Coffee Water Chemistry Explained:How Hardness, Minerals and Alkalinity Affect Your Coffee
Why does the same coffee taste completely different when you brew it at home, in a café or somewhere else?
You might blame the grinder. You might blame the recipe. You might even blame the coffee.
But there is another ingredient in your cup that is easy to overlook: water.
Coffee is mostly water. Once brewed, roughly 98% of a typical cup is water, which means the chemistry of that water can have a surprisingly large influence on how your coffee tastes.
The minerals dissolved in your water don't simply sit there doing nothing.
They affect how compounds are extracted from the coffee, how acidity is perceived and, at higher concentrations, what happens inside your kettle or espresso machine.
This is why two people can use the same coffee, the same grinder and the same recipe — and still produce noticeably different cups.
So what actually makes good coffee water? And is filtered water always better?
Let's get into it.
What is coffee water chemistry?
Coffee water chemistry is the study of how the minerals and other dissolved substances in your brewing water interact with coffee during extraction.
The important variables include:
Total hardness
Magnesium
Calcium
Alkalinity
Bicarbonate
Total dissolved solids (TDS)
pH
Chloride and sulfate
Sodium
You don't necessarily need to understand all of these to make better coffee.
In fact, there are two concepts worth understanding first:
Hardness and alkalinity.
Once those make sense, most of the rest starts to fall into place.
What is water hardness?
Water hardness is primarily associated with dissolved calcium and magnesium.
If you've ever seen white limescale around a kettle, shower head or tap, you've already encountered the effects of hard water.
Hard water contains higher concentrations of minerals, particularly calcium and magnesium.
Soft water contains fewer of them.
But when we're talking about coffee, hardness isn't simply a case of: Hard water = bad. Soft water = good
It's more complicated. Those minerals can actually help extract flavour compounds from coffee. The question is how much is present and which minerals are present.
Why does magnesium matter in coffee?
Magnesium is particularly interesting for coffee.
A landmark 2014 study by Christopher Hendon, Lesley Colonna-Dashwood and Maxwell Colonna-Dashwood examined how dissolved minerals interact with compounds found in coffee.
Their research showed that magnesium and calcium can interact with coffee compounds during extraction, with magnesium showing stronger interactions in the computational model used in the study.
The authors concluded that the mineral composition of brewing water plays an important role in how coffee compounds are extracted.
In simple terms:
The minerals in your water help determine what you get out of the coffee.
This is one reason specialty coffee professionals became increasingly interested in water recipes rather than simply saying "use filtered water."
Magnesium vs calcium: what's the difference?
Both magnesium and calcium contribute to hardness, but they don't behave identically.
Research suggests that magnesium can be particularly effective at interacting with coffee's flavour compounds.
Calcium also participates in extraction, but the two minerals can produce different extraction behaviour.
This doesn't mean you should simply maximise magnesium.
More extraction isn't automatically better extraction.
The same 2014 research makes an important point: there isn't one mineral composition that produces the ideal flavour from every roasted coffee. The balance between mineral content and alkalinity matters.
That's important.
There is no universal “perfect coffee water”.
What is alkalinity?
Alkalinity is one of the most misunderstood parts of coffee water chemistry. People often talk about pH when they're really talking about alkalinity.
They're not the same thing.
pH tells you the current acidity or basicity of the water.
Alkalinity tells you how much acid the water can neutralise.
For coffee brewing, that distinction is extremely important.
The Specialty Coffee Association's research on coffee water specifically highlights alkalinity as an important variable when considering the acidity of the finished cup.
Think of alkalinity as a buffer.
And one of the most important components of that buffering capacity is bicarbonate.
What does bicarbonate do to coffee?
Bicarbonate can react with acids.
Coffee contains a large number of organic acids, and those acids contribute significantly to the flavour and perceived acidity of the cup.
When your brewing water contains more bicarbonate, it has a greater ability to neutralise some of those acids.
That means:
More alkalinity → more buffering → less perceived acidity
This doesn't mean high-alkalinity water automatically produces bad coffee.
It means the water can change the balance of the cup.
This becomes particularly important with coffees that naturally have bright, complex acidity.
A delicate washed Ethiopian coffee, for example, can respond very differently to highly buffered water than a darker roasted espresso.
Why can hard water make coffee taste flat?
If your water contains a lot of minerals and particularly a lot of bicarbonate, it can change the balance of the extracted coffee.
The acidity that gives many specialty coffees their brightness and complexity can become muted.
The result can be perceived as:
Flat, Dull, Heavy, Bitter, Muted and Lacking clarity.
This is one reason that a coffee can taste fantastic at the roastery but disappointing at home.
