For every 100 mg/l of calcium carbonate hardness removed from the water, 46 mg/l of sodium is added.
Naturally soft water typically contains between 10 to 50 sodium parts-per million (ppm). The sodium content of any water is considered safe to drink by the Drinking Water Inspectorate up to 200 ppm.
When softening water, sodium is added to the concentration, not salt. The equivalent increase of salt intake by the average individual drinking two litres of softened water a day is 0.696 g.
This is about one tenth of a teaspoonful.
Water containing up to 435 mg/l of calcium carbonate hardness can be softened. The original concentration of sodium in the water supply also needs to stay below the 200 mg/l threshold.
The easiest way to check the levels in your current water supply is to call your water supplier. Ask them for the sodium level and calcium carbonate hardness in mg/l. Divide the calcium carbonate hardness by 2.175 and add the sodium level, the result should be no more than 200 mg/l.
In Greater London average softened water will contain an additional 138 milligrams of sodium to natural water levels. Generally if the calcium carbonate levels in your supply are over 400 mg/l, your softened water will probably be close to or above the sodium limit.
Salt intake from softened water:
The average total daily salt intake for an adult is about 9,500 milligrams.
The contribution from softened water would be about 7% – in a typical hard water area and assuming a two litre daily consumption.
In comparison, there is five times more salt in a glass of milk than a glass of softened water. Making it considerably below the threshold recommended by the Foods Standards Agency.