Water hardness in Rennes
16.2 °fH hard
Tap water in Rennes is hard: 16.2 °fH (162 mg/L · 9.5 gpg · 9.1 °dH).
Source: Ministère de la Santé (ARS), contrôle sanitaire SISE-Eaux, commune 35238 (2026-08) · data checked 2026-10-19
Water in the Rennes area comes from rivers and reservoirs, such as the Couesnon and the Rophémel dam, treated at the Mézières and Villejean plants. The Breton bedrock, poor in limestone, gives lightly mineralised water: 162.0 mg/L CaCO₃. Scale stays discreet.
TH en °fH : douce < 15, moyennement dure 15–25, dure 25–35, très dure > 35 °fH (classes usuelles ARS).
Water data
| Total hardness | 16.2 °fH |
|---|---|
| Utility | Spl Ebr |
How Rennes compares
- Tap water in Rennes is 32 % softer than the median of the 33641 cities we track in France (23.9 °fH).
- By hardness it ranks 23301 of 33641 in France (1 = hardest); 30 % of these cities have softer water.
- In Ille-et-Vilaine it is number 182 of 324 cities; the regional median is 16.9 °fH.
- Nearest city with clearly softer water: Saint-Thurial, 21 km away (10.1 °fH).
- Nearest city with clearly harder water: Chevaigné, 12 km away (20.5 °fH).
Ranking of the hardest and softest water in France →
What it means at home
- Fast scale on taps, shower screens and kettle; white spots on glassware.
- Water heater: efficiency loss and element failures within years if unprotected.
- Dry skin and dull hair after showering are common.
- Softener recommended; jug or reverse osmosis for drinking.
How much salt will you use for softening?
Recommended, especially with a heater or boiler: scale cuts efficiency and shortens its life.
Descale a coffee machine or kettle about every 2 months (typical manufacturer guidance for this hardness).
Estimates for planning, not a quote. Check your appliance manual for exact settings.
Dishwasher setting in Rennes
| Brand | Setting for this water |
|---|---|
| Bosch | H02 |
| Miele | 9 °dH |
| AEG / Electrolux | 2 |
| Beko | 3rd |
| Hoover | S1 |
| 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 Rennes water hard or soft?
It is hard: 162 mg/L as CaCO₃ (9.5 gpg, 9.1 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Rennes 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 Rennes water?
9.5 gpg, equal to 9.1 °dH, 16.2 °fH or 162 mg/L calcium carbonate.
What dishwasher salt setting should I use in Rennes?
For 162 mg/L (9.1 °dH) check your manufacturer's table: level 4–5.
Do I need a water softener in Rennes?
Recommended if you have a water heater, boiler or tankless unit: it saves repairs and detergent.
Source
Source: Ministère de la Santé (ARS), contrôle sanitaire SISE-Eaux, commune 35238 (2026-08). Method: own measurement. Ministère de la Santé (ARS), contrôle sanitaire SISE-Eaux, commune 35238 · Outdated figure? Tell us