GPG to mg/L — water hardness converter
One grain per gallon (gpg) equals 17.118 mg/L as calcium carbonate — so to convert gpg to mg/L you multiply by 17.118, and mg/L equals ppm exactly. For example, 15 gpg × 17.118 = 257 mg/L, which is very hard water. Softeners speak gpg; lab reports speak mg/L.
Softeners speak grains per gallon; lab reports speak mg/L — this converts between them.
Convert gpg to mg/L
Enter a number to convert; the result shows its band and how it ranks among 2,977 US counties.
Chart
gpg to mg/L chart
| gpg | mg/L | Band |
|---|---|---|
| 1 | 17 | Soft |
| 2 | 34 | Soft |
| 3 | 51 | Soft |
| 5 | 86 | Moderately hard |
| 7 | 120 | Moderately hard |
| 10 | 171 | Hard |
| 12 | 205 | Very hard |
| 15 | 257 | Very hard |
| 20 | 342 | Very hard |
| 25 | 428 | Very hard |
| 30 | 514 | Very hard |
Formula
The formula, and why
mg/L = gpg × 17.118. The grain per gallon is one grain of calcium carbonate — 64.79891 mg — dissolved in one US gallon — 3.785412 L: 64.79891 ÷ 3.785412 = 17.118 mg/L. Charts often round this to 17.1; we compute at 17.118 and round only the displayed result, so a 17.1-based chart can differ by about 0.1%.
Reverse: mg/L to GPG · ppm→gpg · gpg→ppm · gpg→°dH · all-units calculator
Questions
Frequently asked
- How many mg/L is 1 gpg?
- 17.118 mg/L — one grain per gallon equals 17.118 mg/L as calcium carbonate.
- Is mg/L the same as ppm?
- Yes — for hardness they are the same number.
- What is 5 gpg in mg/L?
- 5 × 17.118 = 86 mg/L, which is moderately hard.
By Marko Visic, MPharm, BSc Physics. Source: Conversion factors per USGS/NIST unit definitions; band data from USGS/EPA WQP. Data last verified . Methodology.