Dehumidifier Size Calculator

How to Size a Dehumidifier

Dehumidifier sizing comes down to two things: the square footage of the space and how damp it actually is. Measure the length and width of the area and multiply for the square footage, then honestly assess the dampness. This is where most people guess wrong, so here is what each level looks like in practice:

• Slightly damp: The space feels heavy or smells faintly musty only during humid weather. No visible moisture. Relative humidity runs 50–60%.
• Moderately damp: A persistent musty smell, and you can spot damp patches on the floor or walls after rain. Cardboard boxes stored here soften over time. Humidity runs 60–70%.
• Very damp: Visible moisture beading on walls or the floor, wet stains, possibly mildew spots on stored items. The space feels damp year-round. Humidity runs 70–85%.
• Wet: Standing water appears on the floor, or water visibly seeps through walls or the slab after rain. Humidity sits at 85–100%. A dehumidifier helps here, but you also have a water-intrusion problem to fix.

The calculator uses capacity rates based on the 2019 DOE test standard — roughly 14 to 23 pints per 500 sq ft depending on dampness — and adds 10% for basements and crawl spaces, where ground moisture constantly wicks through concrete and soil. It then rounds up to the nearest standard retail size. Sizing up is the safe direction: an undersized unit runs constantly and never catches up, while a slightly oversized one simply cycles less.

Dehumidifier Size Chart

Use this chart to cross-check the calculator. Capacities are in pints per day under the current (2019) DOE test standard:

For basements and crawl spaces, lean toward the higher number in each range. Above roughly 1,500 sq ft or in wet conditions, consider two smaller units placed at opposite ends of the space, or a ducted whole-house dehumidifier installed alongside your HVAC system — see our HVAC cost guide for what whole-house units cost installed.

Old vs New Pint Ratings (Why "70 Pint" Reviews Are Confusing)

If you have shopped for a dehumidifier and noticed that older reviews rave about "70 pint" units while today's shelves top out around 50 pints, nothing has shrunk — the test changed. Before 2019, the DOE rated dehumidifiers at 80°F and 60% relative humidity. The 2019 standard tests at 65°F and 60% humidity, conditions much closer to a real basement. Cooler air holds less water, so the same machine extracts fewer pints per day under the new test.

The rough conversion: an old-standard 70 pint unit corresponds to about 50 pints under the new rating, an old 50 pint to roughly 35 new pints, and an old 30 pint to about 20 new pints. This matters when you read reviews or hand-me-down advice written before 2019 — if a guide recommends "a 70 pint unit for a damp 1,200 sq ft basement," today's equivalent purchase is a 50 pint model, not something bigger. This calculator uses new-standard (2019) capacities throughout, so the number it gives you matches what is printed on boxes in stores today.

Where to Put a Dehumidifier

Placement affects performance almost as much as sizing. Put the unit as close to the center of the space as practical, with at least 6–12 inches of clearance around the air intake and exhaust — shoved into a corner, it just recirculates its own dry exhaust. Keep doors and windows to the space closed while it runs, and set the humidistat to 45–50% relative humidity. Lower settings waste electricity chasing dryness you will not notice; anything held below 60% prevents mold growth.

Decide early how you will handle the water. The built-in bucket on a mid-size unit fills in less than a day in a damp basement, and the unit shuts off when it is full — miss a day of emptying and you lose a day of drying. If there is a floor drain or sump pit nearby, run a continuous-drain hose (nearly every model has a gravity drain port). If the drain point is above the unit — a utility sink, or a window to the outside — buy a model with a built-in condensate pump, which can push water up 12–15 feet. A cheap hygrometer placed across the room tells you whether the unit is actually holding the target humidity.

Fixing Moisture at the Source

A dehumidifier treats the symptom. If your space rates "very damp" or "wet," some amount of liquid water is getting in, and no appliance can out-run active seepage. Start outside: extend downspouts at least 4–6 feet from the foundation, clean the gutters, and re-grade soil so it slopes away from the house at least 6 inches over the first 10 feet. Inside, seal visible foundation cracks with hydraulic cement or polyurethane sealant, and consider a vapor-barrier coating on bare concrete walls.

For chronic seepage, an interior perimeter drain and sump system is the durable fix — size the trench and gravel with our French drain calculator. And if the end goal is livable space downstairs, get the moisture problem fully solved first; our basement finishing guide walks through the moisture-testing steps to take before any framing or flooring goes in.