Water hardness converter
Type a hardness figure in any unit and the other four update, along with the USGS classification. The formulas are printed under the tool, because a converter you cannot check is just a box that says a number.
The formulas, in full
Hardness is reported as a concentration of calcium carbonate. Every unit below is a different way of writing the same concentration, so converting between them is arithmetic with fixed factors and involves no estimation.
| From | To | Formula | Where the factor comes from |
|---|---|---|---|
| ppm or mg/L | gpg | gpg = ppm ÷ 17.1 | One grain of CaCO₃ per US gallon. Austin Water's 2025 report states the same factor, about 17.1 ppm per gpg |
| gpg | ppm or mg/L | ppm = gpg × 17.1 | The same relationship, reversed |
| ppm or mg/L | °dH | °dH = ppm ÷ 17.848 | One German degree is 10 mg/L of CaO, which is 17.848 mg/L as CaCO₃ |
| ppm or mg/L | °f | °f = ppm ÷ 10 | One French degree is defined as 10 mg/L as CaCO₃ |
| ppb or µg/L | mg/L | mg/L = ppb ÷ 1000 | A milligram is a thousand micrograms. Denver Water publishes hardness in ppb, so this row is not hypothetical |
| ppm | mg/L | 1 ppm = 1 mg/L | Identical for dilute water. Reports use the two labels interchangeably |
Rounding is the one judgement here. This tool shows gpg to two decimals and ppm to one, because reports themselves rarely publish more precision than that. It does not round your input.
The classification the tool applies
The band names come from the USGS Water Science School, retrieved 2026-10-16, quoted exactly:
| Class | USGS wording | In gpg at 17.1 |
|---|---|---|
| Soft | Soft: 0-60 milligrams per liter (mg/L) as calcium carbonate | 0 to 3.5 gpg |
| Moderately hard | Moderately hard: 61-120 mg/L as calcium carbonate | 3.6 to 7 gpg |
| Hard | Hard: 121-180 mg/L as calcium carbonate | 7.1 to 10.5 gpg |
| Very hard | Very hard: more than 180 mg/L as calcium carbonate | above 10.5 gpg |
Note that other bodies publish different band edges. New York City's own report footnote calls up to 3 gpg soft and 3 to 9 gpg moderately hard, which does not line up with the USGS mg/L bands. This tool uses the USGS bands throughout and says so rather than silently picking a favourite.
Where to get a figure to convert
If you do not have a number yet, your utility's report is the place to start, and how to read a water quality report walks through finding the hardness line. 10 systems already have their published figure transcribed on this site, so you can check whether yours is one of them. If you are on a private well, no utility report covers you at all, which this guide explains.