Grains per gallon, and why it is 17.118 ppm
American water treatment measures hardness in grains per gallon. Laboratories measure it in milligrams per litre. Both describe the same water, and converting between them is the first thing anyone reading a water report has to do.
Where the unit comes from
The grain is one of the oldest units still in use, originally the mass of a single grain of barley, and it survives in exactly two places: the weight of bullets, and the hardness of water. It is defined as one seven-thousandth of an avoirdupois pound.
- 1 pound = 453.59237 grams, so 1 grain = 0.064799 grams
- 1 US gallon = 3.785411784 litres
- 1 grain per gallon = 0.064799 g / 3.785411784 L = 0.017118 g/L
- which is 17.118 mg/L, and mg/L is the same number as ppm
Most references round this to 17.1, which is accurate to about a tenth of a percent and is entirely adequate for choosing a softener.
Conversion table
| Grains per gallon | ppm (mg/L) | Class | What it means in practice |
|---|---|---|---|
| 0.5 | 9 | Soft | No treatment needed. Soap lathers easily and there is no scale. |
| 1 | 17 | Soft | No treatment needed. Soap lathers easily and there is no scale. |
| 2 | 34 | Soft | No treatment needed. Soap lathers easily and there is no scale. |
| 3 | 51 | Soft | No treatment needed. Soap lathers easily and there is no scale. |
| 3.5 | 60 | Soft | No treatment needed. Soap lathers easily and there is no scale. |
| 5 | 86 | Moderately hard | Noticeable but usually tolerable. Some spotting on glassware. |
| 7 | 120 | Moderately hard | Noticeable but usually tolerable. Some spotting on glassware. |
| 7.5 | 128 | Hard | Scale in kettles and on fixtures, more soap needed, shorter appliance life. |
| 10 | 171 | Hard | Scale in kettles and on fixtures, more soap needed, shorter appliance life. |
| 10.5 | 180 | Hard | Scale in kettles and on fixtures, more soap needed, shorter appliance life. |
| 12 | 205 | Very hard | Persistent scale, water heater efficiency loss, most households soften. |
| 15 | 257 | Very hard | Persistent scale, water heater efficiency loss, most households soften. |
| 20 | 342 | Very hard | Persistent scale, water heater efficiency loss, most households soften. |
| 25 | 428 | Very hard | Persistent scale, water heater efficiency loss, most households soften. |
| 30 | 514 | Very hard | Persistent scale, water heater efficiency loss, most households soften. |
The thresholds in grains
The classification thresholds are round numbers in mg/L and awkward ones in grains, which is why the boundaries look arbitrary on an American water report: