How to Measure Humidity Levels

Purchase a hygrometer from a hardware store or online. These are relatively inexpensive. For example, according to the Amazon website, hygrometer prices range from $5 to $25 in 2010. Digital and mechanical versions are available. This guide refers to using a mechanical hygrometer.

Calibrate the hygrometer. Place the device in a large plastic box. Take a teaspoon of table salt and add two drops of water. Place this in the box with the hygrometer. Seal the box and leave for at least four hours. Remove the hygrometer. It should read exactly 75 percent. This is the humidity that salt + water creates in a controlled atmosphere. Mechanical hygrometers usually have a small screw or wheel that you can turn to adjust the settings. If the reading is more or less than 75 percent, adjust the device to read 75 exactly.

Place the hygrometer in the environment where you want to measure the humidity. For example, if measuring a particular room, set the hygrometer up in there. Allow the device to stabilize to the environment by leaving it at the location for a few hours.

Check the marker on the hygrometer. It will point to a number on the scale from 0 to 100. This refers to the percentage of water vapor in the air, or humidity. Remember--humidity levels vary according to the season and according to your location. However, ideal home humidity levels are less than 60 percent in summer and between 25 and 40 percent in winter, according to the U.S. Environmental Protection Agency.