Hard water ยท Piece
Cambridge water sits in the national map's top band
Cambridge water hardness falls in the top band on the DWI's national map, over 300 mg/L. It is around 330 mg/L CaCO3 on a secondary data set; ask your water company for your postcode's current figure.
Most useful for: Householders, landlords and tenants in Cambridge and South Cambridgeshire
Compiled from the sources at the end. Updated .
Neither number describes your street. A dated figure from the water company itself was out of reach for this page, so what follows rests on a national map and one secondary data set. The Drinking Water Inspectorate (DWI) warns of local variation that its map does not show, and it sends readers to their water company for that reason.
What do the hardness bands mean?
DWI uses five bands, in mg/l of calcium carbonate: soft up to 100; slightly hard 100-150; moderately hard 150-200; hard 200-300; very hard over 300. A mg/L is the same as a part per million, so ppm and mg/L figures can be compared directly.
DWI hardness bands, mg/l as calcium carbonate
The Cambridge area appears in the top band on the DWI map. Part L treats water above 200 ppm as hard. Bar lengths show the order of the bands, not a measured scale.
The chart also marks a building rule. Approved Document L defines hard water as total hardness "greater than 200ppm" CaCO3. On DWI's hardness map, Crown copyright 2009, the Cambridge area appears in the top band, "Very Hard: Over 300". That comes from looking at a national map dated 2009, so treat it as the broad picture.
Where does the 330 figure come from?
TapWater.uk, a secondary source that sells softener quotes, gives a CB postcode area median of 330 mg/L, with a range of 278-499, across 7 measured districts. It shows 330 on the CB1, CB4, CB21, CB22, CB23 and CB24 pages, yet its CB1 page says no reading is published for that district and 330 is the area's typical value. The hardness figure carries no sample date, and the site's data page does not name Cambridge Water among the companies on the portal it relies on.
| Area | mg/L CaCO3 | DWI band |
|---|---|---|
| CB area median | 330 | Very hard |
| CB area range | 278-499 | Hard to very hard |
| CB25, Anglian-supplied | 317 | Very hard |
| CB6, Anglian-supplied | 278 | Hard |
| Anglian Water region generally | 200-350 | Hard to very hard |
The CB25, CB6 and regional rows show why one number will not do. CB6 at 278 falls in the band below the other two. Every row still sits at or above 200, the line Part L uses for hard water.
Seeing degrees Clark instead?
You may meet an older scale on product labels and in manuals. Wikipedia gives 1 degree Clark as 1 grain of calcium carbonate per imperial gallon, or 14.254 ppm; no official source for it was reached. Worked out from that conversion:
- 200 mg/L is 14.0 degrees Clark
- 300 mg/L is 21.0 degrees Clark
- 330 mg/L is 23.2 degrees Clark
Why does 200 mg/L matter in a house?
There is no UK standard for hardness, calcium or magnesium. DWI cites the World Health Organization (2017) for two thresholds: 200 mg/l or more produces scale, and 100 mg/l or less is more corrosive to pipework. Water at Cambridge's level is on the scale-forming side of that divide.
That is where the practical pages on this site start. Installers fitting a new boiler have to think about limescale in a boiler, householders deal with kettles, taps and shower heads, and some weigh up a softener. Older homes have one more connection: DWI says harder water forms a protective scale that can reduce lead, which is one reason to read the page on lead pipes before changing the water in a pre-1970 house.
Does hardness affect health?
DWI says there are "no health-based guidelines" for hardness. It also cites "some limited evidence" of a link between hardness and cardiovascular health, which is why companies that artificially soften water are advised to keep at least 150 mg/l. That is not enough to call hard water good or bad for you, and this site does not. Softened water raises separate questions about sodium and baby feeds, covered on the softener page.
Who settles what
- Your postcode's current hardness figure
- Your water company
- Scale measures when a new heating appliance goes in
- The installer, under Approved Document L
- Whether a softener needs notice to the water company
- Confirm with your water company
- A water complaint the company has not resolved
- CCW, the consumer body
Other questions on this
Is 330 mg/L the figure for my street?
Nothing here can show that. It is an area value from a secondary source, and on at least one district page it stands in for a reading that was not published. How to find your water hardness sets out how to get a figure for your address.
Is there a legal limit on how hard tap water can be?
No. DWI says there is no UK standard for hardness. There is a separate standard for sodium, 200 mg Na/L at consumers' taps, and that matters once water has been through a softener.
Does the answer change if Anglian supplies me rather than Cambridge Water?
It can. TapWater.uk shows lower figures for the Anglian-supplied CB25 and CB6 districts, though districts are too coarse to settle a single village. The page on who supplies whom explains how to check, and your bill names the company to ask.
Sources, read 4 November 2026: Drinking Water Inspectorate: water hardness; Drinking Water Inspectorate: hardness map; Drinking Water Inspectorate: standards and regulations; Drinking Water Inspectorate: filters and softeners; Approved Document L, Volume 1 (2021 with 2023 amendments); TapWater.uk: hardness map (secondary, sells softener quotes); TapWater.uk: CB1 (secondary); TapWater.uk: CB25 (secondary); TapWater.uk: CB6 (secondary); TapWater.uk: hardness overview (secondary); TapWater.uk: data sources (secondary); Wikipedia: hard water (secondary, Clark conversion only).