Coffee is approximately 98 to 99 percent water. That fact alone should settle the question of whether water quality matters. When you taste your cup of coffee, you are tasting whatever minerals, compounds and characteristics your water brings along for the ride, combined with the extracted coffee compounds. If your water tastes odd, your coffee will taste odd. If your water is very hard or very soft, it will extract coffee differently, and the result in the cup will be noticeably different to what the same coffee tastes like brewed with ideal water.
This is not just theory. Professional baristas competing in world championships select their water with as much care as they select their coffee. The effect is measurable and real. For everyday home brewing, you do not need to go to those lengths, but understanding the basics will help you make one or two simple changes that will genuinely improve your daily cup.
What Water Chemistry Does to Coffee Extraction
Water extracts coffee compounds through a combination of chemistry and physics. Not all minerals in water interact with coffee in the same way. The two most relevant minerals for coffee brewing are magnesium and calcium.
Magnesium has an affinity for the aromatic compounds in coffee that give it its flavour and complexity. Water with a good level of magnesium tends to extract coffee more thoroughly and produce a more flavourful cup. Calcium also plays a role in extraction but its relationship with coffee is more complex: moderate calcium is beneficial, but high calcium (as found in very hard water) can suppress extraction of certain flavour compounds and produce a flat, dull tasting brew.
The total hardness of your water (measured in parts per million or milligrams per litre of dissolved minerals) is therefore important. Very hard water has too many minerals and can suppress extraction. Very soft water (like distilled water, which has almost no minerals at all) also extracts poorly and can produce a flat, sour cup. The sweet spot is water with a moderate mineral content: enough to facilitate good extraction but not so much that it interferes with it.
Water Hardness in the UK: Why Location Matters
The UK has significant regional variation in water hardness, and this directly affects how your coffee brews at home. Broadly speaking:
- London and the South East: Very hard water, typically 250 to 400 mg per litre or above. This is some of the hardest tap water in the country. It scales up kettles and coffee machines quickly and can noticeably flatten coffee flavour. Filtering is recommended.
- East Midlands: Hard water, similar issues to London though slightly less extreme.
- South West, Wales, Scotland and Northern Ireland: Generally softer water that is closer to ideal for coffee brewing. Tap water in many of these areas can be used with good results.
- Yorkshire and North West England: Variable, often moderately hard.
You can check the hardness of your local water on your water supplier's website. Most publish this information and it is worth knowing, not only for coffee but for your kettle and any water using appliances.
What to Do About Hard Water
If you live in a hard water area, the simplest and most effective fix is a jug filter with an ion exchange cartridge. Brita is the most familiar brand in the UK, and the standard Maxtra Plus filters reduce temporary hardness (calcium and magnesium carbonate) while retaining some beneficial minerals. The result is water that typically falls much closer to the ideal range for coffee brewing.
Filter jugs do need their cartridges replaced regularly, usually every 4 to 8 weeks depending on your water hardness and usage. Ignoring this leads to a filter that provides less benefit and potentially releases scale back into your water. Set a reminder and replace on schedule.
For those who want to go further, in line scale filters for kettle connections are available, as are filter taps that treat all water coming through them. These are more expensive upfront but more convenient for households that use a lot of filtered water.
Water Temperature: The Other Critical Variable
Mineral content is not the only water variable that matters. Temperature is equally important. The Specialty Coffee Association recommends brewing at 90 to 96 degrees Celsius, measured at the point of contact with the coffee grounds.
Boiling water at 100 degrees Celsius over extracts coffee, pulling harsh, bitter compounds into the cup. Using water that is too cool (below 88 degrees Celsius) produces under extraction: a sour, thin, underdeveloped cup. The ideal range of 90 to 96 degrees Celsius is where the extraction is balanced and the full range of flavour compounds are properly solubilised.
For most brewing methods, the practical approach is to boil your kettle and then let it sit for 30 to 60 seconds before brewing. This drops the temperature from 100 degrees Celsius to around 93 to 96 degrees Celsius, right in the ideal zone. If you have a temperature controlled kettle (increasingly common and not particularly expensive), set it to 93 degrees Celsius for a reliable result across most brewing methods. Use 91 degrees Celsius for light roast coffees where you want to avoid over extraction of the more delicate aromatic compounds.
- Coffee is 98–99% water. Water quality directly affects extraction and flavour in every cup you make.
- Ideal brewing water has moderate mineral content (50–150 mg/L TDS) — enough to extract well, not so much it suppresses flavour.
- London and the South East have very hard tap water (250–400 mg/L). A Brita jug filter noticeably improves coffee in these areas.
- Ideal brewing temperature is 90–96°C. Let boiled water sit 30–60 seconds before brewing for a reliable result.
- Scotland, Wales and the South West generally have softer water that is closer to ideal for coffee brewing straight from the tap.
Frequently Asked Questions
Can I use tap water for coffee in the UK?
Is filtered water or bottled water better for coffee?
Does water temperature affect coffee flavour?
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