The coffee hasn't necessarily changed.
The water has.
The SCA notes that high levels of bicarbonate can buffer coffee acids and significantly influence flavour.
What does soft water do to coffee?
Soft water contains lower concentrations of calcium and magnesium.
This can be useful for coffee because excessive mineralisation isn't necessarily desirable.
However, going too far in the opposite direction can also create problems.
Very low-mineral water doesn't necessarily extract coffee in the same way as appropriately mineralised brewing water.
So the goal isn't: “Remove everything from the water.”
The goal is: “Use water with a mineral composition that allows the coffee to extract well and taste balanced.”
That's a much more useful way of thinking about coffee water.
What is TDS in coffee water?
TDS stands for Total Dissolved Solids.
You'll often see it expressed in:
ppm — parts per million
A TDS meter measures the electrical conductivity of water and uses that measurement to estimate the concentration of dissolved material.
But there's an important catch.
Two waters can have the same TDS and behave differently in coffee.
For example, one water might contain more magnesium while another contains more calcium or sodium.
The number on the TDS meter could be similar while the actual chemistry is different.
The 2014 Hendon research highlighted why simply treating all dissolved minerals as interchangeable isn't sufficient for understanding coffee extraction.
So:
TDS is useful.
But TDS alone doesn't tell you everything you need to know.
Is pH important for coffee?
Yes — but perhaps not in the way you've been told.
A common piece of coffee advice is:
> “You want water with a pH of around 7.”
That's not completely useless, but it misses the bigger picture.
For brewing coffee, alkalinity can be more informative than pH because alkalinity tells us about the water's buffering capacity.
The SCA's research explains that water with similar pH values can have very different capacities to neutralise acids.
So if you're trying to improve your brewing water, don't obsess over pH alone.
Look at alkalinity and hardness as well.
Why does coffee water affect acidity?
This is where coffee gets really interesting.
When you taste a coffee and describe it as:
Lemon-like, Bright, Juicy, Wine-like, Tropical & Crisp.
You're experiencing a combination of compounds and sensory interactions. Many coffee acids contribute to these perceptions.
The water you use determines, in part, how those compounds are extracted and how their acidity is perceived.
This is particularly relevant to modern specialty coffee, where producers and roasters often work hard to preserve distinctive acidity.
We've written about this extensively in our articles on Thermal shock coffee processing and Fruit-juice co-fermentation, where fermentation and processing can fundamentally change the acids, aromas and flavour precursors present in the green coffee.
The water used at the brewing stage is another part of that chain.
Think of coffee flavour as a chain
A useful way to understand this is:
Variety → Farm → Processing → Drying → Roasting → Grinding → Water → Extraction → Cup
Every stage contributes something.
A producer might carefully control fermentation to develop particular aromatic compounds.
A roaster then develops the green coffee to preserve or transform those compounds.
The barista grinds the coffee and controls extraction.
And then comes the ingredient that makes up the vast majority of the finished drink: Water.
This is why we get so interested in the chemistry behind coffee. The final cup isn't created by the roaster alone. It's created by everything that happens between the coffee plant and the cup.
Can bad water ruin good coffee?
Absolutely.
You can have: Excellent green coffee, Exceptional processing, Careful roasting, A great grinder, A good espresso machine and still make disappointing coffee.
If your water contains excessive alkalinity, the coffee can lose some of its acidity and definition.
If the mineral composition is unsuitable, extraction can change.
If the water contains chlorine or other compounds that affect taste, you may notice those flavours directly.
And if the water is very hard, you also have another problem:
Your coffee equipment. Coffee water and limescale. The same minerals that influence your coffee can also build up inside your equipment. Calcium deposits are particularly associated with limescale.
Over time, this can affect: Kettles, Espresso machines, Boilers, Heating elements, Valves, Flow restrictors
This isn't just an equipment problem.
Scale can affect temperature stability and water flow, which can ultimately affect brewing consistency.
The Specialty Coffee Association has highlighted the importance of monitoring water chemistry not only for flavour but also for equipment protection.
So getting your water right can potentially benefit both: your coffee and your coffee machine.
What water should I use for coffee?
There isn't one answer that works for everyone. But as a starting point, look for water that is: Clean and pleasant tasting, Free from noticeable chlorin, Not excessively hard, Not excessively alkaline, Moderately mineralised and onsistent from day to day
The SCA has historically used water around 150 ppm dissolved mineral content for brewing equipment testing, with defined ranges for hardness and pH.
More recent SCA research also emphasises that alkalinity and brewing ratio need to be considered rather than relying on one universal number.
