Updated September 22, 2026
Convert water hardness from mg/L to GPG
Check that your report gives total hardness as calcium carbonate (CaCO₃). The conversion below does not apply directly to a calcium-only measurement or a total dissolved solids reading.
The conversion
GPG = hardness in mg/L as CaCO₃ ÷ 17.1. To go the other way, multiply GPG by 17.1. Pentair’s water-hardness guidance provides the unit relationship. For dilute household water, this calculator treats ppm as equivalent to mg/L.
| mg/L as CaCO₃ | GPG |
|---|---|
| 85.5 | 5.00 |
| 120 | 7.02 |
| 171 | 10.00 |
| 250 | 14.62 |
| 342 | 20.00 |
Carry units through the sizing calculation
At 171 mg/L, hardness is 10 GPG. If 300 US gallons pass through the softener daily, the hardness load is 300 × 10 = 3,000 grains/day. Seven treatment days plus our separate one-day reserve scenario requires 24,000 grains.
Avoid these errors
- Do not enter 171 with GPG selected when the laboratory reported 171 mg/L.
- Do not substitute TDS for hardness.
- Keep full precision during calculations and round only the displayed result.
The calculator accepts either unit, so you can enter the report value directly. A unit conversion does not establish drinking-water safety.