What hardness needs a softener?

Above about 120 ppm, or 7 grains, is where scale becomes a nuisance. Above 180 ppm most people notice it on fittings and in appliances.

Conversionsscience

Water Hardness Converter

Water hardness across ppm, grains per gallon and the German, French and Clark degree scales. All measure the same calcium carbonate concentration and differ only in units.

Also called: grains per gallon converter, water hardness scale.

ppm as CaCO₃

180

180 ppm, 10.53 grains per gallon, 10.11 °dH or 18 °fH. That is very hard. At this level scale builds visibly on fittings and inside appliances, and a softener is worth considering.

Grains per gallon

10.53

German degrees

10.11

French degrees

Clark degrees

12.59

Millimoles per litre

1.8

Classification

very hard

On treatment

At this level scale builds visibly on fittings and inside appliances, and a softener is worth considering.

How this is calculated

Method and background

How this is calculated

Hardness is the concentration of calcium and magnesium, conventionally expressed as though it were all calcium carbonate. The scales differ only by unit: a grain per US gallon is 17.1 ppm, a German degree 17.8 ppm and a French degree 10 ppm exactly. Softener capacity is usually rated in grains, which is why the grains figure matters even in places that quote ppm, and sizing a softener needs hardness times daily water use.

every scale expresses the same calcium carbonate concentration in different units

c: Concentration

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

180 ppm

Hardness: 180
In: ppm or mg/l as CaCO₃

ppm as CaCO₃180

180/17.1 and 180/10

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grains convert back

Hardness: 10.53
In: Grains per US gallon

ppm as CaCO₃180

boundary: the inverse of the first case

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zero hardness is soft water

Hardness: 0
In: ppm or mg/l as CaCO₃

ppm as CaCO₃0

degenerate case

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Method and limits

What it assumes

Total hardness expressed as calcium carbonate equivalent.

What it deliberately does not model

Temporary and permanent hardness behave differently in a kettle and are not separated here.
The classification bands are conventional rather than regulatory.

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Frequently asked questions

What hardness needs a softener?: Above about 120 ppm, or 7 grains, is where scale becomes a nuisance. Above 180 ppm most people notice it on fittings and in appliances.