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Do you need a water softener? Very hard and highly mineralised water under UK rules

UK law sets no limit for hardness [1], so a softener is about scale, not safety. It replaces calcium and magnesium with sodium: total dissolved solids stay the same [2] and sodium rises towards the 200 mg/l limit [3]. The Drinking Water Inspectorate advises keeping an unsoftened kitchen tap and not using softened water to make up baby feeds [4]. If the water is salty rather than just hard, the answer is reverse osmosis, not a softener.

Updated 2026-09-29 · every regulation and price links to its source at the bottom of the page. A guide, not a construction design.

How hard is very hard water?

“The Drinking Water Directive and the UK drinking water quality regulations do not specify standards for hardness, calcium or magnesium.” [1] The DWI’s classes, in mg/l as calcium carbonate, run from soft (up to 50) through slightly hard (100–150), moderately hard (150–200) and hard (200–300) to very hard (above 300).

Mineral content is judged differently: conductivity has an indicator value of 2,500 µS/cm at 20 °C, and chloride and sulphate 250 mg/l each [5]. On taste, the World Health Organization rates water with 300–600 mg/l of dissolved solids as good and over 1,200 mg/l as unacceptable [6].

What does a softener do, and what doesn’t it?

How much sodium will a softener add?

About 23 mg/l of sodium for each milliequivalent per litre of hardness removed [8], roughly 0.46 mg/l per mg/l of hardness as CaCO₃; German guidance gives the same, 8.2 mg/l per German degree [11].

Example calculation: water with 400 mg/l of hardness and 30 mg/l of sodium, fully softened, gains 400 × 0.46 ≈ 184 mg/l and ends at about 214 mg/l, over the 200 mg/l limit [3]. One UK manufacturer’s installation guide calls an unsoftened drinking tap essential when softened water would exceed 200 mg/l of sodium, which happens above roughly 425 mg/l of hardness [12].

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How soft should the water be?

There is no UK standard for residual hardness. The DWI warns that water at or below 100 mg/l as CaCO₃ is more aggressive to lead, copper and brass pipes and fittings [4]. German guidance is not to soften to zero: aim for about 1.5 mmol/l (150 mg/l as CaCO₃), set with the blending valve, and stop before sodium reaches 200 mg/l [11].

Does a softener work on highly mineralised water?

Less and less well. US industry research shows that the higher the dissolved solids, the harder it is to soften. At 2,000 mg/l it takes about 9 lb of salt per cubic foot of resin (about 144 g per litre) to get down to 1 grain per gallon, at 3,000 mg/l about 17 lb (about 272 g per litre), and above 3,000 mg/l a single household softener gives little deep softening [13].

When do you need reverse osmosis instead?

When the problem is the salt itself (high dissolved solids, chloride, sodium, nitrate or fluoride), reverse osmosis or distillation is the tool [14]. It usually sits under the kitchen sink for drinking and cooking, and a softener in front of the membrane extends its life [15]. The DWI notes that RO wastes 20–30% of the water and that its water is generally not considered suitable for drinking without remineralisation [16].

Softener, reverse osmosis or both? A quick guide by analysis

What the analysis showsWhat usually fits
Hardness above 300 mg/l, dissolved solids moderateSoftener, leaving some residual hardness [1] [11]
Sodium after softening would pass 200 mg/lSoftener plus an unsoftened drinking tap, or RO for drinking [12] [16]
Dissolved solids over about 1,200 mg/l (poor taste)RO under the sink for drinking and cooking [6] [15]
Dissolved solids over 3,000 mg/lA single softener gives little deep softening; get specialist design [13]
Hard water with rusty (oxidised) ironIron filter first, then the softener [9]

What does a softener cost to buy and run?

About £1,250 installed on average; basic models £540–1,000, mid-range £940–1,900, premium £1,750–2,900 (approximate, as of September 2026) [17]. German guidance for softeners built to EN 14743 asks for economical regeneration of at least 4 mol of hardness per kg of salt (400 g as CaCO₃), automatic disinfection of the resin at least every 3 days and a blending valve for residual hardness [18].

Example calculation: a household softening 600 litres a day of water at 350 mg/l removes 210 g of hardness a day; at 400 g per kg that is up to about 0.5 kg of salt a day, some 190 kg a year.

