Home Cleaning Calculators
Water hardness converter.
Enter a hardness reading in any unit and see it in ppm, mg/L and grains per gallon, along with the USGS hardness class. Runs in your browser; nothing is sent or stored.
Your numbers
Result
Fill the form and press Calculate.
Worked example
A reading of 10 grains per US gallon is 10 x 17.1 = 171 mg/L (171 ppm). That falls in the USGS "hard" class (121 to 180 mg/L). In Imperial grains it is 12.0. A reading of 85 mg/L is 85 / 17.1 = 4.97 gpg and is moderately hard (61 to 120 mg/L).
How it works
mg/L = US grains per gallon x 17.1. The Water Quality Association glossary states that one grain per gallon equals 17.1 ppm or 17.1 mg/L, and that grains per Imperial gallon equals 14.3 mg/L. For water, ppm and mg/L are used interchangeably.
Classes. The USGS table of hardness classes (from Hem, 1985) gives soft as 0 to 60, moderately hard as 61 to 120, hard as 121 to 180 and very hard as more than 180 mg/L as calcium carbonate. The tool puts values between the printed bands (for example 60.5) in the higher class. The WQA scale-deposits page cites NSF/ANSI 44 and NSF/ANSI 330 as defining soft water as under 1 grain per gallon (under 17.1 mg/L), a stricter threshold than the USGS "soft" class, so the tool shows both.
Units. Hardness must be reported as calcium carbonate equivalent. Some reports use other scales (such as German degrees) that this tool does not convert.
Sources and basis (pages opened 2026-10-08): Water Quality Association, Glossary of Terms; Water Quality Association, Scale Deposits; USGS WRIR 02-4045, Tables 14 & 15: hardness classification (classes from Hem, 1985).
FAQ
How do I convert ppm to grains per gallon?
Divide by 17.1. One US grain per gallon equals 17.1 ppm (mg/L) as calcium carbonate.
What is hard water on the USGS scale?
Soft is 0 to 60 mg/L, moderately hard 61 to 120, hard 121 to 180 and very hard more than 180, all as calcium carbonate.
Is ppm the same as mg/L?
For water they are used interchangeably, so the tool shows the same number for both.
What counts as soft water for a softener?
NSF/ANSI 44 and 330 define soft water as under 1 grain per gallon (under 17.1 mg/L), according to the Water Quality Association.
Is the Imperial grain different?
Yes. Grains per Imperial gallon equal 14.3 mg/L, per the WQA glossary, so the tool converts that unit separately.