What the NHS Says About Tap Water for Babies
The NHS recommends that all tap water used for infant formula should be boiled and cooled, regardless of hardness. This is not about hardness — it's about eliminating microbiological risk for very young infants whose immune systems are still developing. For babies over 6 months, standard municipal tap water from a reliable supply is generally considered safe.
Water hardness is a separate question: it refers to dissolved calcium and magnesium minerals, which are not themselves harmful. However, very high mineral concentrations can affect how formula powder mixes, slightly alter the nutritional balance of the final feed, and may cause digestive discomfort in some sensitive infants.
Water Hardness Guidelines for Infant Formula
WHO (2022)
No specific hardness limit for infants. Recommends water from reliable, properly treated municipal supplies. For formula, boiling is advised for infants under 6 months.
NHS (UK)
Boil and cool all tap water used for formula, regardless of hardness. No upper hardness limit specified, but advises consulting a health visitor for high-hardness areas.
FSA (UK Food Standards Agency)
For areas with softeners, do not use softened water for infant formula — the sodium exchange process raises sodium levels above the safe infant threshold of 200 mg/L.
EU Directive 2020/2184
Drinking water hardness is not regulated at the EU level. Member states set their own guidance. All municipal supplies must meet microbiological and chemical safety standards independently.
Important: Do Not Use Water Softener Output for Baby Formula
Ion-exchange water softeners work by replacing calcium and magnesium ions with sodium ions. This increases the sodium content of the water significantly — sometimes above 200 mg/L, which exceeds the safe threshold for infant formula. If your home has a water softener, always use water from the dedicated unsoftened drinking tap (usually installed in the kitchen) for formula preparation. Never use hot tap water for formula either — hot water tanks leach more minerals and can harbour bacteria.
Do I Need a Filter?
In most UK cities, tap water is safe for formula without a filter — simply boiling is sufficient for under-6-month infants. However, if your local hardness is above 200 mg/L (which applies to London, Birmingham, Leeds, and most of South East England), a filter can reduce mineral content and improve the taste and consistency of formula.
If you use a filter, you must still boil the filtered water for infants under 6 months. Filters remove minerals but cannot guarantee microbiological safety without a boiling step. Replace filter cartridges per manufacturer guidelines — an old filter can harbour bacteria.
Hardness by City: UK Quick Reference
| City | Hardness | Formula Guidance |
|---|---|---|
| London | ~360 mg/L | Filter recommended for daily use |
| Birmingham | ~291 mg/L | Filter recommended |
| Leeds | ~320 mg/L | Filter recommended |
| Bristol | ~150 mg/L | Boiling sufficient |
| Manchester | ~68 mg/L | Boiling sufficient |
| Edinburgh | ~20 mg/L | Boiling sufficient |
| Glasgow | ~14 mg/L | Boiling sufficient |
Values are approximate municipal averages. Check your specific city page for verified data.