Water Hardness Converter

1 grain per gallon (gpg) equals 17.1 mg/L, also written as 17.1 ppm, of hardness as calcium carbonate. To convert ppm to gpg, divide by 17.1. So 171 ppm is 10 gpg, and 15 gpg is about 257 ppm. Water above 7 gpg (120 ppm) counts as hard.

Water hardness

15.00 gpg

Very hard on the WQA scale

mg/L (ppm): 256.8
mmol/L: 2.565
German degrees (°dH): 14.39
French degrees (°fH): 25.7
WQA class: Very hard
USGS class: Very hard

If you soften this water, it adds about 118 mg of sodium per liter.

Show the math

input 15 gpg = 256.8 mg/L as CaCO3

gpg = 256.8 ÷ 17.1181 = 15.00

mmol/L = 256.8 ÷ 100.087 = 2.565

°dH = 256.8 ÷ 17.848 = 14.39

°fH = 256.8 ÷ 10 = 25.7

sodium added = 15.00 × 7.864 = 118 mg/L

15.00 gpgBack to calculator

How to use this calculator

Type the hardness number from your test strip, lab report or water utility report.
Pick the unit it's given in. Utility reports in the US usually use mg/L or ppm. Softener manuals use gpg.
Read the other units and where your water sits on both hardness scales.

The formula

Every unit here measures the same thing, the calcium and magnesium in your water expressed as calcium carbonate. A grain is 64.8 mg and a gallon is 3.785 liters, so 1 gpg = 17.1 mg/L.

gpg = mg/L ÷ 17.1

mg/L = gpg × 17.1

mmol/L = mg/L ÷ 100.1

German degrees (°dH) = mg/L ÷ 17.85

French degrees (°fH) = mg/L ÷ 10

Example 1: 15 gpg × 17.1181 = 256.8 mg/L. That's 2.57 mmol/L, 14.4 °dH or 25.7 °fH. It's very hard on both scales.

Example 2: a city report shows 120 mg/L. 120 ÷ 17.1181 = 7.0 gpg, which sits at the top of "moderately hard" on both the USGS and WQA scales.

What this calculator doesn't cover

Test strips that report in their own color steps. Use the number printed next to the matching color.
"Hardness as calcium" (not as calcium carbonate). Multiply calcium mg/L by 2.5 first.
TDS (total dissolved solids) readings. TDS meters don't measure hardness.
Temporary versus permanent hardness.

Water hardness scale

Class (WQA)	gpg	mg/L (ppm)	USGS class
Soft	under 1.0	under 17.1	Soft
Slightly hard	1.0 to 3.5	17.1 to 60	Soft
Moderately hard	3.5 to 7.0	60 to 120	Moderately hard
Hard	7.0 to 10.5	120 to 180	Hard
Very hard	over 10.5	over 180	Very hard

Quick conversions

gpg	mg/L (ppm)	mmol/L	°dH
1	17.1	0.17	1.0
3	51.4	0.51	2.9
5	85.6	0.86	4.8
7	119.8	1.20	6.7
10	171.2	1.71	9.6
12	205.4	2.05	11.5
15	256.8	2.57	14.4
20	342.4	3.42	19.2
25	428.0	4.28	24.0
30	513.5	5.13	28.8

Tips

US water utilities publish hardness in their yearly Consumer Confidence Report. Search your utility's name plus "water quality report".
Softener controls ask for gpg. If your report says 200 mg/L, enter 11.7 gpg.
Well water can change with the seasons. Test again after a wet spring or a drought.
If you add iron to the hardness setting on your softener, add 4 to 5 gpg for every 1 ppm of iron.

FAQ

How do I convert ppm to gpg?

Divide ppm by 17.1. For example, 200 ppm ÷ 17.1 = 11.7 gpg. This works when the ppm figure is hardness as calcium carbonate, which is how US water reports list it.

Is ppm the same as mg/L for water hardness?

For water, yes. One liter of water weighs about one kilogram, so 1 mg per liter is 1 part per million.

What is a good water hardness level?

Under 1 gpg (17.1 mg/L) is soft. Between 1 and 7 gpg most homes see few problems. Above 7 gpg (120 mg/L) is hard, and above 10.5 gpg (180 mg/L) is very hard, where scale builds up quickly.

What is 300 ppm hardness in gpg?

300 ÷ 17.1 = 17.5 gpg. That's very hard water on both the USGS and Water Quality Association scales.

How do I convert German degrees of hardness to gpg?

Multiply °dH by 1.043. So 10 °dH is about 10.4 gpg. One German degree equals 17.85 mg/L as calcium carbonate.

How much sodium does softened water contain?

A salt-based softener adds about 7.9 mg of sodium per liter for every gpg of hardness it removes. Softening 15 gpg water adds about 118 mg per liter.