Convert between mg/L and grains per gallon, with a general hardness category.
Disclaimer: All results are estimates for planning only. County and state rules govern — check with your local authority.
How to use this converter
- Enter the value you have.
- Choose the direction: mg/L to grains per gallon, or grains per gallon to mg/L.
- Click Convert and read the value and the hardness class.
How the result is calculated
1 grain per gallon (GPG) = 17.1 mg/L. To go from mg/L to GPG, divide by 17.1. To go from GPG to mg/L, multiply by 17.1. Classes in this tool: 0-1 Soft, 1-3.5 Slightly hard, 3.5-7 Moderately hard, 7-10.5 Hard, over 10.5 Very hard.
Worked example: 171 mg/L = 10.00 GPG, class Hard.
Frequently asked questions
What does GPG mean?
Grains per gallon is the US unit commonly used for water hardness and softener sizing. mg/L (milligrams per litre) is the metric unit used on many laboratory reports.
Is 10 GPG hard water?
Yes, in this tool’s classes 10.00 GPG is Hard. Very hard starts above 10.5 GPG.
Does hardness mean the water is unsafe?
No. Hardness is about calcium and magnesium. It does not by itself show whether water is safe to drink. Use a qualified laboratory test for safety decisions.
Why do I need GPG for a softener?
Softener sizing in the US usually uses GPG. If your report is in mg/L, convert it first so the softener calculator uses the right input.
Can I convert ppm to GPG the same way?
For hardness, ppm is treated as mg/L in this tool, so the same 17.1 conversion applies. If your report uses a different basis, check its notes.
When to call a professional
Call a water treatment professional if hardness is very high, you see scaling or appliance damage, or you are unsure how to interpret a laboratory report or size a softener.
Related tools: Water Softener Size Calculator – All free tools