For a more accurate reading, invest in a hard water test strip set. Test strips have small amounts of chemicals on them that react to the minerals in hard water. Some manufacturers make different strips for testing drinking water and aquariums, so make sure you buy the right type for your purpose.
To use a test strip, simply dip the designated end into a glass or bowl of water and hold it there for about a second, until the end changes color. You shouldn’t have to hold it in the water for long, so pull it out once you have your reading.
You can also take a few test strips to different parts of your home, to see if water hardness varies in one part of your house or another.
Once you have completed the test, compare the color on the tested strip to the legend or explanatory scale that came with the kit to get a precise estimate of your water hardness. Simply match the color on the end of the strip to the closest color on the given scale. Usually, a darker color means harder water.
As an alternative to test strips, test kits with tablets are another way to test water hardness. The tablets are made up of a chemical compound that will react to change the color of the water, based on the hardness. For most tests, this involves partially filling a small receptacle (to about 10 ml of water), adding the tablet to the water, sealing the receptacle and shaking for a few seconds.
Usually, a red color indicates a hardness level of 20 mg/L or above, while a green-ish color means a lower hardness level, but consult the instructions and legend on the test kit for an exact reading.
Checking With Your Water Provider
A further, very simple method to estimating your water hardness is to just ask your water supplier or local government. Often, they keep fairly current measures of water hardness and can provide them to you at no charge.
Of course, the measure they have might not mirror the exact hardness level coming out of your faucet. And this will not always be an option, especially for those who use a private or semi-private water source.
Read: Do I Really Need a Water Filter?