What Are Grains per Gallon (gpg)? Water Hardness Units

Utilities and labs usually report hardness in mg/L, while softener manuals and many test strips ask for grains per gallon. It’s the same measurement in an older unit: the grain is a tiny unit of mass, and the gallon here is the US gallon.

How do you convert mg/L to grains per gallon?

Divide by 17.1. The National Institute of Standards and Technology lists one grain as 64.79891 milligrams and one US gallon as 3.785412 liters, and gives the conversion directly: one grain per gallon is 17.11806 mg/L. Since ppm and mg/L are the same for water, a result of 171 ppm is about 10 gpg. To go the other way, multiply gpg by 17.1.

How many grains per gallon is hard water?

Converted from the USGS classes, which are set in mg/L as calcium carbonate:

• Soft: up to 60 mg/L, about 3.5 gpg
• Moderately hard: 61 to 120 mg/L, about 3.5 to 7 gpg
• Hard: 121 to 180 mg/L, about 7 to 10.5 gpg
• Very hard: more than 180 mg/L, about 10.5 gpg and up

Not every chart draws the lines in the same place. A University of Nebraska extension guide, adapted from the Water Quality Association, calls water under 1 gpg soft and 1 to 3.5 gpg slightly hard, then follows the same 7 and 10.5 gpg breaks. Check which scale a test kit uses before you read too much into its label.

Why does the number matter for a softener?

Because you set the softener to it. The Minnesota Department of Health says that above 7 gpg (120 mg/L) you might need a softener, that softening is a personal choice, and that a softener set too high costs more to run and wastes water. Write your number down in gpg before you shop. Our guide to hard water covers whether you need one at all.