Water Hardness Converter (ppm to Grains)
Convert water hardness between ppm (mg/L as CaCO₃), grains per gallon, German, French and English degrees, mmol/L and meq/L, see its class on the USGS, WQA or German scale, and get the softener hardness setting with an iron allowance.
How it works
gpg = ppm ÷ 17.118 · °dH = ppm ÷ 17.848 · °fH = ppm ÷ 10 · °e (Clark) = ppm ÷ 14.254 · mmol/L = ppm ÷ 100.09 · meq/L = ppm ÷ 50.04 · softener setting = ⌈gpg + iron (ppm) × 3–5⌉ · (ppm = mg/L expressed as CaCO₃)
Hardness is the calcium and magnesium dissolved in water, reported as the equivalent amount of calcium carbonate so that one number covers both. Every unit on this page measures that same CaCO₃ equivalent, so converting is a single division: one grain per US gallon is 64.799 mg in 3.785 L, or 17.118 mg/L (17.1 ppm); one German degree (°dH) is 10 mg/L of CaO, which equals 17.848 mg/L of CaCO₃; a French degree (°fH) is 10 mg/L; an English or Clark degree (°e) is one grain per imperial gallon, 14.254 mg/L; and a millimole of CaCO₃ per litre is 100.09 mg/L (two milliequivalents). Enter your value in whichever unit your test or water report uses and every other unit is shown at once. The class depends on the scale: the USGS and WHO use soft up to 60 ppm, moderately hard 61–120, hard 121–180 and very hard above 180; the Water Quality Association grades in grains per gallon (under 1 soft, 1–3.5 slightly hard, 3.5–7 moderately hard, 7–10.5 hard, above 10.5 very hard); and Germany labels water soft below 8.4 °dH, medium to 14 °dH and hard above. Water softeners are set in grains per gallon rounded up, plus an allowance of 3–5 gpg for every ppm of dissolved iron, because iron uses up resin capacity too.
Worked example
120 ppm ÷ 17.118 = 7.01 grains per gallon. The same water is 120 ÷ 17.848 = 6.72 °dH, 12 °fH, 8.42 °e and 120 ÷ 100.09 = 1.20 mmol/L. On the USGS scale 120 ppm is the top of moderately hard; on the WQA scale 7.01 gpg is just into hard. With 0.5 ppm of iron the softener setting is 7.01 + 0.5 × 4 = 9.01 → 10 gpg.
Sources: USGS — Hardness of water · Water Quality Association — glossary (1 gpg = 17.1 mg/L)
Frequently asked questions
Why is hardness given "as CaCO₃"?
Hardness comes mostly from calcium and magnesium ions of different masses, so water chemists express the total as an equivalent mass of calcium carbonate. This common reference lets a single number represent the combined hardness regardless of the ion mix, and it is why ppm, grains and degrees convert with fixed factors.
How many ppm is one grain per gallon?
One grain per US gallon is 17.1 ppm (17.118 mg/L as CaCO₃). So divide ppm by 17.1 to get grains, or multiply grains by 17.1 to get ppm: 10 gpg is about 171 ppm and 200 ppm is about 11.7 gpg. A grain per imperial (UK) gallon — the English or Clark degree — is smaller, 14.25 ppm.
What softener setting matches my water?
Most softeners are set in grains per gallon. Convert your lab or test-strip figure with this tool, round up, and add 3–5 gpg for each ppm of clear-water iron your report lists (4 is a common middle value). The result appears as the softener hardness setting; open “More options” to enter iron.
Is 7 grains per gallon hard?
It depends on the scale. On the USGS scale 7 gpg (120 ppm) is the top of moderately hard; the WQA scale calls 7–10.5 gpg hard. You may notice some scale and reduced soap lather, and softening becomes more worthwhile above about 10 gpg.
Can I use a TDS meter reading as hardness?
No. A TDS meter measures all dissolved solids — sodium, chloride, sulfate and more — and reports them in ppm, so its number is usually higher than the hardness and cannot be converted to grains. Use a hardness test strip, a titration kit or your utility’s water quality report.
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This calculator is for educational purposes. Double-check important results before acting on them.