How Hard Is Portsmouth's Water?

How hard is Portsmouth water in mg/l and Clark degrees?

Between 263 and 310 mg/l as calcium carbonate, or about 18 to 22 °Clark, depending on the zone. Those are 2026 averages from Portsmouth Water's zone table, which also gives each zone in German and French degrees and as mg/l of calcium. The Portsmouth city zones come out at 277 mg/l (19 °Clark) in Farlington Central and 282 mg/l (20 °Clark) in Farlington South.

The units trip people up. Milligrams per litre as calcium carbonate is the figure UK regulators use, and parts per million (ppm) means the same thing. Clark degrees are the older British unit; Portsmouth Water's own table pairs 300 mg/l with 21 °Clark, so one degree is roughly 14 mg/l. German degrees (°dH) appear in boiler manuals, and the zone table puts Portsmouth's zones at 15 to 17 °dH.

On the DWI's hardness scale, water is soft up to 100 mg/l, slightly hard from 100 to 150, in a middle band from 150 to 200, hard from 200 to 300 and very hard above 300. So the local supply is hard to very hard. Portsmouth Water's prose describes it as moderately hard, though its own 2026 table marks every zone H (hard) or VH (very hard).

Why is Portsmouth water so hard?

Because most of it comes out of chalk. Portsmouth Water says around 85% of its supply is groundwater from boreholes and springs beneath the South Downs, and the other 15% is taken from the River Itchen, which is itself fed by groundwater (water quality page). Rain is soft when it falls; as it soaks through the rock it dissolves calcium and some magnesium.

The British Geological Survey's baseline study of the Hampshire Chalk puts it plainly: chalk reactions "produce a hard water where Ca predominates". That matches the zone table, where calcium alone runs from 105 to 124 mg/l. Much of the hardness is the temporary kind, carbonates and bicarbonates that the DWI says come out of the water as scale when it is boiled.

Which zones are hardest?

Worlds End is the hardest at 310 mg/l (22 °Clark), followed by Littleheath at 302 mg/l and Lovedean at 296 mg/l. The table flags all three VH, but Lovedean's average is under 300, so this site calls it near the very-hard line rather than very hard. The softest zone average is Walderton, at 263 mg/l.

The spread is narrow: 47 mg/l separates the softest zone from the hardest. Moving house across the area will not change much about how quickly a kettle furs up.

How do you find your own zone's hardness?

Use Portsmouth Water's Zone Lookup map, which finds your zone from an address, then open that zone's summary on the results page. Zones are Portsmouth Water's own divisions, their names are not town names, and each serves no more than 100,000 people.

The 2025 summaries, dated 20 March 2026, show more than hardness: nitrate, sodium, lead, chlorine and each zone's pass rate. Southsea, for example, is named in the Farlington South summary. A dishwasher's softener setting or a boiler installer's checklist needs the zone figure, so it is worth writing down.

What does hard water do in a house?

It leaves scale wherever water is heated or dries. The DWI lists scaling in hot water systems, kettles, irons and appliances, which shortens heating elements' life and makes them less efficient, plus less lather from soap, tide marks on basins and baths, and scum on tea. The limescale page covers removal, and the combi boilers page covers scale on a boiler's heat exchanger.

Building guidance treats this area as hard water. Approved Document L, para 8.8, says feed water to water heaters and the hot-water circuit of combination boilers should be treated to reduce limescale where hardness is over 200 ppm. That can mean a softener, covered on the water softeners page, or a scale reducer.

For comparison, the SoftPro ECO City softener (SoftPro Water Systems, a US shop with the same owners as this site) and the SoftPro Elite HE softener (Quality Water Treatment, a US shop with the same owners as this site) are ion-exchange salt softeners listed in 32,000 to 64,000-grain sizes in US shops. US listings measure hardness in grains per US gallon; one grain per gallon is about 17.1 mg/l, so Farlington South's 282 mg/l is about 16.5. A UK installation must meet the Water Fittings Regulations and be notified to Portsmouth Water under regulation 5, so ask the installer to show that any unit complies. The sizing page works through the numbers.

Is hard water safe to drink?

There is no legal limit for hardness in UK drinking water: the DWI says the regulations "do not specify standards for hardness, calcium or magnesium". It notes "some limited evidence" of a link between hardness and heart health, which is why water companies that soften a supply are asked to keep at least 150 mg/l. Portsmouth Water's leaflet makes stronger health claims than that; this site sticks to the DWI's wording.

Very soft water brings its own issue. The DWI, citing the WHO, says water at or below 100 mg/l is more corrosive to pipework. If you soften at home, the official advice is to keep an unsoftened kitchen tap for drinking, set out on the drinking water tap page.

Hardness soundings

• 263–310 mg/lzone averages as calcium carbonate, 2026Portsmouth Water hardness values
• 15supply zones in the tablePortsmouth Water hardness values
• 85%share of the water drawn from chalk boreholes and springsPortsmouth Water
• 300 mg/lstart of the DWI's very hard bandDWI
• 740,000people suppliedPortsmouth Water Key Facts

Further questions

Keep reading

• Limescale: causes, removal and preventionWater and limescale
• Water softeners: worth it on hard water?Water and limescale
• Combi boilers and hard waterBoilers and hot water
• Portsmouth water and limescale hubSection