That's an important distinction. Coffee water isn't about chasing one magic number.
It's about finding a sensible chemical environment for extraction.
What if I live in a hard-water area?
This is particularly relevant in the UK. Tap water varies significantly between regions, so there is no single “UK tap water” profile.
The first thing to do is find your local water quality report.
Look for information about:
Calcium
Magnesium
Hardness
Alkalinity
Bicarbonate
Sodium
Chloride
Once you know what's coming out of your tap, you can decide whether you actually need to change it. You may not need an expensive bottled water solution.
Does a Brita filter make better coffee?
Yes, it can. But it depends on what's in your water to begin with.
A typical carbon filtration system can improve taste by reducing substances such as chlorine, while some filtration systems are also designed to reduce hardness.
The important question isn't:
“Is filtered water better?”
It's:
“What is my starting water, and what does my filter actually remove?”
If your tap water is already relatively soft and low in alkalinity, aggressively reducing minerals may not improve your coffee.
If you're dealing with hard, highly mineralised water, appropriate filtration may make a much bigger difference.
Can bottled water be better for coffee?
It can, but bottled water isn't automatically better. Different bottled waters have dramatically different mineral compositions.
Some are extremely low in minerals. Others are highly mineralised. Look at the label.
If the mineral content is listed, compare:
Calcium, Magnesium, Bicarbonate, Sodium & Total mineralisation
You can then start to understand why two bottles of water can produce completely different coffee.
What is the best water for espresso?
Espresso creates a slightly different problem because the coffee-to-water relationship is much more concentrated than in a typical filter brew.
The SCA has highlighted how beverage ratio changes the influence of alkalinity. In a concentrated espresso preparation, the amount of water relative to the coffee is much lower than in a typical filter brew, which changes how much buffering capacity is available relative to the extracted acids.
This is one reason there isn't a single water recipe that should simply be copied across every brewing method.
Water that produces a beautiful V60 might not produce the same result in espresso.
What is the best water for filter coffee?
Filter coffee uses substantially more water relative to the coffee than espresso.
That means the water's mineral composition and alkalinity can have a particularly noticeable influence on the finished beverage.
If you're brewing light roasted specialty coffee, you may want water that allows the coffee's acidity and aromatic complexity to remain clear without excessive buffering.
This is one reason specialty coffee brewers sometimes use custom water recipes.
The aim isn't simply to extract more.
It's to extract the right things in the right balance.
Can you make your own coffee water?
Yes. There is now a whole world of coffee-water recipes built around combinations of minerals such as:
Magnesium sulfate
Calcium chloride
Sodium bicarbonate
Potassium bicarbonate
Magnesium chloride
Specialty coffee professionals have developed recipes specifically for brewing.
Barista Hustle, for example, publishes different brew-water recipes and explains how mineral concentrates can be combined with low-mineral water.
But we wouldn't recommend blindly mixing chemicals into water for an espresso machine.
Some DIY recipes are designed for brewing rather than for prolonged contact with boilers and metal components, and certain low-mineral recipes can create corrosion risks in equipment.
For most home brewers, understanding your existing water and choosing an appropriate filter is a much simpler place to start.
How do I test my water for coffee?
You don't need a laboratory. Start with your local water supplier's published water-quality information.
For more control, inexpensive water-hardness tests can give you useful information about hardness.
More advanced coffee-water testing can measure:
Total hardness
Calcium hardness
Magnesium
Alkalinity
Chloride
Sulfate
pH
TDS
Coffee professionals increasingly use dedicated water-testing equipment to understand and monitor these variables. The SCA's 2026 Best New Product Awards, for example, included a brewing-water test system designed specifically to measure parameters including hardness, alkalinity, chlorine, chloride and sulfate.
You don't need all of that equipment to make good coffee.
But it demonstrates just how important water chemistry has become within specialty coffee.
A simple experiment you can do at home
Here's the easiest way to understand coffee water chemistry without becoming a chemist.
Take one coffee.
Keep everything identical:
Same beans. Same grinder. Same dose. Same water temperature. Same brew recipe.
Then brew it using two different waters.
For example:
Water A: Your normal tap water.
Water B: A filtered or bottled water with a different mineral profile.
Taste them side by side.
Don't worry about trying to identify every mineral.
Instead ask:
Which cup is more acidic?
Which is sweeter?
Which has more clarity?
Which tastes flatter?
Which has more bitterness?
Which has more body?
Which has a longer finish?
You may be surprised by how different the same coffee can taste. Why this matters with experimental coffee processing
This becomes even more interesting when you're drinking coffees produced using modern fermentation techniques.
