What size dehumidifier do I need

Put in the floor area and how damp the space is now. The answer comes out in pints per day, which is the number every unit is rated in.

The formula, in full

pints per day = (square feet / 100) x moisture factor, plus fixed additions.
Moisture factor: Slightly damp 1.4 · Moderately damp 1.9 · Very damp 2.5 · Wet 3.1 · Extremely wet 3.7.
Additions: Vented crawl space, open to outside air +5 · Sump pit or standing water +4 · Bare dirt floor, no vapour barrier +6 · Laundry or utility sink in the space +3.

This is a planning estimate for choosing a size, not a specification. Manufacturers publish their own sizing tables and they do not all agree, so treat the number as the class of unit to shop in and check the maker's table before you buy.

Why condition beats floor area

A 1,500 sq ft space that is only slightly damp needs 21 pints per day. The same space at very damp needs 38, which is a different class of machine (50 pint). Doubling the area moves the answer less than getting the condition wrong does, which is why the first thing to buy is a hygrometer, not a dehumidifier.

See AlorAir crawl space and basement units

Start from your space

• Crawl space: bare earth and cold air, the two things that break normal sizing.
• Basement: cool walls, a sump pit, and a load that never quite goes away.
• Garage: spiky rather than steady.
• Whole house: ducted, sized on the conditioned area.

Or start from a number you already have

• 30 pint: what it covers and where it stops.
• 50 pint: what it covers and where it stops.
• 70 pint: what it covers and where it stops.
• 90 pint: what it covers and where it stops.
• 120 pint: what it covers and where it stops.
• 198 pint: what it covers and where it stops.

Before you spend anything

1. Fix water you can see. Gutters and grading are cheaper than any machine.
2. Lay a vapour barrier if there is bare earth. It removes load permanently.
3. Read humidity for a week. One reading after rain sizes you a unit that short cycles for the rest of the year.
4. Decide where the water drains before you decide which unit to buy.