Calculator
Water-hardness converter
UK water companies publish hardness in mg/l, while coffee machines ask for a level from a test strip. Enter the value you have, in any unit, and see it in all four.
In short
Everything converts through mg/l as calcium carbonate (CaCO3): 1 °Clark = 14.25 mg/l, 1 °dH = 17.85 mg/l and 1 °fH = 10 mg/l. ppm is treated as equal to mg/l, and calcium (Ca) in mg/l is multiplied by 2.497.
Convert
- mg/l CaCO3 (ppm)
- 285.0
- °Clark
- 20.0
- °dH
- 16.0
- °fH
- 28.5
UK band: Hard. Most machines set hardness in °dH bands from a test strip, so check your manual for its band table.
How it works
Hardness is mostly calcium and magnesium dissolved in the water, and it is expressed as the amount of calcium carbonate (CaCO3) it equals. One English degree (°Clark) is 14.25 mg/l, one German degree (°dH) is 17.85 mg/l and one French degree (°fH) is 10 mg/l. Parts per million (ppm) is treated as the same as mg/l.
Some water companies list calcium (Ca) rather than calcium carbonate. Multiply mg/l calcium by 2.497 to get mg/l CaCO3, then convert as usual. Find your figure on your water company's website by postcode, or use the test strip that came with your machine.
| UK band | mg/l CaCO3 | °Clark | °dH |
|---|---|---|---|
| Soft | 0 to 50 | 0 to 3.5 | 0 to 2.8 |
| Moderately soft | 50 to 100 | 3.5 to 7.0 | 2.8 to 5.6 |
| Slightly hard | 100 to 150 | 7.0 to 10.5 | 5.6 to 8.4 |
| Moderately hard | 150 to 200 | 10.5 to 14.0 | 8.4 to 11.2 |
| Hard | 200 to 300 | 14.0 to 21.1 | 11.2 to 16.8 |
| Very hard | over 300 | over 21.1 | over 16.8 |
A harder setting makes the machine ask for descaling sooner. If you fit a water filter, many machines want you to switch the filter on in the menu instead. More on this in Descaling and filters.