How to Measure Relative Humidity


Relative humidity is the amount of moisture that is present in the air in your environment. Moisture enters your home through activities such as showering, cooking and washing dishes, and can also seep into the walls through the soil at a house's foundation. Heating systems are one of the main causes of low humidity levels in a home. Optimal humidity levels in your home are neither too moist nor too dry. It is important for comfortable breathing and optimal health, but is also essential to the health of your indoor plants.

Step 1

Get a hygrometer. This small device will tell you how much relative humidity is in the environment. There are two types of hygrometers available--mechanical and electronic. Both will work well in a home.

Step 2

Calibrate the hygrometer. Test the calibration by combining half a cup of table salt with one-quarter cup of tap water in a coffee mug and stirring to dissolve some of the salt. Put the mug inside a plastic bag with the hygrometer and seal the bag so it is airtight. Place the airtight plastic bag with the items inside in a place that has no gusting air and no direct sunlight, and where the temperature shouldn't change. Check the reading on the hygrometer after 12 hours. The reading should be approximately 75 percent. If it isn't, adjust the hygrometer if it can be adjusted. If it is not adjustable, take note of the difference between the reading and 75 and apply the same difference to all future readings. For instance, if the hygrometer reads 80, add 5 to all readings.

Step 3

Place the hygrometer in a location in your home where you have seen the most signs of humidity issues. Signs of too much humidity in the home include condensation on windows, water spots on walls and musty smells. Signs of too little humidity include chapped skin, a scratchy throat and static electricity.

Step 4

Bring humidity to a mid-level in your home. Humidifiers can be used to raise humidity levels and de-humidifiers and ventilation fans can help reduce humidity. To raise the humidity levels for your plants only, place plants together in a 2-inch tray lined with 1-inch of gravel. Add water so that it fills the tray to just below the level of the gravel. As the water evaporates, it will create humidity for the plants above.

Step 5

Try to keep the relative humidity in your home between 30 and 50 percent for the health of both human and plant residents, with 45 percent being optimal. Humidity below 30 degrees will prevent plants from thriving, while humidity levels above 50 will cause mold to form on your houseplants. Keep the humidity level closer to 30 percent when the temperature outside drops below 14 degrees.

Tips and Warnings

  • It will take up to two hours for a hygrometer to adjust to a new location and give an accurate reading. During calibration, do not let the salt water come into contact with the hygrometer. Keep the hygrometer away from radiators, stoves and anywhere that the device may be affected by direct heat.

Things You'll Need

  • Hygrometer
  • Table salt
  • Tap water
  • Coffee mug
  • Plastic airtight bag


  • Retirees.UWaterloo.CA
  • Reducing Humidity in the Home
Keywords: relative humidity, hygrometer, home

About this Author

Alexis Lawrence has been writing professionally for six years and has been published on Associated Content. She is also a freelance filmmaker and photographer with extensive experience in digital video, book publishing and graphic design. Lawrence attended the University of Central Florida where she earned a Bachelor of Science in English.