, humid lab air. Your crawl space is cooler, so the same unit removes less water than the sticker claims.
The moisture load is how much water your crawl generates a day. The AHAM rating is what a unit removes at 80°F. Because the crawl is cooler, you shop for a rating higher than the load — that gap is the cool-crawl derate.
A vapor barrier and sealed vents can cut the load 40 to 60 percent. Sizing a unit for a bare, vented crawl means buying and running far more capacity than a sealed one needs — sealing usually pays for itself in a smaller unit.
A dehumidifier makes gallons of condensate a day. Drain it by gravity to a low point or the sump, or add a condensate pump when there's no fall — a unit with nowhere to drain overflows or shuts off.
When it's the wrong first fix
If the crawl has standing water, an active leak, or a drainage problem, that comes first. A dehumidifier controls humidity in the air after the water sources are handled.
Reach for drainage, grading, or a sump before sizing a unit when you see standing water, a torn or missing vapor barrier over wet soil, or humidity that stays high right after rain. Once the water sources are addressed, come back and size the dehumidifier to hold 45 to 55 percent.
Related planning pages
Handle the moisture sources first, then hold humidity with the right-sized unit.
When you need one, what it should hold, and how it's installed.
Learn moreSealing the crawl cuts the moisture load and the unit size you need.
Learn moreEstimate the liner and accessories to seal the soil first.
Learn moreFAQs
Short answers for homeowners using the calculator before comparing quotes.
Size it from the air volume and how damp the space is, then adjust for temperature. A rough start is about 15 pints per day for every 500 square feet of a sealed crawl, more for bare soil, open vents, or a wet space. Because units are rated at 80°F but a crawl runs cooler, plan on a higher pints-per-day rating than the load alone suggests.
A sealed 1,500 square foot crawl often lands around a 50 to 70 pint-per-day unit, and a 2,000 square foot crawl closer to 70 to 100, once the cool-crawl derate is included. Bare soil, open vents, or standing dampness push both higher; a fully encapsulated space needs less.
Yes. A crawl with a good vapor barrier over the soil and sealed vents can need roughly 40 to 60 percent less dehumidification than bare, vented earth, because far less moisture enters the air. Sealing first almost always lets you buy a smaller, cheaper-to-run unit.
Manufacturers rate pints per day at AHAM conditions — 80°F and 60% relative humidity — which a crawl space rarely reaches. At a typical 60°F crawl, a unit delivers roughly 20 to 35 percent less than its sticker rating, so you size up: the calculator shows both the moisture load and the higher AHAM rating to actually shop for.
Aim for 45 to 55 percent relative humidity. That is dry enough to stop mold and wood decay without over-drying, and it lets the unit cycle rather than run constantly. Lower setpoints waste energy and rarely add protection.
By gravity to a low point or into the sump when there is fall to reach one, or through a small condensate pump when there is not. Keep the line sloped and clear, and route the discharge where it won't pool against the foundation or freeze at the outlet.
Often a smaller one. Encapsulation cuts the moisture load a lot, but sealed crawls can still drift above 55 percent in humid weather, so many need a modest dehumidifier to hold the target. What a dehumidifier can't do is fix standing water or leaks — that's a drainage job first.
Leave it set and let it run on its own. A crawl space unit works from a built-in humidistat: set the target near 55 percent and it cycles on and off to hold it, running more in humid weather and barely at all in dry spells. You don't schedule it or switch it off at night — you let the humidistat decide, and check the reading monthly.
A crawl space dehumidifier draws roughly 300 to 700 watts while running. On a humidistat it cycles rather than running flat out — often 30 to 60 percent of the time — so a common planning range is about $10 to $40 a month at typical rates near 13 to 15 cents per kWh. A leaky or vented crawl runs it harder and costs more; a sealed crawl costs less. It adds to the bill, but far less than heating or cooling.
Sources
These calculator assumptions use public cost references and crawl space building-science guidance. Results are planning ranges, not contractor quotes.
Last reviewed: August 4, 2026.