A Guide to Water Composition and Coffee

There are a lot of things to consider when brewing a cup of coffee. The type of beans, water temperature, bean freshness, brewing duration and many other factors can all make a difference to the final taste.

Arguably the most important factor – and also one that’s commonly overlooked – is the water you use. Despite its transparent appearance, the hidden balance of chemicals and minerals in water can greatly influence taste.

Many people assume that pure water is the best for coffee, which is why they often add pure water to their coffee machine. There is some truth to this, but the presence of calcium and other hard minerals can have a positive effect on flavour.

So, to help you improve your coffee, here’s an overview of how water affects taste.

An Introduction to TDS and Water Chemistry

Water chemistry doesn’t sound like the most exciting topic – but it shouldn’t be overlooked by coffee enthusiasts. You need to know what terms like “hard water” and “total dissolved solids” mean before you can optimise the water you use, after all.

The acronym TDS stands for “total dissolved solids.” It’s a way of measuring hardness, as it includes the quantity of all hard minerals (such as calcium and magnesium) the water contains.

TDS is given in units of parts per million (ppm). The typical range for tap water is 50-180ppm, with the higher end being considered hard water.

How much is recommended for coffee though?

As I mentioned earlier, a lower TDS isn’t always better. In fact, the SCAA gives a recommended value as 150ppm. Anything between 80-240ppm is considered acceptable for brewing coffee though.

This is only part of the story though.

Coffee cup

Which Minerals are Important?

There’s a problem with TDS. While it gives a broad measure of water hardness, it doesn’t tell you how much of each mineral the water contains. The quantity of certain types of minerals often matters more than the total amount of all minerals – especially as some minerals are desirable for a great cup of coffee.

With that in mind, here’s an overview of some of the most common minerals found in UK water – and how they affect coffee:

  • Iron. Iron enters the water supply via old pipes. As you might guess, it can cause coffee to taste metallic, so it should be filtered out whenever possible.
  • Copper. Copper is similar to iron, as it seeps out from pipes and causes a metallic taste. Like iron, it’s important to filter it out for best taste.
  • Carbonates. Carbonate is a substance that soaks up acid if levels start to get too high, so it plays an important role in taste. High levels can cause calcium carbonate to form on equipment though – especially if the water also contains a lot of calcium.
  • Calcium. Calcium is one of the most common hard minerals in UK water supplies. It’s also the mineral behind most limescale problems. Surprisingly, calcium in water can help extract flavour from beans, so you don’t want to filter it all out.
  • Magnesium. Magnesium is less common than calcium in the UK, but has similar properties. An advantage of magnesium is that it helps coffee extraction.
  • Chlorides. Salt (sodium chloride) isnot a good idea in coffee, but chlorides without sodium can also affect your brew. While low levels may make your coffee taste a little sweeter, they can damage stainless steel elements. It’s usually best to filter chlorides.

There are other factors that affect the quality of water. Acidity, for example, can greatly affect the bitterness of the final taste. You should aim for a neutral acidity to prevent this.

How to Get the Ideal Water Composition

As you can tell, there are a lot of factors affecting “good” and “bad” water for making coffee. The ideal water shouldn’t be too hard, but it needs a significant amount of calcium and magnesium to extract maximum flavour. It also shouldn’t be acidic or have high levels of chlorides.

The most cost effective way to get ideal water is to treat it yourself. There are many filters available for water, including ones that remove carbon, sediment, chlorine and other undesirable minerals while leaving calcium and magnesium.

These filters range from simple filters up to reverse osmosis systems. The latter usually removes all hard minerals, including calcium, but newer models allow you to control the amount that remains in the coffee. Some coffee machines include filters as standard (check out my favourites here), although this depends on the brand.

Are these filters really worth it though? If you live in a hard water area, or just want complete control over your water mineralisation, a reverse osmosis system that’s designed for coffee can be a worthwhile investment. Simpler filters can do a decent job and cost much less though.

Of course, you could always buy bottled water. This is a great option – but the cost adds up quickly. There’s also no guarantee that the water composition is ideal for coffee.

Other Factors

As you already know, water composition isn’t the only factor that affects the taste of your coffee.

A good example is water ratio. There’s a lot of science behind a ratio of 1 unit of coffee to 17.4 units of water. This allows the solids to fully dissolve while providing strong coffee – although you may need to tweak this ratio depending on your tastes.

Another factor is the brewing temperature. In general, a temperature around 200 degrees Fahrenheit (or 5 degrees either side) provides optimal extraction rate. If the water is too cold, coffee isn’t extracted fast enough so it’ll taste flat. If the water is too hot, you lose much of the taste.

If you’d like to learn more about the science of coffee, check out my in-depth article here.


Many people consider brewing coffee to be an art. There’s truth to this – it takes experience to brew the perfect coffee – but science can teach us a lot about why coffee tastes the way it does.

Water composition is a great example. By understanding how the quantity of minerals in water affects the taste – including which are desirable and which should be filtered – you can optimise water content for the perfect taste. This isn’t always easy, but it explains why coffee often tastes too acidic, flat or metallic even when the brewing process seems to be perfect.

Do you have any questions about water composition in coffee? Or any tips about how to filter water to improve taste? I’d love to hear them in the comments.

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