Free Converter
Water Hardness Converter
Convert water hardness between ppm, mg/L, grains per gallon and French, German and English degrees — and see instantly where the result lands on the hardness scale.
Built by SubcontractorHub — the software plumbing contractors use to quote, finance, and run every job.
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Labs and utilities report ppm. Softeners are rated and programmed in grains per gallon. This converts between them and every other unit you will meet.
The number from your water test or utility report.
Labs and utilities normally use ppm or mg/L. Equipment uses gpg.
📋 Important: All calculator results are ballpark estimates
The conversion factors used here are the standard industry values, but the classification bands are conventions rather than regulation and a single hardness figure does not describe water on its own. These results are for general planning purposes only and are not a substitute for a certified water analysis. Always confirm with a licensed installer or certified water treatment professional before specifying equipment.
The Water Hardness Scale and Its Units
The conversion, in one line
ppm ÷ 17.1 = grains per gallon
grains per gallon × 17.1 = ppm
One grain per gallon is 17.1 parts per million of hardness expressed as calcium carbonate. Every other unit on this page is the same quantity measured against a different reference volume, which is why they all route through ppm.
Water hardness scale
The classification used across the US water treatment industry:
| Grains per gallon | ppm / mg/L | Classification | What you notice |
|---|---|---|---|
| 0 – 1 | 0 – 17 | Soft | No scale. Soap lathers freely. No treatment needed. |
| 1 – 3.5 | 17 – 60 | Slightly hard | Rarely noticed. Treatment is optional. |
| 3.5 – 7 | 60 – 120 | Moderately hard | Faint spotting on glassware, mild soap scum. |
| 7 – 10.5 | 120 – 180 | Hard | Visible scale, soap scum, stiff laundry. Softener justified. |
| 10.5 – 15 | 180 – 257 | Very hard | Scale in kettles and heaters, measurable appliance wear. |
| 15+ | 257+ | Extremely hard | Rapid scaling. Water heater life visibly shortened. |
A softener starts paying for itself around 7 gpg and is clearly worthwhile above 10.
ppm to grains per gallon: quick reference chart
| ppm / mg/L | Grains per gallon | Classification |
|---|---|---|
| 17.1 ppm | 1 gpg | Slightly hard |
| 34.2 ppm | 2 gpg | Slightly hard |
| 51.3 ppm | 3 gpg | Slightly hard |
| 85.5 ppm | 5 gpg | Moderately hard |
| 120 ppm | 7 gpg | Moderately hard |
| 171 ppm | 10 gpg | Hard |
| 205 ppm | 12 gpg | Very hard |
| 257 ppm | 15 gpg | Very hard |
| 342 ppm | 20 gpg | Extremely hard |
| 428 ppm | 25 gpg | Extremely hard |
| 513 ppm | 30 gpg | Extremely hard |
The other units, and the one that catches people out
| Unit | Equals | Where you meet it |
|---|---|---|
| 1 grain per gallon (gpg) | 17.1 ppm | US equipment ratings and valve programming |
| 1 ppm | 1 mg/L | Lab reports, municipal water quality reports |
| 1 French degree (°fH) | 10 ppm | French and Italian equipment |
| 1 German degree (°dH) | 17.8 ppm | German equipment and datasheets |
| 1 English / Clark degree (°e) | 14.3 ppm | Older UK documentation |
Watch the German degree. At 17.8 ppm it sits only 4% away from a US grain per gallon at 17.1 ppm. The two are close enough that a mix-up never looks obviously wrong — it just leaves a unit slightly undersized for years.
Calcium carbonate equivalent, and why it matters
Hardness is conventionally reported “as CaCO3” — as though all of it were calcium carbonate — so that calcium and magnesium can be added into one number. If a lab report lists calcium at 100 mg/L rather than hardness as CaCO3, that is not the same figure: multiply calcium by about 2.5 and magnesium by about 4.1 to get their calcium carbonate equivalents, then add them. Feeding a raw calcium number straight into a sizing calculation understates the load by more than half.
Why the wrong unit shows up as a broken softener
The single most common programming fault in the field is a ppm figure entered into a valve that expects grains per gallon. A 250 ppm supply is really 14.6 gpg. Enter 250 instead and the valve believes the water is seventeen times harder than it is, so it calculates capacity as spent almost immediately and regenerates daily — burning salt and water while the homeowner is told they need a bigger unit.
On any call where a softener is cycling far too often, check the programmed hardness against a current test before anything else. Then size from the converted figure with the water softener sizing calculator and confirm the schedule with the regeneration calculator.
Frequently Asked Questions
How do you convert ppm to grains per gallon?
Divide ppm by 17.1. One grain per gallon equals 17.1 parts per million of hardness expressed as calcium carbonate, so 171 ppm is 10 gpg and 250 ppm is about 14.6 gpg. To go the other way, multiply grains per gallon by 17.1. The conversion matters because the two units are used by different people for the same water: laboratories and municipal water reports almost always give ppm or mg/L, while softener equipment is rated and programmed in grains per gallon. Entering a ppm figure into a valve expecting grains is one of the most common causes of a softener that regenerates every single day.
Is ppm the same as mg/L for water hardness?
Yes, for practical purposes they are identical. One milligram per litre equals one part per million in dilute water solutions, because one litre of water weighs approximately one million milligrams. Water quality reports use the two interchangeably and you can read mg/L as ppm with no conversion. What does matter is what the number is expressed as: hardness is normally reported as calcium carbonate equivalent, written CaCO3. A report giving calcium alone rather than calcium carbonate equivalent is a different figure and must be multiplied by about 2.5 before it can be compared.
What is the water hardness scale?
The classification widely used in the United States runs: soft is 0 to 1 gpg (0 to 17 ppm), slightly hard is 1 to 3.5 gpg (17 to 60 ppm), moderately hard is 3.5 to 7 gpg (60 to 120 ppm), hard is 7 to 10.5 gpg (120 to 180 ppm), very hard is 10.5 to 15 gpg (180 to 257 ppm), and anything above 15 gpg or 257 ppm is extremely hard. In practice a softener starts paying for itself around 7 gpg, becomes clearly worthwhile above 10, and on well water above 15 gpg the scaling is fast enough to shorten water heater life noticeably.
What are French, German and English hardness degrees?
They are regional hardness units you will meet on imported equipment and European datasheets. One French degree equals 10 ppm of calcium carbonate. One German degree equals 17.8 ppm. One English or Clark degree equals 14.3 ppm. A German degree and a US grain per gallon are close enough in size to be confused, which is a real hazard: 17.8 ppm against 17.1 ppm is only 4% apart, so the error hides until a unit sized from the wrong figure underperforms slightly for years.
What hardness level needs a water softener?
Scale becomes a practical problem from about 7 grains per gallon, or 120 ppm. Below 3.5 gpg most households notice nothing and a softener is hard to justify. Between 3.5 and 7 gpg it is a preference rather than a need. From 7 gpg upward you get visible scale, soap scum and stiffer laundry, and from 10.5 gpg the cost shows up in shortened appliance and water heater life. Hardness alone does not settle it on well water, though: iron, manganese, pH and total dissolved solids change which equipment is correct, and a hardness reading on its own will not reveal any of them.
Turn a Water Test Into a Priced Proposal
Water treatment dealers close far more when the quote goes out while the test result is still in front of the customer. Build good/better/best options on site, with financing beside the cash price.
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Conversion factors are the standard industry values and the arithmetic is exact, but classification bands are conventions and a hardness figure alone does not describe water. All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a certified water analysis, a professional assessment, or applicable local code requirements.