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Water

How hard is my water?

Short answer

Hardness is the amount of lime (calcium and magnesium). The Netherlands mostly uses °dH or mmol/l, Belgium mainly French degrees (°F). Your water company gives the value per address.

Hardness scale: Hardness classes 0–4–8–12–18–30 °dH, where 1 mmol/l ≈ 5.6 °dH.
Hardness scale — Hardness classes 0–4–8–12–18–30 °dH, where 1 mmol/l ≈ 5.6 °dH. (The text inside the figure is in Dutch.)

Explanation

Hard water leaves limescale in boilers, shower heads and appliances. It is not unhealthy. You need the hardness to set the dishwasher, washing machine and any softener correctly.

Rules of thumb

  • Very soft 0–4 °dH; soft 4–8; medium 8–12; fairly hard 12–18; hard 18–30; very hard above 30 °dH. [SW-S33]
  • 1 mmol/l corresponds to about 5.6 °dH. [SW-S34]
  • Example: Dunea supplies water of about 1.42 mmol/l (7.9 °dH). [SW-S33]

Pitfalls

  • Appliances sometimes use °dH and sometimes °F; check which unit the manual asks for. [SW-S35]

By region

Belgium
Hardness per address via De Watergroep, Farys, Pidpa, water-link, Vivaqua or SWDE. [SW-S35]
Netherlands
Many Dutch companies soften water centrally. [SW-S34]

Search terms

These search terms are in Dutch; only terms that also find a topic in English are links.

waterhardheid kalk °dH °F mmol/l

Read also

General information, not personal advice. Check the source and reference date.

Sources

  1. SW-S33 Waterhardheid — Dunea (drinkwaterbedrijf) (Reference date 2026-09-27)
  2. SW-S34 Praktijkcode hardheid (totaal): calcium en magnesium — Praktijkcodes Drinkwater (NL) (Reference date 2026-09-27)
  3. SW-S35 Waterhardheid en ontharding — veelgestelde vragen — De Watergroep (Reference date 2026-09-27)

Reference date: 2026-09-27 · Translated with AI from the Dutch original.