💧 Water
Water hardness ppm to gpg and degrees
The same water can be described as 300 ppm, 17.5 grains per gallon, 16.8 °dH or 30 °f depending on which country the test kit came from. They are all the same hardness, and converting between them is arithmetic, not chemistry.
What hardness actually measures
Hardness is the concentration of dissolved calcium and magnesium, expressed as if it were all calcium carbonate (CaCO₃). Nothing in the sample need be carbonate; the convention exists so results from different labs, kits and countries can be compared on one scale.
The conversion from a full mineral analysis is mg/L as CaCO₃ = 2.497 × Ca (mg/L) + 4.118 × Mg (mg/L). Water with 80 mg/L calcium and 12 mg/L magnesium is 2.497 × 80 + 4.118 × 12 = 249 mg/L, conventionally written 249 ppm.
Because most drinking water is close to 1 kg/L, 1 mg/L and 1 ppm differ by about 0.2% and the two are used interchangeably. Every hardness figure in this guide is w/v, which is what a test kit reports.
The four scales, and the exact factors
Each scale is a different "grain" or "degree" definition, and each converts to calcium carbonate equivalent by an exact factor:
| Unit | ppm as CaCO₃ | Where it is used |
|---|---|---|
| 1 mg/L (ppm) | 1 | Scientific reports, WHO, EU and US utilities |
| 1 US grain per gallon (gpg) | 17.118 | US softeners, US test strips |
| 1 imperial grain per gallon (Clark) | 14.254 | Older UK water reports |
| 1 German degree (°dH) | 17.848 | Germany, Austria, Switzerland, much of Eastern Europe |
| 1 French degree (°f) | 10 | France, and most European test kits |
| 1 mmol/L | 100.087 | Water treatment and laboratory work |
The US grain figure comes from two exact definitions: one grain is 64.79891 mg and one US gallon is 3.785411784 L, so 64.79891 ÷ 3.785411784 = 17.118. The imperial grain uses the 4.54609 L gallon, giving 14.254. The German degree is defined as 10 mg/L of calcium oxide, and CaO to CaCO₃ is a 56.077 to 100.087 mass ratio, so 10 × 100.087 ÷ 56.077 = 17.848.
| From | ppm | US gpg | °dH | °f |
|---|---|---|---|---|
| 1 ppm | 1 | 0.0584 | 0.0560 | 0.100 |
| 1 US gpg | 17.12 | 1 | 0.959 | 1.712 |
| 1 °dH | 17.85 | 1.043 | 1 | 1.785 |
| 1 °f | 10 | 0.584 | 0.560 | 1 |
US grains and German degrees sit within 4% of each other, which is why a softener set in gpg and a test reported in °dH often appear to disagree slightly. That is not drift; they are different units.
Turning a test result into a softener setting
US softeners are rated in grains of capacity and set in grains per gallon. If a lab reports 240 mg/L, divide by 17.118 to get 14.0 gpg and set the dial to 14. Rounding up to 15 is normal and costs a little extra salt.
Then check the capacity maths. A 32,000-grain unit at 14 gpg removes 32,000 ÷ 14 = 2,286 gallons between regenerations at 100% efficiency. Real resin is derated — a common working figure is 75–85% — so plan on about 1,700–1,950 gallons, and add iron if present, since 1 mg/L of dissolved iron typically costs about 5 gpg of capacity, depending on the manufacturer's chart.
Two settings people get wrong. A softener does not reduce TDS much, so a TDS pen before and after will barely move; the calcium is replaced by sodium, not removed. And a "hardness" setting above the measured value does not soften better — it just regenerates more often.
How hard is hard?
| Band | ppm | US gpg | °dH | What it feels like |
|---|---|---|---|---|
| Soft | 0–60 | 0–3.5 | 0–3.4 | Lathers easily, no scale |
| Moderately hard | 60–120 | 3.5–7.0 | 3.4–6.7 | Some soap scum, light filming |
| Hard | 120–180 | 7.0–10.5 | 6.7–10.1 | Visible scale on taps, kettle furring |
| Very hard | 180+ | 10.5+ | 10.1+ | Heavy scale, soap barely lathers |
Those bands are the widely used US Geological Survey classification. The World Health Organization takes a different view: it sets no health-based guideline, notes that hardness above roughly 100–300 mg/L is acceptable to most consumers, and treats taste complaints as the main issue. Very soft water can taste flat and can be more corrosive to metal pipework.
Temporary and permanent hardness
Temporary hardness is the part that can be removed by boiling: calcium and magnesium bicarbonate, which precipitates as carbonate scale. Permanent hardness is the sulphate and chloride part, which stays dissolved. Total hardness is the sum, and a test kit reports total. This is why boiling water softens it but never fully softens it, and why a kettle furs up faster in a hard-water area than the cold tap suggests.
Alkalinity is a separate measurement — the water's capacity to neutralise acid, reported as CaCO₃ too. High hardness with low alkalinity behaves quite differently from high hardness with high alkalinity, which is why aquarium and brewing advice always asks for both.
Scale, soap and appliances
Scale is calcium carbonate coming out of solution as water is heated or evaporates. On a heating element it acts as an insulator; the US Department of Energy has long cited a few millimetres of scale as a several-percent efficiency penalty, and thicker deposits cost more. In a home the practical symptoms are kettles that fur, shower heads that block, and washing machines that need more detergent.
Detergent use is the other half of the story. Hardness minerals bind to soap and form scum, so more detergent is needed to get the same lather — a common rule of thumb is that each gpg of hardness adds roughly 0.5–1 g of detergent per wash, which is why detergent dosing tables are often printed by hardness band.
Sodium, and who should not drink softened water
Ion-exchange softening swaps two sodium ions for each calcium ion. Because the hardness is expressed as CaCO₃, each gpg removed adds about 7.9 mg/L of sodium, so 14 gpg adds roughly 110 mg/L. UK drinking-water guidance is stricter than US guidance: softened water is not recommended for mixing infant formula or for people on a medically restricted sodium diet, and a hard-water tap is usually left unsoftened for drinking and cooking.
If you want the number for your own supply, multiply gpg removed by 8 and add it to the sodium already in the water — most UK and US supplies start between 5 and 50 mg/L.
Why a TDS meter is not a hardness test
A TDS pen measures electrical conductivity and converts it with an assumed factor. It sees every ion: sodium, chloride, nitrate, sulphate. Two waters can have the same TDS and completely different hardness. After softening, TDS often barely changes because one calcium ion is replaced by two sodium ions.
For a real number, use a titration kit (drop count, EDTA) or a lab report. A drop-count kit is accurate to about ±1 gpg, which is enough for a softener setting but not for a report.
Frequently asked questions
Sources
- World Health Organization, Hardness in Drinking-water (background document for drinking-water quality guidelines).
- US Geological Survey — hardness classification and the calcium/magnesium to CaCO₃ factors.
- NIST Special Publication 811 — the grain (64.79891 mg) and the US and imperial gallons.
- Drinking Water Inspectorate and UK water-industry guidance on softened water, sodium and infant formula.
- US Department of Energy — scale and water-heater efficiency.