Water hardness in Harrow (South East England): 36.1°fH, quality index 3/10. Official UK data.
✓ Compliant with DWI standardsHarrow has very hard water (36.1°fH). Monthly descaling of all appliances is essential. A water softener pays for itself in 2 to 3 years.
UK tap water is subject to the Water Supply (Water Quality) Regulations 2018, overseen by the Drinking Water Inspectorate (DWI). The sanitary compliance indicator shows whether the latest official analyses found any non-conformities on parameters such as bacteriology, nitrates, pesticides or heavy metals.
The TH (Total Hardness) measures calcium and magnesium content. Below 10°fH: soft. From 10 to 20: moderately soft. From 20 to 27: slightly hard. From 27 to 35: hard. Above 35: very hard. Harrow's water is very hard at 36.1°fH.
Maintenance budget: ~£101/year
For 2 persons, water at 36.1°f deposits around 1.7 kg of limescale per year in your appliances. Estimated cost in descalers, salt and filters.
Calcium: ~58 mg per litre
Your water provides 58 mg of calcium/L, which is 6% of the recommended daily intake.
Filter jug: cartridge every 3 months
At 36.1°f, replace the cartridge every 3 months. An under-sink filter lasts 6 to 12 months.
The tap water hardness in Harrow is 36.1°fH, corresponding to very hard water. In comparison, the England average in our dataset is 18.1°fH and the South East England region average is 26.9°fH.
Yes. The water in Harrow meets DWI sanitary standards (Water Supply (Water Quality) Regulations 2018). The hardness of 36.1°fH affects taste and appliance maintenance, not safety. Water in Harrow is supplied and monitored by Affinity Water.
With 36.1°fH, a water softener is a worthwhile investment in Harrow. Limescale reduces the lifespan of boilers, washing machines and coffee makers and increases energy consumption by 10 to 15% per mm of scale. An anti-scale filter is a minimum at this hardness level.
A hardness of 36.1°fH corresponds to approximately 58 mg of calcium per litre, or 6% of the recommended daily intake. This calcium is not dangerous; hard water can even contribute to your daily calcium intake.
Water in Harrow is supplied by Affinity Water. Data source: Affinity Water 2025.
Data: Affinity Water official zone-level hardness reports (2025). Values verified via DWI drinking water quality database.