Which UK town has the hardest tap water?

Part of our hard water guide

London gets the blame for hard water, but the published figures put other towns higher.

By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026

A zone figure is an average, and the band at your own tap can differ. You can check it in about a minute with a 20-in-1 strip. Below are the hardest zones we found, why chalk country is hard, why your tap may not match the figure for your area, and how to find your own number.

Which zones are the hardest we found?

Ipswich South, at 402 mg/l of calcium carbonate on Anglian Water's lookup, was the hardest town-centre zone we found. The table takes one zone per town, the one serving a town-centre postcode, with each figure as the company publishes it. Hull is converted from Yorkshire Water's calcium figure using its own factor of 2.5. Periods differ and some companies state none, so read it as a snapshot. The Sources give what each states.

Just outside the ten were Southampton at 300 (Southern Water publishes 120 mg/l calcium, converted with its own factor of 2.5), Portsmouth at 294 and Southend at 278. Lower down, Severn Trent publishes degrees Clark only. At 1 Clark = 14.3 mg/l, our conversion gives Nottingham 227, Leicester 207, Derby 182 and Coventry 161.

London is hard but not top ten. Thames Water's checker gave five central zones at 261 to 273 ppm (Whitechapel 273, Bloomsbury 270, City 266, Parliament 264, Lambeth North 261). ppm of calcium carbonate is the same as mg/l. Thames Water says the figures are for the previous calendar year and are updated once a year. Our guide on why London water is so hard goes further.

Why is chalk and limestone country hard, and what does it change?

Because water that runs through chalk and limestone dissolves calcium on the way. The Drinking Water Inspectorate (DWI) says hardness is dissolved calcium and magnesium and varies with local geology. South East Water says most of its Southern England supply comes from underground chalk aquifers. Southern Water says areas with granite or peat have softer water. Anglian Water's fact sheet says its whole region is hard to very hard because of the chalk in its rocks.

The DWI groups hardness in mg/l of calcium carbonate as soft (up to 100), slightly hard (100 to 150), moderately hard (150 to 200), hard (200 to 300) and very hard (over 300). It says UK regulations set no standard for hardness, calcium or magnesium, so there is nothing to pass or fail. What the DWI says hardness changes is scaling in hot water systems, kettles and irons, which it says shortens heating element life and makes appliances less efficient, and weaker lather from soap. Our guide on why kettles get limescale covers the effects. At the other end, the DWI says water at 100 mg/l or less is more corrosive to pipework.

That soft end is where Birmingham sits, at about 52 to 58 mg/l by our conversion of Severn Trent's degrees Clark. A Severn Trent news release describes the 72-mile Elan Valley Aqueduct from Wales as the sole source of water into the city when it was written. See is Birmingham water hard or soft.

What one reader found

A commenter in a HealthUnlocked Healthy Eating thread about UK tap water says "Where we live in London we have a lot of limescale". They say that unless the water is treated, limescale can damage washing machines, dishwashers and other appliances, and that it looks bad on shower screens and tiles unless cleaned regularly, which they do. Another commenter says London has always had hard water and that the South West, where they now live, is quite soft and lathers better. These are personal accounts, not measurements.

Shared on HealthUnlocked

Why might your tap differ from the area figure?

The area figure is an average. Your tap can differ for reasons the sources give. A figure for the area does not tell you what comes out of your own tap.

“If your tap is the only one you actually drink from, I would treat the area figure as a starting point and not the final word. A softener in the house or a different supply zone can make your water read quite differently, so a quick check at your own tap is worth doing.”

• Local variation. The DWI says its hardness map does not show areas with local variation, and that for an accurate measurement you should ask your water company directly. Severn Trent says individual results may vary with natural changes in the raw water and with changes within the domestic plumbing system.
• One town, several zones. Maidstone's Forstal zone reads 323 while two other Maidstone postcodes return the Boxley zone at 243. Ipswich East reads about 375 against Ipswich South's 402.
• A softener in the house. The DWI says the common household softener swaps calcium and magnesium for sodium, and that you should keep an unsoftened outlet for drinking. A softened tap would not match its zone.
• No hardness sampling. Yorkshire Water says it does not investigate or collect samples for hardness, though it can give detailed information on request.

How can you check the band at your own tap?

Fill a clean glass or cup from the cold tap, dip one strip, wait 30 to 60 seconds and match the hardness pad to the chart. You can also enter the colours in the free results checker, or scan the QR code on the chart, with no app or account. The hardness pad has steps at 0, 25, 50, 100, 250 and 425 mg/l, so it indicates a band, not an exact figure. Our guide to testing water hardness at home covers the method.

