Water hardness in Poissy
18.5 °fH very hard
Tap water in Poissy is very hard: 18.5 °fH (185 mg/L · 10.8 gpg · 10.4 °dH).
Source: Ministère de la Santé (ARS), contrôle sanitaire SISE-Eaux, commune 78498 (2026-08) · data checked 2026-10-19
In the Yvelines, in Poissy, drinking water comes mostly from the Seine, treated at a plant. It measures 185.0 mg/L CaCO₃, a value lower than in the east of the Paris urban area. Scale is still visible on hot surfaces.
TH en °fH : douce < 15, moyennement dure 15–25, dure 25–35, très dure > 35 °fH (classes usuelles ARS).
Water data
| Total hardness | 18.5 °fH |
|---|---|
| Utility | Suez Eau France |
How Poissy compares
- Tap water in Poissy is 23 % softer than the median of the 33641 cities we track in France (23.9 °fH).
- By hardness it ranks 21406 of 33641 in France (1 = hardest); 36 % of these cities have softer water.
- In Yvelines it is number 170 of 222 cities; the regional median is 23 °fH.
- Nearest city with clearly softer water: Maisons-Laffitte, 8 km away (9.8 °fH).
- Nearest city with clearly harder water: Andrésy, 6 km away (34.4 °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 Poissy
| Brand | Setting for this water |
|---|---|
| Bosch | H03 |
| Miele | 10 °dH |
| AEG / Electrolux | 2 |
| Beko | 3rd |
| Hoover | S2 |
| LG | H2 |
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 Poissy water hard or soft?
It is very hard: 185 mg/L as CaCO₃ (10.8 gpg, 10.4 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Poissy 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 Poissy water?
10.8 gpg, equal to 10.4 °dH, 18.5 °fH or 185 mg/L calcium carbonate.
What dishwasher salt setting should I use in Poissy?
For 185 mg/L (10.4 °dH) check your manufacturer's table: maximum level (5–7).
Do I need a water softener in Poissy?
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 78498 (2026-08). Method: own measurement. Ministère de la Santé (ARS), contrôle sanitaire SISE-Eaux, commune 78498 · Outdated figure? Tell us