Calcium and magnesium are probably the two most discussed minerals in specialty coffee water.
You will often hear simple descriptions:
Magnesium extracts more.
Calcium adds body.
Magnesium brings fruit.
Calcium brings roundness.
These ideas are useful starting points.
But they are not the whole story.
The science is more complicated — and that makes calcium and magnesium much more interesting.
Why are calcium and magnesium important?
Calcium and magnesium are both divalent cations.
In simple terms:
Ca²⁺
Mg²⁺
Both contribute to what we commonly call water hardness.
But they are not interchangeable.
They have different chemical properties, interact differently with dissolved compounds, and can be introduced into brewing water through different salts.
So two waters with the same total hardness can still have very different calcium-to-magnesium compositions.
And potentially very different sensory behaviour.
The classic magnesium story
Magnesium became particularly famous in coffee-water science after the work of Hendon, Colonna-Dashwood and Colonna-Dashwood.
Their 2014 study used computational chemistry to investigate interactions between dissolved cations and compounds found in coffee.
Their model suggested that magnesium could interact strongly with certain coffee compounds and potentially facilitate their extraction.
This became one of the foundations for the modern idea that magnesium-rich water can be a powerful extraction tool.
It is also one reason why specialty coffee water recipes often favour magnesium.
But there is an important distinction:
A plausible molecular interaction is not the same thing as proof that increasing magnesium in real brewing water will always increase extraction or improve flavour.
What does newer research say?
A 2024 experimental study by Bratthäll, Figueira and Nording investigated calcium and magnesium chloride directly.
Instead of modelling molecular interactions, they brewed coffee using different concentrations of calcium and magnesium and measured organic acids using GC-MS and NMR.
At concentrations typically found in drinking water, the researchers found only limited changes in acid content.
When the concentrations were increased to roughly ten times those levels, the differences became more pronounced.
Even more interestingly, comparisons between adding the minerals before brewing and adding them after brewing produced similar acid content in most cases.
The authors concluded that extraction of the measured acids appeared to proceed largely independently of the divalent ions.
They proposed that some flavour differences may instead arise from interactions occurring after extraction.
So the simple equation:
More magnesium = more extraction
is not established science.
The question is still open.
Does that mean magnesium doesn't matter?
Not necessarily.
It means we should be more careful about explaining why it matters.
Water composition can still influence the resulting beverage.
The effect may involve extraction, post-extraction interactions, buffering, mineral composition, or several mechanisms at once.
And sensory work can reveal differences even when the underlying chemical mechanism isn't fully understood.
This distinction is important.
We can observe a sensory effect without knowing exactly why it happens.
What does sensory research suggest?
APAX's sensory work takes a different approach.
Rather than focusing only on extraction chemistry, their Ion Triangle uses sensory tasting to associate individual ions with perceived characteristics in coffee.
Their work associates magnesium-rich profiles with characteristics such as brightness, fruitiness and sweetness.
Calcium is associated with a fuller, rounder sensory direction.
These observations are valuable.
But they should be described as sensory observations, not universal chemical laws.
In other words:
Research can tell us what mechanisms are plausible.
Sensory experiments can tell us what trained tasters perceive.
The two shouldn't automatically be treated as the same thing.
Magnesium: a direction, not a guarantee
If you increase magnesium in a water profile, don't assume the coffee will automatically become:
more fruity
more sweet
more extracted
or better.
The result depends on the entire water composition and the coffee being brewed.
A washed Ethiopian and a dense natural Brazil are not the same chemical system.
Neither are a light roasted Gesha and a dark roasted blend.
The same water change can therefore produce different results in different coffees.
That's why we prefer the word:
direction
rather than:
rule.
Magnesium may move a coffee in a particular sensory direction.
It does not guarantee a particular flavour.
Calcium: more than "body"
Calcium is often described as the mineral that gives coffee body or roundness.
This is a useful sensory shorthand.
But calcium has a much more complicated role in water chemistry.
It contributes to hardness.
It can interact with coffee compounds.
