350 ppm to gpg: 20.45 gpg, Very Hard on Both Scales

Problem: the same water gets a different number on every label

The test strip stops at 15. The water softener manual asks for grains per gallon, the utility report lists mg/L, the coffee machine from Europe wants German degrees. Read as grains, that 15 is 257 mg/L; read as German degrees, 268 mg/L; read as French degrees, only 150 mg/L; and read as mg/L, it is soft water at 0.88 gpg. Same digits, four different waters. This calculator takes a reading in any of seven units, converts it to all of them, and rates it on both US hardness scales.

The two traps. First, grains per gallon and German degrees are so close (17.12 and 17.85 mg/L per unit) that mixing them up looks harmless, yet it is a 4% error that can push a reading over a class line. Second, the USGS and the Water Quality Association draw their lines in different places, so 120 mg/L is "moderately hard" to one and "hard" to the other.

Solution: one reading in, every unit and both ratings out

Unit of the reading - whatever is printed next to the number: gpg, mg/L or ppm as CaCO3, German, French or Clark degrees, mmol/L or mEq/L.
Hardness reading - the number itself, decimals allowed, from the utility's annual water quality report or a test strip. "1,250" is read as one thousand two hundred fifty.
Daily water use - optional, in gallons per day. Your water bill gives gallons for the billing period; divide by its number of days.
Read the result - gpg, mg/L and German degrees on top, the USGS and WQA ratings, all seven units in a table, and with water use the grains of hardness per day and pounds per year.

The factors are derived from molar masses rather than copied from a chart. A German degree is defined as 10 mg of calcium oxide per liter; with calcium carbonate at 100.086 g/mol and calcium oxide at 56.077 g/mol, that is 17.848 mg/L as CaCO3. A grain is 64.79891 mg and a US gallon 3.785411784 L, so one grain per gallon is 17.118 mg/L, the figure the WQA glossary rounds to 17.1. A French degree is 10 mg/L by definition, and a Clark degree, a grain per imperial gallon, is 14.254 mg/L.

USGS or WQA: which label to quote

The USGS classifies by mg/L as calcium carbonate; the Water Quality Association, whose glossary the softener trade uses, classifies by grains per gallon. The rows below are the class lines of both, run through the calculator, with the label each scale gives.

gpg	mg/L as CaCO3	German degrees	USGS	WQA
1.00	17.1	0.96	Soft	Slightly hard
3.50	59.9	3.36	Soft	Moderately hard
7.00	120	6.71	Moderately hard	Hard
10.50	180	10.1	Hard	Hard
15.00	257	14.4	Very hard	Very hard
20.00	342	19.2	Very hard	Very hard

The rule of thumb that falls out of it: below about 60 mg/L both scales agree the water is soft or close to it, and above about 180 mg/L both call it very hard. In between, quote the scale your reader uses.

Six readings people actually bring

Utility report, 180 mg/L. Converts to 10.52 gpg and 10.1 German degrees. The USGS says hard; the WQA, whose very hard band starts above 10.5 gpg, says very hard.

Report in mg/L, 250, household using 80 gallons a day. That is 14.6 gpg, very hard on both scales, and 1,168 grains of hardness a day, 8,178 a week, for a softener to remove; about 61 lb a year counted as calcium carbonate.

Well water test, 20 gpg, 300 gallons a day. Equals 342 mg/L, very hard, and 6,000 grains a day, 42,000 a week. That weekly load is the number to set against a softener's rated capacity between regenerations.

Espresso machine manual asks for German degrees, report says 150 mg/L. Enter mg/L and 150: the answer is 8.40 German degrees, or 8.76 gpg, hard on both scales.

European detergent label in mmol/L, water at 2.5. That is 250 mg/L, 14.6 gpg and 14.0 German degrees, very hard by US standards.

Low reading, 45 mg/L. 2.63 gpg: soft for the USGS, slightly hard for the WQA, whose soft band ends at 1 gpg. This is where most disagreements about "soft" water come from.

Conversion cheat card

One unit of each scale expressed in the others. Multiply a reading by the factor in its row to get the column unit.

1 unit of	mg/L	gpg	German deg.	mmol/L
gpg	17.12	1	0.959	0.171
mg/L (ppm)	1	0.0584	0.0560	0.00999
German degree	17.85	1.043	1	0.178
French degree	10	0.584	0.560	0.0999
Clark degree	14.25	0.833	0.799	0.142
mmol/L	100.1	5.85	5.61	1
mEq/L	50.04	2.92	2.80	0.5

What readers write in about hard water

Is hard water bad for my health?

The USGS describes hardness as a property of water that is not a health concern, but it can be a nuisance. Calcium and magnesium are minerals people need, and the WHO counts drinking water as a possible source of both.

Then why bother converting it?

Because of the nuisance part. According to the USGS, heated hard water leaves calcium carbonate scale that can shorten the life of a water heater, raise the cost of heating water and clog pipes, and soap reacts with calcium into scum, so more soap or detergent is needed. Appliances and softeners are set by number, and the number has to be in their unit.

Are ppm and mg/L the same thing?

For hardness in drinking water, yes: a liter of water weighs about a kilogram, so one milligram per liter is one part per million. Both are normally given "as calcium carbonate", the convention that adds calcium and magnesium into one number.

My report lists calcium and magnesium separately. Can I add them?

Not directly. Each has to be converted to its calcium carbonate equivalent first, as the WQA glossary shows: calcium in mg/L times about 2.5, magnesium times about 4.1. Add the two results and enter the sum in mg/L.

What does the daily water use field add?

Grains per gallon times gallons per day gives the grains of hardness that pass through the house each day. That is the load a softener removes, which is why softener capacity is rated in grains between regenerations. The pounds-per-year figure is the same load as calcium carbonate by weight, not the amount of scale; only part of it settles out, mostly where water is heated.

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350 ppm to grains per gallon: is it hard water?

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