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Water hardness units explained: ppm, mg/L and degrees
Read hardness as calcium carbonate, convert German, French and Clark degrees, and avoid confusing calcium-only figures, TDS and appliance setting bands.
What does “as calcium carbonate” mean?
Water hardness principally reflects dissolved calcium and magnesium. Reporting it as calcium carbonate, written CaCO₃, expresses their hardness contribution on a common chemical-equivalent basis. It does not mean every milligram in the value is a visible particle of chalk floating in the glass.
Keep three pieces together when copying a result: the number, the unit and what it represents. “250 mg/L as CaCO₃ total hardness” is useful; “250 water reading” is not. A report may also list calcium or magnesium separately, which are different figures even when the unit is mg/L.
For dilute domestic water, ppm and mg/L are close enough to be used numerically interchangeably when both describe the same hardness basis. That approximation does not allow a ppm display on a TDS meter to become a hardness measurement. The instrument is estimating a different property.
Supplier labels such as hard and very hard use classification ranges that can differ between organisations. The numerical value and unit are more useful for setting an appliance than copying a category from an unrelated map. Ask for the supplier’s figure if the result is only described in words.
Convert degrees using one consistent reference
The factors below are rounded for practical reference and assume hardness expressed on the CaCO₃-equivalent basis. More decimal places do not make a broad supplier average more precise. For an appliance, use its own table and the precision it requires.
| Unit | Equivalent in mg/L as CaCO₃ | Convert a value to mg/L |
|---|---|---|
| 1 ppm as CaCO₃ | About 1 mg/L | Use approximately the same number |
| 1 German degree, °dH | 17.8 mg/L | Multiply by 17.8 |
| 1 French degree, °fH | 10 mg/L | Multiply by 10 |
| 1 English / Clark degree, °e or °Clark | 14.3 mg/L | Multiply by 14.3 |
| 1 mmol/L hardness on the matching equivalent basis | 100.1 mg/L | Multiply by about 100.1 |
| 1 meq/L hardness | 50.0 mg/L | Multiply by about 50.0 |
To convert in the other direction, divide by the relevant factor. Check a report’s definition before using mmol/L or a calcium concentration: similar-looking unit labels can describe different quantities. Do not mix millimoles and milliequivalents, which differ by the ionic charge convention.
As an example, 250 mg/L as CaCO₃ is approximately 14.0°dH, 25.0°fH or 17.5°Clark using these rounded factors. These are different expressions of the same example hardness, not three conflicting assessments of the water.
Why a calcium-only value is not interchangeable
Calcium and magnesium both contribute to ordinary hardness. If a report lists calcium in mg/L, copying that number directly into a CaCO₃-hardness field will usually be wrong. Even multiplying calcium by its equivalent factor omits magnesium unless the report has already included it elsewhere.
For ordinary calcium-and-magnesium hardness, an approximate analytical calculation is 2.497 × calcium (mg/L) + 4.118 × magnesium (mg/L), giving mg/L as CaCO₃. This is useful for understanding the reporting basis, not a reason to replace an available supplier total with a rough guess.
For illustration, calcium of 80 mg/L and magnesium of 10 mg/L produce about 241 mg/L as CaCO₃ by that calculation. The result is not 90 mg/L total hardness, and it is not obtained by adding the two concentrations without their equivalent factors. These invented values do not describe a particular postcode.
If the report mixes calcium, hardness and alkalinity, ask the supplier which value applies to the appliance setting you need. Alkalinity and total hardness can be related in a water supply but are not interchangeable labels. A TDS reading cannot resolve that distinction.
Use the maker’s bands after identifying the correct unit
A dishwasher or coffee machine may ask for °dH, °fH, Clark degrees or a numbered setting band. The setting number is specific to the appliance’s table; it is not another universal hardness unit. Do not enter 14 simply because the water is about 14°dH if the machine expects a level from one to five.
Follow the appliance manual’s instructions for the water actually feeding it. A jug on the worktop does not change the dishwasher supply. An installed softener may change that feed, but its settings and any appliance adjustments should be agreed with the installer and appliance guidance.
If a reported value lies near a band boundary or the supply varies, ask the supplier and use the appliance maker’s rule for that situation. Avoid inventing a setting from the word “hard”. Preserve the original value and any conversion in your household record so later adjustments are understandable.
Do not treat a higher hardness category as evidence that the water is unsafe. UK guidance distinguishes hardness-related scale and soap effects from a health-protection requirement to filter a compliant mains supply.
Use the hardness figure to choose the scope of change
For kettle water in a compatible MAXTRA PRO vessel, a scale-focused cartridge is one option to assess. It can reduce new deposits in the water you filter, but it does not descale an already coated appliance or change the rest of the plumbing.
BRITA MAXTRA PRO Limescale Expert · 6 pack
A MAXTRA PRO option for a hard-water kettle-scale priority, without changing a compatible jug or Flow dispenser.
- Pack
- 6 cartridges
- Family
- MAXTRA PRO
- Limit
- 150L / 4 weeks
Good forComparing a scale-focused refill for a compatible jug.
Watch forWater conditions can shorten the stated maximum life.
BRITA positions this type for hard to very hard water. It treats the water you filter through the jug; it does not soften the whole household supply.
For a wider household scale concern, a softener is a different project with regeneration, salt, drainage and an appropriate retained drinking-water supply. The BWT example below needs an installer’s assessment of demand and feed conditions rather than a purchase based solely on a converted number.
BWT WS455 electronic water softener
A UK domestic ion-exchange softener using block or tablet salt, with electronic control and regeneration.
- Resin volume
- 10L
- Salt
- Block or tablet
- Control
- Electronic
Good forFor an assessed whole-home scale concern
Watch for10 litres is resin volume, not a drinking-water reservoir.
For an installer-assessed hardness-reduction plan. Retain an appropriate unsoftened drinking-water supply; confirm drain, overflow, power, fittings and salt access.
Read filters versus softeners for that distinction, and finding the supplier report if your original hardness value is uncertain. The useful outcome of conversion is a correct practical decision, not an unnecessarily alarming label.
Common questions
Are ppm and mg/L always identical?
They are approximately numerically equivalent for dilute household water when both express the same hardness basis. Do not transfer that approximation between different measured quantities.
What is 250 mg/L as CaCO₃ in German degrees?
Approximately 14.0°dH using 17.8 mg/L per German degree. Use the appliance maker’s table to turn that into its setting.
Can I use the calcium concentration as total hardness?
No. Calcium-only concentration has a different basis and excludes magnesium’s contribution. Use the supplier’s total-hardness figure where available.
Is a dishwasher level number a universal hardness unit?
No. It is a model-specific setting band. Find the correct unit and then use that appliance’s instructions.
Does a TDS meter tell me the hardness setting?
No. Conductivity-derived TDS estimates are not exact calcium-and-magnesium hardness measurements.