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Why is hardness expressed as different measures?

In the UK, there are different ways of measuring water hardness. Parts per million (ppm) is the modern metric system. Before that, the degree Clark method was used to measure hard water

  1. Parts per million

This scale is used to measure very small amounts of something in a larger quantity of liquid. It is used to measure dilute concentrations of chemical substances. One litre of water weight 1 million milligrams (mg). So 1 part per million (ppm) would mean the chemical is one millionth of the solution, or 1mg per litre. 

For water hardness levels, we measure parts per million of minerals including calcium carbonate (CACO3) in the water. Calcium carbonate is the compound in hard water that causes limescale build-up. Soft water typically has less than 50 ppm of calcium carbonate. Hard water has over 200ppm. 

  1. Degree Clark

This imperial measure of hard water is also called grain per gallon (gpg). 1 grain is 64.8mg of calcium carbonate per gallon of water (3.79 litres). 1 grain per gallon or 1 degree Clark is roughly equal to 17.1 parts per million. On this scale, soft water has fewer than 3.5 grains per gallon, hard water has over 10. 

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. 

  1. Other measures

There are other water hardness measurement scales, but they are not as common in general use. You might come across the Degree of General Hardness (dGH), German degree (dH) or French degree (fH or f). 

We have compiled a handy chart below which shows the relative range of PPM and it’s representation in GPG and °Clark measurement.

PPM (Parts per Million) GPG (Grains per US Gallon) °Clark (Degree Clark)
50 2.9 3.5
100 5.8 7.0
150 8.8 10.5
200 11.7 14.0
250 14.6 17.5
300 17.5 21.0

Across the UK, many people suffer from the effects of hard water. If you’re unsure whether you have hard water in your home, try our water quality test to find out how hard the water is in your area. Explore the hardness of the water in your area