Water hardness in Cergy
33 °fH very hard
Tap water in Cergy is very hard: 33 °fH (330 mg/L · 19.3 gpg · 18.5 °dH).
Source: Ministère de la Santé (ARS), contrôle sanitaire SISE-Eaux, commune 95127 (2026-06) · data checked 2026-10-19
Range across supply zones: 269–436 mg/L.
In the Val-d'Oise around Cergy, drinking water is mostly drawn from the Oise and the Seine, treated at a plant. Paris Basin limestone gives it a median of 330.0 mg/L CaCO₃, with a wide gap from 269.0 to 436.0 mg/L. Check the value for your neighbourhood before buying a softener.
TH en °fH : douce < 15, moyennement dure 15–25, dure 25–35, très dure > 35 °fH (classes usuelles ARS).
Water data
| Total hardness | 33 °fH |
|---|---|
| Utility | Cyo'; Sfde - Agence De Cergy |
How Cergy compares
- Tap water in Cergy is 38 % harder than the median of the 33641 cities we track in France (23.9 °fH).
- By hardness it ranks 4550 of 33641 in France (1 = hardest); 86 % of these cities have softer water.
- In Val-d'Oise it is number 61 of 149 cities; the regional median is 30.6 °fH.
- Nearest city with clearly softer water: Neuville-sur-Oise, 2 km away (19.5 °fH).
- Nearest city with clearly harder water: Ennery, 5 km away (41.7 °fH).
Ranking of the hardest and softest water in France →
What it means at home
- Severe scale: heater elements and washing-machine heaters fail early.
- Detergent use is very high; taps and screens coat within days.
- Dishwasher at maximum salt setting.
- An ion-exchange softener is practically essential; RO for drinking water.
How much salt will you use for softening?
Yes. At this hardness a softener pays for itself through appliance life and detergent.
Descale a coffee machine or kettle about every 1 months (typical manufacturer guidance for this hardness).
Estimates for planning, not a quote. Check your appliance manual for exact settings.
Dishwasher setting in Cergy
| Brand | Setting for this water |
|---|---|
| Bosch | H05 |
| Miele | 18 °dH |
| AEG / Electrolux | 4 |
| Beko | 5th |
| Hoover | S3 |
| LG | H3 |
From each brand's manual for the model family named in the source; your model may differ, so check its manual. All brands and how to change the setting →
FAQ
Is Cergy water hard or soft?
It is very hard: 330 mg/L as CaCO₃ (19.3 gpg, 18.5 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Cergy safe to drink?
Hardness is not a health issue; it is about limescale, taste and appliances. Mains water must meet national drinking-water standards. TH en °fH : douce < 15, moyennement dure 15–25, dure 25–35, très dure > 35 °fH (classes usuelles ARS).
How many grains per gallon is Cergy water?
19.3 gpg, equal to 18.5 °dH, 33 °fH or 330 mg/L calcium carbonate.
What dishwasher salt setting should I use in Cergy?
For 330 mg/L (18.5 °dH) check your manufacturer's table: maximum level (5–7).
Do I need a water softener in Cergy?
Yes, unless you are only passing through: scale damages appliances within a few years.
Source
Source: Ministère de la Santé (ARS), contrôle sanitaire SISE-Eaux, commune 95127 (2026-06). Method: own measurement. Ministère de la Santé (ARS), contrôle sanitaire SISE-Eaux, commune 95127 · Outdated figure? Tell us