At Swan Song, we're increasingly working with coffees where producers are deliberately manipulating fermentation, oxygen, temperature, yeast and drying to influence the final flavour.
Our thermal shock coffee article looks at how temperature manipulation can influence fermentation.
Our fruit-juice co-fermentation article explores how changing the fermentation environment can alter microbial activity and the chemical compounds produced during processing.
And our work with CO₂-flush and controlled fermentation coffees looks at another way producers manipulate the fermentation environment.
The brewing water becomes the final part of that equation.
A producer might spend months developing a coffee's flavour at origin.
A roaster then spends hours profiling and refining the roast.
You can then change the final expression of that coffee simply by changing the water.
That's what makes coffee so fascinating.
Does better water make better coffee?
Not necessarily.
It makes different coffee.
The goal is not to find the most expensive water or the water with the highest mineral content.
It's to find water that allows the particular coffee you're brewing to express itself well.
A bright washed coffee might benefit from a different balance than a chocolate-heavy natural.
A light roast may behave differently from a darker roast.
Espresso may respond differently from filter.
And a highly processed experimental coffee may respond differently again.
There is no single answer. That's the point.
At Swan Song, we're interested in coffee as a complete system. We don't just want to tell you that a coffee tastes like peach, chocolate or black tea.
We want to explain why. The variety matters. The soil matters. The altitude matters. The processing matters. The fermentation matters. The roast matters. And then, finally, the water matters.
If you're buying specialty coffee and finding that it doesn't taste the same at home as it did when you first tried it, don't automatically assume you've bought the wrong coffee.
Start with the basics.
Check your grinder, check your recipe, check your extraction and then check your water.
You might discover that the biggest change you can make to your coffee isn't another £500 piece of equipment.
It might simply be what's coming out of your tap.
The simple version
If you remember nothing else from this article, remember these five things:
1. Water isn't just water.
The minerals dissolved in your water affect coffee extraction and flavour.
2. Hardness comes mainly from calcium and magnesium.
Both can influence extraction, but they don't behave identically.
3. Alkalinity is different from pH.
Alkalinity describes the water's ability to buffer acids and can have a major effect on perceived coffee acidity.
4. TDS doesn't tell the whole story.
Two waters can have similar TDS but different mineral compositions and produce different cups.
5. There is no universal perfect coffee water.
The best water depends on the coffee, roast and brewing method.
And if you're struggling to get the same flavour at home that you tasted from your roaster? Check the water.
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Frequently Asked Questions About Coffee Water Chemistry
What is the best water for brewing coffee?
There isn't one universal best water. Good brewing water should be clean-tasting, reasonably mineralised and have an appropriate balance of hardness and alkalinity for the coffee and brewing method.
Does hard water make coffee taste bad?
Not necessarily. Calcium and magnesium can help extract coffee compounds, but excessive mineral content or alkalinity can alter the balance of the cup and contribute to a flatter or more bitter profile.
Is soft water better for coffee?
Not automatically. Very soft water can also extract coffee differently. The goal is balanced mineralisation rather than simply removing all minerals.
Is magnesium or calcium better for coffee?
Both can influence extraction, but research has shown that magnesium interacts strongly with coffee compounds. The ideal balance depends on the coffee and the desired flavour profile.
What does bicarbonate do to coffee?
Bicarbonate contributes to alkalinity and acts as a buffer, neutralising some of the acids extracted from coffee. Higher alkalinity can therefore reduce perceived acidity.
What is TDS in coffee water?
TDS means Total Dissolved Solids. It provides an estimate of the amount of dissolved material in water, but it does not tell you exactly which minerals are present.
Does filtered water make better coffee?
It can, particularly if your tap water contains high levels of hardness, alkalinity or chlorine. But the effectiveness depends on what your starting water contains and what the filter removes.
Should I use bottled water for coffee?
You can, but bottled water varies considerably in mineral composition. Check the mineral analysis on the label rather than assuming bottled water is automatically better.
Can I use distilled water for coffee?
Pure distilled or demineralised water isn't generally the simplest choice for brewing because it contains very few dissolved minerals. Coffee extraction depends partly on the mineral composition of the brewing water.
Does coffee water affect espresso?
Yes. Espresso uses a concentrated beverage ratio and is particularly sensitive to changes in extraction. Water chemistry can influence both flavour and equipment maintenance.
How can I improve my coffee without buying new equipment?
Start with your water, grinder, recipe and extraction. Testing or researching your local tap water can be a surprisingly inexpensive way to identify a problem.
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Good coffee deserves good water.
The next time your coffee tastes different from the cup you had at the roastery, look beyond the beans. The answer might be coming straight from your tap.
