How this calculator works (no black box)
The base numbers are the AHAM sizing chart used by ENERGY STAR, in 2019-DOE-standard pints: for example, 500 sq ft that's moderately damp needs ~20 pints/day; 2,500 sq ft with a standing-water history needs ~65. We interpolate between chart rows, extend the chart's own slope above 2,500 sq ft, and add10 pints for an unsealed or dirt-floor crawl space — ground moisture is a continuous source the chart's room assumptions don't cover.
Unit matching works on coverage claims, not pints, because crawl-space units and retail portables are rated under different test conditions and comparing their pint numbers directly is exactly the mistake this site exists to prevent. The dampness margin (up to 1.6× your square footage for standing-water spaces) is why a "1,800 sq ft" unit isn't recommended for your 1,600 sq ft swamp.
Before you buy anything
- Fix liquid water first. Standing water or active seepage is a grading/gutter/sump problem. No dehumidifier outruns a spring.
- Wet and standing-water spaces want a pump model — gravity drains need slope you probably don't have down there.
- Crawl spaces want low-temp capability (units rated to operate near 49°F) — cheap portables ice up and quit exactly when you need them.
- Target 50–55% relative humidity. Lower wastes electricity; higher grows things you don't want to meet.