You need to know the square footage of your basement in order to purchase a dehumidifier. When you measure your basement, it is better to overestimate rather than underestimate the size. Underestimating could result in the purchase of a dehumidifier too small to cover the entire basement. Manufacturers provide capacity guidelines for individual dehumidifier models.
The condition of your basement -- moderately damp, very damp, wet or extremely wet -- should also help you determine the size of dehumidifier you need. With the size of your basement and this classification, you can use the chart on the Energy Star webpage (see Resources) to determine the size of dehumidifier you need. Dehumidifier capacity of pints per 24 hours ranges from 10 to 44.
You can use a hygrometer to determine the relative humidity in your basement. Take a humidity reading in several places in the basement to get a good understanding of where the humidity is highest and lowest. The ideal relative humidity (RH) level is between 30 and 50 percent. Bacteria can grow at higher than 50 percent. During winter months in cold climates the RH level should not exceed 40 percent to prevent window condensation.
Before purchasing a dehumidifier, do an audit of your basement's environment. Look for factors causing a high level of humidity that you can eliminate on your own. Check that the gutter system on your house works properly, seal off unused crawl spaces and vents, and confirm correctly functioning dryer ventilation. Also, make sure that windows in the basement are in good condition.