Water Hardness Converter
Same value in every unit
| Unit | Value |
|---|
The scale, in the two units you will meet
| Description | ppm as CaCO₃ | grains per gallon |
|---|---|---|
| Soft | 0–60 | 0–3.5 |
| Moderately hard | 60–120 | 3.5–7 |
| Hard | 120–180 | 7–10.5 |
| Very hard | 180+ | 10.5+ |
Why everything is expressed "as CaCO₃"
Hardness is caused by several dissolved minerals — mostly calcium and magnesium compounds — with different molecular weights. Reporting them each separately would make two water reports incomparable, so the convention is to state the equivalent amount of calcium carbonate. Every unit here follows it, which is what allows a German degree and a grain per gallon to be converted at all.
Sizing a softener
Softener capacity is rated in grains, so the arithmetic is: hardness in grains per gallon × gallons used per day × days between regenerations. Getting the hardness unit wrong at the start puts the whole calculation out by a factor of seventeen, which is the most common reason a unit ends up badly sized.
What hard water actually costs you
Scale in kettles, boilers and pipework, more detergent for the same result, and marks on glass and taps. It is not a health concern — the minerals involved are ones you eat — but scale genuinely shortens the life of water heaters, which is where most of the money goes.
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Frequently Asked Questions
Broadly: under 60 ppm is soft, 60 to 120 moderately hard, 120 to 180 hard, and above 180 very hard. In grains per gallon those boundaries are roughly 3.5, 7 and 10.5. Different bodies draw the lines in slightly different places.
Because softener capacity is rated in grains and American practice measures hardness the same way. If your water report is in ppm you have to convert before sizing anything, which is what this page is for.
One °dH is 10 mg of calcium oxide per litre, which works out at about 17.85 ppm as calcium carbonate. French degrees are simpler — one °fH is exactly 10 ppm — and English or Clark degrees are about 14.25 ppm.
No. It is a nuisance rather than a health problem — scale in kettles and pipes, more soap needed, spots on glassware. The calcium and magnesium that cause it are dietary minerals.
Because hardness comes from several minerals with different molecular weights, so it is conventionally reported as the equivalent amount of calcium carbonate. Every unit on this page uses that convention, which is what makes them comparable.