WaterWhat Is Water Hardness?
A complete guide to understanding water hardness in the UK — what causes it, how it's measured, and what it means for your home.
Water Hardness Explained
Water hardness is a measure of the dissolved mineral content in your water supply, primarily calcium carbonate (CaCO₃) and magnesium carbonate. When rainwater passes through rock formations like chalk and limestone, it dissolves these minerals and carries them into the water supply.
Hard water is not harmful to health — in fact, the minerals can be beneficial. However, it causes limescale buildup in pipes, kettles, boilers, and other appliances, which can increase energy costs and reduce the lifespan of household equipment.
How Is Water Hardness Measured?
Water hardness is most commonly measured in parts per million (ppm) of calcium carbonate, also written as mg/L CaCO₃. There are several measurement scales used worldwide:
| Scale | Unit | Conversion from ppm |
|---|---|---|
| PPM (mg/L) | Parts per million | 1 ppm = 1 mg/L |
| Clark Degrees (°Clark) | UK traditional | 1° = 14.286 ppm |
| German Degrees (°dH) | European standard | 1° = 17.848 ppm |
| French Degrees (°fH) | French standard | 1° = 10 ppm |
| Grains per Gallon (gpg) | US standard | 1 gpg = 17.118 ppm |
Water Hardness Classification
The standard UK classification for water hardness is:
| Classification | PPM Range | Clark Degrees |
|---|---|---|
| Soft | 0 - 60 ppm | 0 - 4.2° |
| Slightly Hard | 61 - 120 ppm | 4.3 - 8.4° |
| Moderately Hard | 121 - 180 ppm | 8.5 - 12.6° |
| Hard | 181 - 300 ppm | 12.7 - 21.0° |
| Very Hard | 301+ ppm | 21.1°+ |
What Causes Hard Water?
The hardness of your water depends almost entirely on the geology beneath your area. Here's why different regions have different hardness levels:
- Chalk and limestone areas (South East, East Anglia, Thames Valley) — Very hard water (250-400 ppm). These sedimentary rocks are rich in calcium carbonate.
- Sandstone and clay areas (Midlands, parts of Yorkshire) — Moderate to hard (150-300 ppm). Less mineral dissolution than chalk.
- Granite and metamorphic rock areas (Scotland, Wales, Devon/Cornwall) — Soft water (10-100 ppm). These ancient, hard rocks barely dissolve.
- Peat and upland areas (Lake District, Pennines) — Soft water (30-100 ppm). Reservoir water from high ground with minimal rock contact.
Water Hardness Across the UK
The UK has some of the most varied water hardness in Europe. London and the South East are among the hardest water areas in the world, while Scotland has some of the softest. The national average is around 200 ppm, classified as hard.
See the 50 hardest water areas or the 50 softest. You can also check your postcode for an instant result.
Why Does Water Hardness Matter?
While hard water is safe to drink, it has practical implications for your home:
- Limescale buildup in kettles, pipes, boilers, and heating elements
- Higher energy bills — limescale acts as an insulator, making your boiler work harder
- Dry skin and hair — hard water makes soap harder to lather and rinse
- Soap scum on shower screens, taps, and bath surfaces
- Reduced appliance lifespan — washing machines and dishwashers scale up faster
Read more about hard water effects or explore solutions for hard water.
Frequently Asked Questions
Is hard water safe to drink?
What PPM is considered hard water?
Why is water hard in some areas?
Can hard water damage my boiler?
How do I find my water hardness?
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