Use the reading, then check the manual.
A hardness result is useful only when its location, units and meaning match the decision you are making. This guide shows how to turn a published water result into a value you can compare with an appliance manual. It does not assign a universal dishwasher setting or descaling schedule.
1. Find the right measurement
Start with your supplier’s current water-quality report for your supply zone. Look for total hardness as calcium carbonate (CaCO₃), its units and the sampling period. A report’s publication date and the dates of the samples can be different. A city estimate is a useful starting point when you do not yet know your zone.
Check whether the number is an average, a minimum–maximum range or a single sample. Do not assume an average is the midpoint of a range. If your home has its own softener or other treatment, the utility’s figure describes the incoming supply rather than necessarily the water reaching the appliance.
For example, Sydney Water’s appliance guidance describes Greater Sydney water as approximately 57 mg/L. Our Sydney page uses that as a reference estimate, not a test of every household.
2. Convert to the unit your manual uses
Suppose your report gives 100 mg/L as CaCO₃. This is a worked example, not a new measurement for any city. Using the factors in our methodology:
| Manual’s unit | Calculation | Result |
|---|---|---|
| German degrees (°dH) | 100 ÷ 17.848 | 5.60 °dH |
| French degrees (°fH) | 100 ÷ 10 | 10 °fH |
| Grains per US gallon | 100 ÷ 17.118 | 5.84 gpg |
| mmol/L total hardness | 100 ÷ 100.0869 | 1.00 mmol/L |
Our manual-entry box accepts ppm, equivalent here to mg/L as CaCO₃. If a strip reports 10 °dH, first multiply by 17.848: enter 178.48 ppm, not 10 ppm. The extra decimal places in a conversion do not make an approximate test more precise.
3. Match the number to your exact model
Open the water-hardness or softener section of the appliance manual. Locate the row containing the converted value, and follow that row’s setting and salt instructions. A label such as “moderately hard” is not a setting number and may use different boundaries from the manufacturer’s table. For an example of a manufacturer’s table, see Bosch document 9000461048; use the document for your own model when choosing its setting.
For a hypothetical manual with a row covering 4–7 °dH, the example value of 5.60 °dH falls in that row. Use the setting printed beside it; this example deliberately supplies no setting code because codes vary between models. If the local range crosses two rows, follow the manufacturer’s instructions and obtain a recent tap-level measurement when needed.
What hardness alone cannot decide
- Coffee machine and kettle: usage, design and the manufacturer’s maintenance programme matter. Follow the machine’s prompts and compatible descaler instructions rather than deriving a calendar interval from the city’s hardness band.
- Laundry: use the detergent label’s hardness guidance together with load size and soil level. A harder-water category is not automatically a reason to use the maximum dose.
- Humidifier: water choice depends on the humidifier type. For ultrasonic and impeller units, EPA recommends low-mineral water to reduce mineral dispersal. It recommends emptying and drying portable tanks daily and cleaning every third day; follow the manufacturer’s cleaning instructions. Soft water does not remove the need for hygiene. Read EPA’s full guidance.
- Drinking-water safety: a hardness number does not measure microbial contamination or other drinking-water hazards. Consult the supplier’s current quality information and any local advisories.
Common reading mistakes
TDS is not hardness. Do not enter a general total-dissolved-solids meter value as total hardness. A hardness test or a report explicitly identifying hardness is needed. Also check that the value is expressed as CaCO₃: a separate calcium concentration is not the same measurement. USGS distinguishes hardness from dissolved solids and conductivity.
A test strip is an estimate too. Follow its sampling and timing instructions, check its expiry date and read the correct unit on its chart. A broad colour band should not be treated as a laboratory result.
Convert your reading → · Find a city source · Water softeners: pros and cons · Report a correction