The checker uses the Drinking Water Inspectorate’s classification bands and converts between mg/l, degrees Clark, French and German degrees. It tells you what your hardness figure means for equipment, and points you at the authoritative source for your supply zone.
What the checker does
- Classifies a hardness figure against the Drinking Water Inspectorate’s official bands – soft, slightly hard, moderately hard, hard, very hard.
- Converts between units. mg/l calcium carbonate, parts per million, degrees Clark, French degrees and German degrees all describe the same thing and appear on different documents.
- Explains what the band means for your boiler, cylinder, immersion heater, appliances and fittings.
- Tells you where to get the authoritative figure for your own supply zone.
You will find websites quoting a specific hardness number for Norwich or for Norfolk. Those figures are third-party aggregations whose provenance we have not been able to verify against the water company, and we are not prepared to publish a number we cannot stand behind. Anglian Water publishes the figure for your supply zone on its own water quality portal, and that is the source to use.
The classification bands
These are the Drinking Water Inspectorate’s published bands and they are what the industry works to:
| Classification | mg/l calcium carbonate |
|---|---|
| Soft | up to 100 |
| Slightly hard | 100 to 150 |
| Moderately hard | 150 to 200 |
| Hard | 200 to 300 |
| Very hard | more than 300 |
Be wary of alternative scales. Some manufacturers publish their own bands with different boundaries, generally arranged so that more customers fall into a band that suits the product being sold. The DWI bands are the authoritative ones.
The conversions
| To convert from mg/l CaCO3 | Divide by |
|---|---|
| Degrees Clark (English) | 14.254 |
| French degrees (fH) | 10 |
| German degrees (dH) | 17.848 |
| Parts per million (ppm) | 1 – they are equivalent here |
One degree Clark equals one grain of calcium carbonate per imperial gallon, which is 14.254 mg/l. It matters because water softener manufacturers specify in different units depending on where the machine was designed, and a softener configured from the wrong unit is badly wrong rather than slightly wrong.
If you are setting up a softener, get the hardness figure from your water company, convert it to whatever unit the machine wants, and set it from that – not from a factory default.
What your band means in practice
- Soft and slightly hard – scale is not a significant issue. No treatment needed on scale grounds.
- Moderately hard – scale will form on heat transfer surfaces over time. Worth protecting a boiler and cylinder; a softener is a comfort choice rather than a necessity.
- Hard – meaningful scale formation. Equipment protection is worthwhile, and the everyday effects on soap, kettles and fittings are noticeable.
- Very hard – significant and continuous. Equipment protection is a strong case, and most households notice the everyday effects considerably.
This region sits in the upper bands, which is why water treatment features more prominently in what we do than it would in a soft water area. See why Norfolk water is hard and what limescale actually does.
Getting your actual figure
- Your water company’s quality portal. Anglian Water supplies most of Norfolk and publishes water quality data by supply zone, including hardness. Search by postcode.
- Your water company directly, who will tell you.
- A test kit, which gives you the figure at your tap rather than at the zone. Useful, and less precise than a laboratory figure.
- A laboratory test, which is what you would want for a private supply.
The DWI’s own guidance notes that there is local variation not captured by regional maps and recommends asking your water company directly. That is good advice and it is why we point you there rather than guessing.
What to do with the answer
If your figure puts you in the hard or very hard bands, the practical options are:
| Option | What it does | Right when |
|---|---|---|
| Nothing | Accept scale as a maintenance cost | Low hot water demand and you do not mind descaling |
| Scale inhibitor | Prevents scale adhering on heat transfer surfaces | Protecting a boiler is the priority, limited space, no drain |
| Water softener | Removes hardness entirely | There is a cylinder to protect and a whole house to treat |
The distinguishing question is usually whether you have a hot water cylinder – see softener or scale inhibitor.
Got your hardness figure? Tell us what hot water system you have and we will tell you whether treatment is proportionate.
Questions we get asked about this
Why does the checker not give me a figure for my postcode?
Because we have not been able to verify the figures circulating online against the water company, and we will not publish a number we cannot stand behind. Anglian Water publishes the authoritative figure for your supply zone.
What units will my water company use?
Usually mg/l calcium carbonate, sometimes with degrees Clark alongside. The checker converts between all the common units.
Are the classification bands standard?
The Drinking Water Inspectorate’s bands are the authoritative UK ones and are what the industry works to. Some manufacturers publish their own with different boundaries – treat those with caution.
Can I test my own water?
Yes, with a test kit, which gives you the figure at your tap. It is useful and less precise than a laboratory figure or the water company’s zone data.
Does hardness vary within Norfolk?
Yes. It varies by supply zone and there is local variation the DWI itself notes is not captured by regional maps. Your supply zone figure is the one to use.
Is the tool free?
Yes, and it requires no email address. Nothing is stored and nothing is sold on.
Sources
- Drinking Water Inspectorate, water hardness classification — https://www.dwi.gov.uk/consumers/learn-more-about-your-water/water-hardness-hard-water/
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.