It can combine with carbonate species and contribute to scale formation.
And its effect depends on concentration and the rest of the water composition.
So:
Calcium = body
is too simplistic.
A better way to think about it is:
Calcium is one of the variables that can influence the overall expression of a water profile.
Calcium and magnesium are not interchangeable
Imagine two waters.
Water A
50 ppm calcium hardness
10 ppm magnesium
Water B
10 ppm calcium
50 ppm magnesium
They could have similar overall hardness.
But they are not the same water.
The ratio of Ca²⁺ to Mg²⁺ has changed.
And so has the ionic composition.
This is one reason we don't describe water simply as:
"50 ppm hardness."
We want to know what is actually making up that hardness.
The salt matters too
This is one of the most important concepts in mineral formulation.
Magnesium doesn't enter water by itself.
It usually comes from a salt.
For example:
MgCl₂
dissociates into:
Mg²⁺ + 2Cl⁻
But magnesium sulfate:
MgSO₄
gives:
Mg²⁺ + SO₄²⁻
The magnesium ion is the same.
The accompanying ion is different.
And therefore the resulting water chemistry is different.
The same principle applies to calcium.
CaCl₂ introduces calcium and chloride.
So when we formulate water, we shouldn't only ask:
"How much magnesium do I want?"
We should also ask:
"What salt am I using to introduce it?"
And:
"What else am I adding with it?"
This is why TDS isn't enough
Suppose two waters both measure:
60 ppm TDS.
One contains mostly:
Mg²⁺ + HCO₃⁻
The other contains:
Ca²⁺ + Cl⁻ + SO₄²⁻
The number may be similar.
The chemistry isn't.
This is exactly why TDS is useful as a checkpoint but insufficient as a complete description of brewing water.
The composition gives the number meaning.
Calcium vs magnesium is not a competition
We sometimes see coffee water discussed as if calcium and magnesium are competing for the title of:
"Best extraction mineral."
We don't think that's the most useful question.
The better question is:
What balance makes sense for this coffee?
A coffee that benefits from a more vibrant and delicate expression may respond differently to a magnesium-forward profile than a coffee where you are looking for more roundness and weight.
Neither mineral is inherently better.
They are different variables.
What about hardness?
Because calcium and magnesium are the major contributors to hardness, changing their concentrations changes hardness.
But hardness is not the same thing as alkalinity.
Hardness is primarily associated with calcium and magnesium.
Alkalinity describes the water's capacity to neutralise acids, with bicarbonate being a major contributor in many brewing waters.
You can therefore have:
high hardness + low alkalinity
or:
low hardness + high alkalinity.
Those waters can behave very differently in the cup.
This is why we separate hardness from buffer when thinking about water design.
What should you actually do?
If you're experimenting with coffee water, don't start by asking:
"Should I use magnesium or calcium?"
Start with the coffee.
Then ask:
What expression am I looking for?
More vibrant?
More delicate?
More rounded?
More structured?
More clarity?
More sweetness?
Then choose a water profile as an experiment.
Keep the other brewing variables as consistent as possible.
Taste.
Record.
Change one variable.
Taste again.
This is much more useful than blindly chasing a mineral number.
The Water Artist approach
We don't think magnesium is the "good" mineral and calcium is the "bad" mineral.
We don't think calcium automatically means body.
We don't think magnesium automatically means extraction.
And we don't think there is one perfect Ca:Mg ratio for every coffee.
Instead, we think about the entire ionic composition.
Coffee.
Water.
Extraction.
Perception.
And the interaction between them.
The important question isn't "which is better?"
It's:
"What happens when I change the balance?"
That is where coffee water becomes interesting.
Because calcium and magnesium don't need to compete.
They give us different ways to explore how a coffee can express itself.
The Water Artist
Science gives us the mechanisms we can investigate.
Sensory work gives us observations.
Brewing gives us the experiment.
And the cup gives us the result.
Don't chase the "best" mineral.
Find the water that lets this coffee speak.
Water is the canvas.
You're the artist.