mg/L to GPG — water hardness converter
Milligrams per litre (mg/L) as calcium carbonate is the standard lab unit for water hardness, and one grain per gallon equals 17.118 mg/L — so to convert mg/L to gpg you divide by 17.118. For example, 150 mg/L ÷ 17.118 = 8.8 gpg, which is hard water. mg/L equals ppm exactly.
Lab reports use mg/L as calcium carbonate; softeners use grains per gallon — the same water, two scales.
Convert mg/L to gpg
Enter a number to convert; the result shows its band and how it ranks among 2,977 US counties.
Chart
mg/L to gpg chart
| mg/L | gpg | Band |
|---|---|---|
| 10 | 0.6 | Soft |
| 25 | 1.5 | Soft |
| 50 | 2.9 | Soft |
| 100 | 5.8 | Moderately hard |
| 120 | 7 | Hard |
| 150 | 8.8 | Hard |
| 200 | 11.7 | Very hard |
| 250 | 14.6 | Very hard |
| 300 | 17.5 | Very hard |
| 400 | 23.4 | Very hard |
| 500 | 29.2 | Very hard |
| 750 | 43.8 | Very hard |
| 1000 | 58.4 | Very hard |
Formula
The formula, and why
gpg = mg/L ÷ 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: GPG to mg/L · ppm→gpg · gpg→ppm · gpg→°dH · all-units calculator
Questions
Frequently asked
- How many gpg is 100 mg/L?
- 100 ÷ 17.118 = 5.8 gpg, which is moderately hard.
- Is mg/L the same as ppm?
- Yes — for hardness they are identical, both expressed as calcium carbonate.
- What is 300 mg/L in gpg?
- 300 ÷ 17.118 = 17.5 gpg, which is very 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.