There are other water hardness measurement scales, but they are not as common in general use. The degree Clark scale isn’t used very often in the UK any more, though you might occasionally find it in the instructions for a dishwasher, in context of measuring the amount of added salt you need to add for effective cleaning in a hard water household. On this scale, soft water has fewer than 3.5 grains per gallon, hard water has over 10. 1 grain per gallon or 1 degree Clark is roughly equal to 17.1 parts per million.

1 grain is 64.8mg of calcium carbonate per gallon of water (3.79 litres). This imperial measure of hard water is also called grain per gallon ( gpg ).

Soft water typically has less than 50 ppm of calcium carbonate. Calcium carbonate is the compound in hard water that causes limescale build-up. So 1 part per million (ppm) would mean the chemical is one millionth of the solution, or 1mg per litre.įor water hardness levels, we measure parts per million of minerals including calcium carbonate (CACO3) in the water. One litre of water weight 1 million milligrams (mg). It is used to measure dilute concentrations of chemical substances. This scale is used to measure very small amounts of something in a larger quantity of liquid. Before that, the degree Clark method was used to measure hard water. Parts per million (ppm) is the modern metric system. In the UK, there are different ways of measuring water hardness.

How to calculate ppm of caco3