This is our reading of the chart. The DWI's hard band runs from 200 to 300 and very hard begins above 300, so the 250 step falls in hard and 425 in very hard. We have not confirmed the pad's unit basis, so treat any comparison with the DWI bands as rough. The midpoint of the 250 and 425 steps is 337.5, so a zone figure like Cambridge's 334.6 sits nearer the 250 step even where a company labels it very hard. That is the chart's coarseness, not a disagreement about the water. A strip result is not a legal sample. If your reading sits far from your company's figure, the reasons above are where to start, and your water company can give the zone figure.

The area figure is an average, and your own tap can differ through local variation, a neighbouring zone or a softener in the house, as the DWI and Severn Trent describe. The hardness pad is the one that matters for this topic. A reading a band or more below your zone's would fit a softened tap or local variation, and a repeat reading weeks later that moves a band would indicate the hardness at your tap has changed. Best pack: 20 strips, enough to check two taps and repeat the readings later, £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit

Your hardest water questions

Which UK town has the hardest tap water?

Of the lookups we checked on 20 October 2026, Ipswich South, at 402 mg/l of calcium carbonate on Anglian Water's figures. We have not checked every zone in the country.

Is London's water the hardest?

No. The five central Thames Water zones we checked read 261 to 273 ppm, below every zone in the table, which starts at 300.7. Thames Water's hardest zone in our sample was Reading Central at 316.

Why is Birmingham soft when the Midlands are hard?

Severn Trent's pages describe Birmingham's supply as coming from the Welsh mountains, and the DWI ties hardness to local geology. Its city-centre zones read 3.64 and 4.04 degrees Clark, about 52 to 58 mg/l by our conversion. The release also says a planned back-up supply comes from other natural sources and may taste or smell slightly different, and we have no hardness figure for that supply.

Is hard water bad for you?

The DWI says UK regulations set no standard for hardness, and Yorkshire Water says there is no known health risk associated with hard water. The DWI says there is some limited evidence linking water hardness and cardiovascular health, possibly through calcium and magnesium in the diet. Our guide is hard water bad for you goes through it.

How do I find my own figure?

Use your water company's postcode lookup, or ask it directly, as the DWI advises. Our postcode hardness guide shows how, and a strip then indicates the band at your own tap.

Sources

1. Drinking Water Inspectorate, Water hardness / hard water (undated page; geology, the five bands, no standard for hardness, scaling effects, WHO 2017 100 mg/l point, softeners and sodium, limited cardiovascular evidence, ask your company for an accurate figure; read 29 October 2026).
2. Cambridge Water, Cambridge City North zone Z1 (334.6 mg/l CaCO3, 23.4 Clark, 2025 samples; undated page; read 29 October 2026).
3. Yorkshire Water, Check your water hardness (no known health risk, no hardness sampling, detailed information on request; undated; read 29 October 2026) and the Hull Central West zone report (147.3 mg/l calcium, 2025 average, very hard; checked 22 October 2026) with its hardness converter (calcium x 2.5 = calcium carbonate).
4. South East Water, Water hardness (chalk aquifers, limescale in kettles, boilers and pipes; undated; read 29 October 2026) and its lookup (Forstal 323, Boxley 243, Canterbury 308 mg/l CaCO3, "hard", no period stated; checked 20 October 2026).
5. Southern Water, How hard is my water? (limestone and chalk harder, granite and peat softer, no UK or European standard; undated; read 29 October 2026) and its lookup (Southampton ZH510 120.0 mg/l calcium, factor 2.5; checked 20 October 2026).
6. Severn Trent, Check my water quality (individual results may vary with raw water and domestic plumbing; data updated to 31 August 2026; read 29 October 2026) and its news release Our work to create a new back up water supply for Birmingham is well underway (Elan Valley Aqueduct, back-up supply; undated; read 29 October 2026).
7. Wessex Water, Wessex Water (Bath, Monkswood 300.7 mg/l CaCO3, "very hard"; checked 20 October 2026). Anglian Water, Affinity Water and Thames Water postcode lookups and Anglian Water's fact sheet (figures as stated in the table; checked 20 October 2026).
8. HealthUnlocked, Is it safe to drink tap water? (Healthy Eating community thread; personal comments, not measurements; read 29 October 2026).

Contains public sector information licensed under the Open Government Licence v3.0.