What’s in Your Kettle? Dialling In UK Tap Water for Filter and Espresso

The cup starts before the coffee

You can spend a small fortune on a burr grinder, weigh every dose and still wonder why the same beans taste flat on a Tuesday. Often the answer is sitting in the kettle. Water makes up more than 90 per cent of a brewed coffee, and in the UK it is not one thing. A flat in Inverness and a flat in Islington can have tap water with wildly different personalities.

This is not about chasing a laboratory-perfect number. It is about giving your coffee a fair hearing. Once you know what your water is doing, you can stop blaming the beans for every muddy cup and make a few small changes that actually help.

Hardness and alkalinity are not the same thing

Two numbers matter most for home brewers: hardness and alkalinity. They are related, but they are not interchangeable.

Hardness is mostly calcium and magnesium. Those minerals help pull flavour compounds out of coffee. Too little and the cup can taste thin or papery; too much and you can get chalky, drying textures. In the UK, hardness is often talked about in milligrams per litre as calcium carbonate, or ppm as CaCO3. A soft Highland supply might sit under 50 ppm. Parts of London and the South East can push well past 250 ppm.

Alkalinity is the water’s ability to neutralise acid. That sounds technical, but it matters because coffee is acidic. If alkalinity is very high, it buffers those bright fruit acids and can leave filter coffee tasting dull, flat or soapy. If alkalinity is very low, coffee can taste sharp and hollow. For most home filter brewing, a gentle middle ground is around 30 to 50 ppm alkalinity. Espresso often likes a touch more buffer, but not a chalk pit.

pH is less useful on its own. A water can have a neutral pH and still be hopeless for coffee if the alkalinity is high. TDS meters are handy for spotting changes, but they do not tell you whether the dissolved solids are hardness, alkalinity or something less helpful.

How to test without a lab coat

You do not need a science kit to get useful information.

Start with your water supplier’s published report. Most UK suppliers publish regional averages and ranges for hardness. They are not precise enough for brewing, but they tell you whether you are in a soft, moderate or hard area.

Next, get a cheap aquarium GH and KH test kit. GH gives you general hardness; KH gives you carbonate hardness, which is a practical stand-in for alkalinity. Follow the instructions, test a fresh glass of cold tap water and note the results. A TDS meter is a useful extra, especially if you use a filter jug, because it shows whether the filter is actually changing anything.

One important caveat: a standard carbon filter jug reduces chlorine and some odours, but it does not reliably remove hardness or alkalinity. Ion-exchange softeners swap calcium and magnesium for sodium, which can reduce hardness but leave alkalinity high. That is why softened water can still make dull coffee. Reverse osmosis removes almost everything, which is why it is a good blank canvas for remineralising.

A region-by-region sketch

These are broad brushstrokes, not guarantees. Water can change street by street, and older buildings can add their own quirks.

• Scottish Highlands and islands: often very soft, low-mineral water. Filter coffee can taste delicate, but very low alkalinity may make bright coffees taste sharper. A small amount of mineral adjustment can add body.
• Central Scotland and Edinburgh: generally soft to moderate, though supplies vary. Chlorine is often the first thing to fix.
• London and the South East: frequently hard and high in alkalinity thanks to chalk aquifers. This is where filtering or dilution with low-mineral water can make a dramatic difference to filter coffee.
• South West England: often soft or moderately soft, with some harder pockets. Bristol and parts of Devon can differ noticeably.
• Wales: mostly soft to moderate, with local variation. Soft water can flatter washed light roasts but may need a little buffer for espresso.
• Midlands: a mixed bag. Birmingham and surrounding areas can be moderate to hard, while other supplies are softer.
• North East and Yorkshire: variable, with some soft upland supplies and harder sources. Test rather than assume.
• North West: generally soft to moderate, but old plumbing and local treatment still matter.

The beginner-friendly starting recipe

If your tap water is hard, high in alkalinity or heavily chlorinated, the simplest upgrade is to start with low-mineral bottled water or reverse-osmosis water and add a little back. You do not need a precision lab, but a cheap 1 ml syringe helps.

Make two concentrates:

1. Epsom concentrate: dissolve 10 g of Epsom salt in 100 ml of distilled or RO water. This gives roughly 100 mg of Epsom salt per ml.
2. Bicarbonate concentrate: dissolve 10 g of bicarbonate of soda in 100 ml of distilled or RO water. This also gives roughly 100 mg per ml.

For 1 litre of low-mineral water, start with:

• 1.5 ml Epsom concentrate
• 0.5 ml bicarbonate concentrate

That lands you in a gentle starting zone of roughly 60 ppm hardness and 30 ppm alkalinity. It is not a sacred formula. It is a sensible place to begin. For espresso, you might prefer a touch more buffer, so try 2 ml Epsom and 0.7 ml bicarbonate per litre, then taste.

Use a clean bottle, shake well and keep it in the fridge. Make small batches at first. The aim is not to become a water chemist; it is to remove a variable so you can taste the coffee more clearly.

Small habits, better cups

Before you rebuild your water, try the cheap fixes. Run the tap for a few seconds, use fresh cold water and descale your kettle regularly. Chlorine and scale are not glamorous topics, but they are common culprits.

Then brew two cups side by side: one with your usual tap water, one with your adjusted water. Use the same coffee, dose, grind and temperature. Taste them warm, not scalding. You may find the adjusted cup tastes sweeter and cleaner, or you may find your soft tap water was already doing a lovely job. Either way, you will know more than you did before.

Better coffee is often less about buying another gadget and more about noticing what is already in the cup. Water is the quiet ingredient. Give it a little attention and the rest of your brew routine starts to make more sense.