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Frequently asked questions

Is softened water safe to drink in the UK?

There is no health standard for hardness, but softening raises sodium; the DWI advises an unsoftened kitchen tap for drinking and cooking, and not using softened water for baby feeds [4].

Does a water softener reduce TDS?

No. It swaps one kind of ion for another and leaves the dissolved solids the same [2].

What hardness counts as very hard in the UK?

Above 300 mg/l as CaCO₃ in the DWI classification [1].

Should I soften water to zero hardness?

No. Water at 100 mg/l or less is more aggressive to pipes [4]; leave some hardness with the blending valve [11].

What is the sodium limit for drinking water in the UK?

200 mg/l [3].

Read also:Water treatment for a private supply UK: filters explainedWater softener installation UK: drain, bypass and brinePrivate water supply testing UK: what to test and cost

Sources

  1. Drinking Water Inspectorate — Private water supplies treatment guide: The physical and chemical properties of water, Hardness, Table 1 (UK) · recommendation · checked 2026-09-29 · source
  2. Montana State University Extension Water Quality — Hardness fact sheet, Treating Hard Water (US) · recommendation · checked 2026-09-29 · source
  3. The Private Water Supplies (England) Regulations 2016 (SI 2016/618), Schedule 1, Part 1, Table B, Part II (national requirements) (UK) · regulation · checked 2026-09-29 · source
  4. Drinking Water Inspectorate — Domestic water filters and softeners, Water softeners (UK) · recommendation · checked 2026-09-29 · source
  5. The Private Water Supplies (England) Regulations 2016 (SI 2016/618), Schedule 1, Part 2, Table C (indicator parameters) (UK) · regulation · checked 2026-09-29 · source
  6. WHO/SDE/WSH/03.04/16 «Total dissolved solids in Drinking-water» (background document for WHO Guidelines for Drinking-water Quality), Organoleptic properties; Conclusions (INT) · recommendation · checked 2026-09-29 · source
  7. CDC — A Guide to Drinking Water Treatment Technologies for Household Use (2012), Reverse Osmosis; Ultraviolet; Water Softeners; Filtration (US) · recommendation · checked 2026-09-29 · source
  8. products.geappliances.com (US) · recommendation · checked 2026-09-29 · source
  9. Penn State Extension — Iron and Manganese in Private Water Systems, Water Softeners (US) · recommendation · checked 2026-09-29 · source
  10. Bayerisches Landesamt für Umwelt, Merkblatt Nr. 1.6/5 «Alternative Wasserbehandlungsgeräte zur Verminderung der Steinbildung in Hausinstallationen» (Stand Juli 2010), 1 Hintergrund; 4 Schlussfolgerungen (DE) · recommendation · checked 2026-09-29 · source
  11. DVGW-Kompaktinfo «Wasseraufbereitung in der Trinkwasserinstallation» (März 2024), Enthärtungsanlagen (DE) · recommendation · checked 2026-09-29 · source
  12. Monarch Water — Ultra HE / Premio HE water softener installation guide (2024), C. Check valve; D. Drinking water facility; F. Pressure; I. Drain (UK) · common practice (not a rule) · checked 2026-09-29 · source
  13. WC&P International — C.F. «Chubb» Michaud, «Total Dissolved Solids Limits for Effective Softening» (01.02.2024), Sodium-to-Hardness Ratio; Figure 3 (US) · recommendation · checked 2026-09-29 · source
  14. University of Nebraska–Lincoln Extension EC703 «Drinking Water Treatment: An Overview» (rev. 2014), Table I (problem → treatment); Table II (methods) (US) · recommendation · checked 2026-09-29 · source
  15. University of Nebraska–Lincoln Extension G1490 «Drinking Water Treatment: Reverse Osmosis» (rev. 2014), Treatment principles; Equipment (US) · recommendation · checked 2026-09-29 · source
  16. Drinking Water Inspectorate — Domestic water filters and softeners, Reverse osmosis systems (UK) · recommendation · checked 2026-09-29 · source
  17. MyJobQuote — Water Softener Installation Cost Guide (2026), Average cost (UK) · price · checked 2026-09-29 · ✓ · source
  18. dvgw.de, Enthärtungsanlagen (DE) · regulation · checked 2026-09-29 · source

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