Water hardness in Derry
92 mg/L moderately hard
Tap water in Derry is moderately hard: 92 mg/L (5.4 gpg · 5.2 °dH · 9.2 °fH).
Source: NI Water, customer tap results 2024,2025 (OpenDataNI, OGL v3.0) (2025-12) · data checked 2026-10-19
Range across supply zones: 86–99 mg/L.
The tap water in Derry is provided by NI Water from one of its 24 treatment works, treating lough, river or reservoir water rather than groundwater. Reported hardness is 92.4 mg/L CaCO₃, ranging from 86.1 to 98.6 mg/L between supply zones. Hardness varies with the catchment each treatment works draws from; it sits in the moderately hard band, where limescale appears on kettles and shower heads over months and dishwasher salt helps.
UK suppliers report mg/L CaCO₃ and often Clark degrees (1 °Clark = 14.25 mg/L).
Water data
| Total hardness | 92 mg/L |
|---|---|
| Utility | Northern Ireland Water |
How Derry compares
- Tap water in Derry is 50 % softer than the median of the 471 cities we track in the United Kingdom (185 mg/L).
- By hardness it ranks 329 of 471 in the United Kingdom (1 = hardest); 30 % of these cities have softer water.
- In Northern Ireland it is number 61 of 110 cities; the regional median is 100 mg/L.
- Nearest city with clearly softer water: Dungiven, 26 km away (70 mg/L).
- Nearest city with clearly harder water: Strabane, 22 km away (120 mg/L).
Ranking of the hardest and softest water in the United Kingdom →
What it means at home
- Visible kettle scale every few weeks; vinegar or citric acid is enough.
- Water heater and washing machine: some long-term scale; a scale inhibitor slows it.
- Dishwasher: medium salt setting.
- Softener optional; pays off mainly with a gas boiler or tankless heater.
How much salt will you use for softening?
Optional. A scale inhibitor or a small softener protects the heater; soap savings are modest.
Descale a coffee machine or kettle about every 3 months (typical manufacturer guidance for this hardness).
Estimates for planning, not a quote. Check your appliance manual for exact settings.
Dishwasher setting in Derry
| Brand | Setting for this water |
|---|---|
| Bosch | H00 |
| Miele | 5 °dH |
| AEG / Electrolux | 2 |
| Beko | 2nd |
| Hoover | S1 |
| LG | H1 |
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 Derry water hard or soft?
It is moderately hard: 92 mg/L as CaCO₃ (5.4 gpg, 5.2 °dH). The usual bands are soft < 60, moderately hard 60–120, hard 120–180 and very hard > 180 mg/L.
Is tap water in Derry safe to drink?
Hardness is not a health issue; it is about limescale, taste and appliances. Mains water must meet national drinking-water standards. UK suppliers report mg/L CaCO₃ and often Clark degrees (1 °Clark = 14.25 mg/L).
How many grains per gallon is Derry water?
5.4 gpg, equal to 5.2 °dH, 9.2 °fH or 92 mg/L calcium carbonate.
What dishwasher salt setting should I use in Derry?
For 92 mg/L (5.2 °dH) check your manufacturer's table: level 2–3.
Do I need a water softener in Derry?
Not essential; a scale inhibitor protects the heater cheaply.
Source
Source: NI Water, customer tap results 2024,2025 (OpenDataNI, OGL v3.0) (2025-12). Method: own measurement. NI Water, customer tap results 2024,2025 (OpenDataNI, OGL v3.0) · Outdated figure? Tell us