A lot of the water we use comes into contact with underground limestone, which is rich in minerals like calcium and magnesium. When the water contains 10 or more grains of these minerals, it’s considered “hard” — and when there are fewer than 3 grains, it’s considered “soft water.”
There are several ways to test the water’s hardness in your area. Some laboratories use the parts-per-million method, while others use the grain-per-gallon method, which dates back to ancient Egypt and is the equivalent of one dry grain of wheat. There’s an easy way to correlate this data. If the water has been tested at parts-permillion (or milligrams-per-liter), just divide by 17 to get the hardness in grains-per-gallon. For example, water that has scale at 200 parts per million has approximately 11.7 grains of hard minerals.