How to Calculate Humidity

Four Parts:Buying a Humidity Meter (Hygrometer)Measuring the Accuracy of your HygrometerMeasuring Humidity with a PsychrometerUsing a Dewcheck

This article will help you calculate or measure relative humidity. Relative humidity is an estimate of the how saturated the air is with water vapour. To figure out what kind to tool you need and how to use it, just follow these steps.

Part 1
Buying a Humidity Meter (Hygrometer)

  1. Image titled Calculate Humidity Step 1
    The easiest way to measure humidity is to buy a Hygrometer. Hygrometers measure humidity from 0% (dry) to 100% (when any more moisture comes out as precipitation or fog.)
    • Relative Humidity (RH) can change throughout the day. Colder air holds less moisture, so the RH level will normally be higher at night. This is why Air Conditioned environments tend to be so dry.
    • Prices of Hygrometers can range from very cheap, to very expensive.
      • You would want a more expensive Hygrometer if you are using it for care of expensive things or need a more precise reading. When purchasing a hygrometer, you get what you pay for, within reason. Weather forecasters, museum curators, cigar collectors, tropical lizard owners, technicians working on HVAC systems, acoustic guitar collectors, and owners of antique Stradivarius Violins want a high quality hygrometer to protect their investments.
      • If you simply want to make sure your basement is not too wet, or someone's room is comfortable when they have a cold, a cheap hygrometer will do.
      • Even if you are OK with the inaccuracy of a cheaper hygrometer, there is another problem: cheap hygrometers can break, and you might not notice. For example, if you are measuring humidity in your basement, and you never go in your basement, a broken hygrometer would be a problem.
    • There are both digital and analog hygrometers.
      • If you need to create a system where a humidifier turns on when the humidity hits a certain level, you want a digital one.
      • Many guitar players will buy a nice analog hygrometer to put in their guitar case. The digital ones would not look nice paired with an acoustic guitar.

Part 2
Measuring the Accuracy of your Hygrometer

  1. Image titled Calculate Humidity Step 2
    The simplest way to test a Hygrometer for accuracy is the damp cloth test. Simply wrap a damp cloth around it and leave it for a few hours. It should read ~95%.
    • Note for cheaper electronic hygrometers, the damp cloth test can make readings incorrect for hours or days afterwards. Higher quality gauges will have a very short time till they are accurate.
  2. Image titled Calculate Humidity Step 3
    A more accurate way to measure hygrometer accuracy (or to help calibrate accuracy) is the "Salt Test." The Salt Test is more accurate, but not appropriate for the most demanding applications.
    • Take a ziplock bag and place your hygrometer(s) into the bag, face up. Then you will take a bottle cap (plastic 2 liter cap works well) and pour some standard table salt into the cap. Next you will add a small amount of distilled water, until the salt is the consistency of slushy snow. Close the bag very tightly. After twelve hours, it should read 75%.
      • If your hygrometer reads 80%, for example, that means its calibrated 5% too high.
      • You may need to take more than one reading to confirm it is consistent.
  3. Image titled Calculate Humidity Step 4
    For advanced applications, you can use the salt test with other related elements, each of which has their own readings.
    • For example, you can also use Magnesium Chloride and Lithium Chloride rather than Table Salt (Sodium Chloride.). Magnesium Chloride hits equilibrium at 33%, Lithium Chloride at 11%. Additional ingredients mentioned should be used based on your experience with chemistry.

Part 3
Measuring Humidity with a Psychrometer

  1. Image titled Calculate Humidity Step 5
    Buy a psychrometer. For more advanced applications, you may choose an electronic psychrometer, or even a DewCheck instead. For educational purposes, Sling Psychrometers are recommended because they are fun for students.
    • Field-piece and General Tools are two reputable electronic psychrometer manufacturers.
  2. Image titled Calculate Humidity Step 6
    Make your own sling psychrometer.

Part 4
Using a Dewcheck

  1. Image titled Calculate Humidity Step 7
    The Dewcheck is a very advanced temperature and humidity monitoring device for industrial applications. It is often using in painting and epoxy, to prevent issues from humidity levels being too high.
  2. Image titled Calculate Humidity Step 8
    The Dewcheck comes with a USB device, so the data can be easily exported. It also has analysis software.
  3. Image titled Calculate Humidity Step 9
    Before using the Dewcheck, you need to program in the tolerances. Depending on your application, they may differ.

Things You'll Need

  • Depending on your needs, you may need a hygrometer, psychrometer, Dewcheck or other materials.


  • When purchasing a device to measure your specific humidity needs, be sure to spend a little more. You will be glad you did. Cheap devices have a shorter life and tend to have a large percentage error. Animals, cigars, coins, wine, and more could benefit from an accurate read.

Article Info

Categories: Meteorology | Heating Cooling and Energy Efficiency