Southeast Michigan basements are damp from May to September, and damp basements are hard on everything stored in them: musty upholstery, mildew on the back of rugs, swollen drawers and rusty appliance feet. Two machines handle most of it: a dehumidifier for moisture and an air purifier for dust, pollen and smoke. Both are sold with numbers on the box that are easy to misread. These two calculators give you the size you need.
Dehumidifier Size Calculator
Area is length × width in feet. A 30′ × 40′ basement is 1,200 square feet.
The 2019 Pint Rating Change
Dehumidifiers are rated in pints of water removed per day. In 2019 the U.S. Department of Energy changed the test from 80°F to 65°F, both at 60% relative humidity, which is much closer to a real basement. Cooler air gives up moisture less easily, so the same machine now earns a lower number. Nothing got weaker; the label changed.
| Old Label (before 2019) | Current Label | Typical Use |
|---|---|---|
| 30 pint | About 20–22 pint | Bedrooms, closets, small damp rooms |
| 50 pint | About 35 pint | Small or moderately damp basements |
| 70 pint | About 50 pint | Most Michigan basements |
If your old 70-pint machine died, the replacement is a 50-pint model today, not a bigger, pricier “whole-house” unit.
Dehumidifier Sizing Chart
| Condition | Up to 1,000 sq ft | 1,000–1,500 sq ft | 1,500–2,500 sq ft |
|---|---|---|---|
| Moderately damp | 20–22 pint | 35 pint | 35–50 pint |
| Very damp | 35 pint | 50 pint | 50 pint |
| Wet | 50 pint | 50 pint | 50 pint or whole-house |
The calculator starts from this chart, which reflects typical manufacturer and industry guidance translated to current ratings, and sizes up one class for extra moisture sources like a washer and dryer, people living in the space, or a house without central air (air conditioning removes moisture as it cools). Damp condition matters more than square footage: a dry 1,500-square-foot basement needs less than a wet 600-square-foot one.
Air Purifier Size (CADR) Calculator
Enter the room the purifier will sit in, not the whole floor.
How CADR Sizing Works
CADR, the Clean Air Delivery Rate, is the cubic feet of filtered air a purifier delivers per minute, measured under a test standard run by the Association of Home Appliance Manufacturers (AHAM). Boxes list three numbers, for smoke, dust and pollen. Smoke particles are the smallest and hardest to catch, so the smoke number is the conservative one to size with.
AHAM’s rule of thumb is a smoke CADR of at least two-thirds of the room’s floor area for an 8-foot ceiling. That works out to about five complete air changes per hour. The calculator scales the number for taller ceilings and adds 50% for allergies or smoke, because the rated CADR is measured on the highest fan setting and most people run their purifier a speed or two lower to keep it quiet.
| Room | Area | Minimum Smoke CADR | For Allergies or Smoke |
|---|---|---|---|
| Small bedroom or office | 100 sq ft | 67 | 100 |
| Average bedroom | 150 sq ft | 100 | 150 |
| Primary bedroom | 250 sq ft | 167 | 250 |
| Living room | 350 sq ft | 233 | 350 |
For buying advice beyond sizing, such as drainage options, filter costs, True HEPA versus “HEPA-type” and protecting stored furniture, see our dehumidifier and air purifier guide.
Frequently Asked Questions
What size dehumidifier do I need for a 1,000 square foot basement?
Under the current (2019 and later) ratings, a moderately damp 1,000-square-foot basement needs about a 22- to 35-pint dehumidifier, and a very damp or wet one needs a 35- to 50-pint unit. Most full Michigan basements do best with a 50-pint model on a hose drain.
Is a 50-pint dehumidifier the same as a 70-pint?
Roughly, yes. A unit sold as 70-pint before the 2019 Department of Energy test change is typically sold as a 50-pint model today. The machine is the same size; the test conditions changed.
What humidity should a basement be?
Keep a basement between about 45 and 50 percent relative humidity. Above 60 percent, mildew and musty smells become likely. Set the dehumidifier humidistat there and check with a separate hygrometer.
What CADR do I need for a 300 square foot room?
With an 8-foot ceiling, at least a 200 smoke CADR. For allergies, asthma or wildfire smoke, look for about 300 so you can run it on a quieter setting and still clean the air.
Can one air purifier clean a whole house?
Not well. Purifiers work on the room they are in, and walls and doors block airflow. Put one in the room where you spend the most time, usually the bedroom, and add others as needed.