Convert ppm water hardness to grains per gallon (gpg), or gpg back to ppm, and size a water softener with yearly salt use and cost.
- All Unit Converters
- PPM to mg/L Calculator
- PPM Calculator
- PPM to Grams Calculator
- PPM to Moles Calculator
- Water Softener Hardness Calculator
PPM to Grains Formula
The calculator uses the first two formulas. The third applies when a lab report lists calcium and magnesium separately.
PPM to GPG:
GPG to PPM:
Calcium plus Magnesium to total hardness:
- ppm = parts per million of hardness, expressed as calcium carbonate (CaCO3). Equal to mg/L for dilute water.
- GPG = grains per gallon (US), the standard unit used to size water softeners.
- Ca = calcium concentration in mg/L or ppm from a lab report.
- Mg = magnesium concentration in mg/L or ppm from a lab report.
- 17.118 = conversion constant. One grain per US gallon equals 17.118 mg/L of CaCO3.
- 2.497 and 4.118 = stoichiometric factors that convert elemental Ca and Mg into CaCO3 equivalents.
The result is reported as CaCO3, which is the convention for water hardness regardless of which mineral causes it.
The first mode converts a hardness value in ppm (mg/L) to GPG. The second does the reverse. The third sizes a water softener from hardness and household size, with yearly salt use and cost.
Reference Tables
Use these to interpret a result and to spot-check a conversion without rerunning the calculator.
| Classification | ppm (mg/L as CaCO3) | GPG |
|---|---|---|
| Soft | 0 to 60 | 0 to 3.5 |
| Moderately hard | 61 to 120 | 3.6 to 7.0 |
| Hard | 121 to 180 | 7.1 to 10.5 |
| Very hard | over 180 | over 10.5 |
| ppm | GPG | grains/L |
|---|---|---|
| 17.118 | 1 | 0.264 |
| 50 | 2.92 | 0.772 |
| 100 | 5.84 | 1.54 |
| 150 | 8.76 | 2.31 |
| 200 | 11.68 | 3.09 |
| 300 | 17.53 | 4.63 |
| 500 | 29.21 | 7.72 |
Worked Examples and FAQ
Example 1: Lab report shows 250 ppm hardness.
GPG = 250 / 17.118 = 14.6 GPG. That falls in the very hard range, so a softener is typically warranted.
Example 2: Test strip reads 8 GPG.
ppm = 8 × 17.118 = 137 ppm as CaCO3. Hard water by classification.
Example 3: Report lists Ca = 60 mg/L and Mg = 20 mg/L, no total hardness given.
Hardness = 2.497 × 60 + 4.118 × 20 = 149.8 + 82.4 = 232 ppm as CaCO3. GPG = 232 / 17.118 = 13.6 GPG.
Why 17.118? One grain equals 64.799 mg, and one US gallon equals 3.7854 L. Dividing gives 17.118 mg/L per grain per gallon.
Is ppm the same as mg/L? For fresh water hardness, yes. The density of water is close enough to 1 g/mL that the two are interchangeable in practice.
Why is hardness reported as CaCO3 when my water has no carbonate? CaCO3 is the reference compound used to put calcium, magnesium, and other divalent cations on a single scale. It is a unit convention, not a statement about what is dissolved.
Which GPG number do I give to a softener installer? Use the total hardness in GPG. If iron is present, add about 5 GPG per 1 mg/L of iron to the figure when sizing.
Does temperature matter? Not for the conversion. The 17.118 factor is a unit conversion only.
