The Water Behind the Cup: Why Water Chemistry Matters for Coffee
Water makes up about 98% of your brewed coffee, yet almost nobody thinks about it. We explain TDS and hardness in plain terms, why both very soft and very hard water cause real problems, and give you a simple at home fix that does not require a lab kit.
You dial in your grind. You weigh your beans. You time your pour to the second. And then you fill the kettle from the tap without a second thought. It is a strange blind spot, because water makes up roughly 98% of a brewed cup of coffee. Everything else, the beans, the grind, the technique, is working with the remaining 2%.
If your coffee tastes flat, dull, or inconsistent even when your recipe is solid, the water is worth a closer look.
TDS and hardness, in plain terms
Two numbers describe most of what matters in brewing water.
TDS (Total Dissolved Solids) measures the total amount of dissolved minerals and other solids in the water, usually in parts per million (ppm). Distilled water has close to zero TDS. Hard tap water can sit well above 300 ppm.
Hardness specifically measures dissolved calcium and magnesium. These minerals are not just filler, they actually help pull flavor compounds out of the coffee during extraction. That is the part most people do not expect: a small amount of hardness is good for coffee, not bad.
The Specialty Coffee Association's guideline aims for water around 150 ppm TDS with a good balance of calcium and magnesium, but you do not need a lab to get close to this. You need to understand the two failure modes on either side of that target.
Why very soft water falls flat
Very soft water (think distilled, reverse osmosis, or naturally very soft tap water) has almost no calcium or magnesium to bond with. Without those minerals to grab onto, fewer flavor compounds get pulled out of the grounds during brewing, even with a perfect grind and temperature.
The result is coffee that tastes thin, muted, and strangely one dimensional, as if you brewed with less coffee than you actually used. People often respond by grinding finer or using more coffee, when the real issue is the water itself.
Why very hard water goes dull, chalky, or scaly
At the other extreme, water with too much calcium and magnesium causes its own problems. Excess minerals can bind with coffee's flavor compounds in ways that mute brightness and leave a chalky, flat aftertaste. Hard water also tends to carry more bicarbonate, which pushes the brew's pH higher and can flatten acidity that would otherwise read as pleasant and lively.
There is a second cost too: limescale. If you have ever descaled a kettle or espresso machine and found a crust of white mineral deposit inside, that is hard water leaving its mark. Over months, scale buildup restricts water flow and heating elements, which changes brew temperature and consistency, and eventually damages the machine.
Tap, filtered, or bottled: what actually changes
Tap water varies enormously by city and even by neighborhood, since it depends on the local water source and treatment. Berlin tap water, for example, tends to run moderately hard. It is often perfectly fine for coffee, but it is worth tasting a cup made with it versus one made with filtered water to see if you notice a difference.
Filtered water (a basic activated carbon filter jug, or an inline filter on your machine) removes chlorine taste and some minerals, generally softening hard water toward a more balanced range. This is the easiest fix for most households and usually the best return for the least effort.
Bottled water is not automatically better. Many bottled mineral waters are far too high in TDS for coffee, since they are marketed for their mineral content, not brewing performance. If you go this route, check the label: look for something in the range of roughly 50 to 150 mg per litre combined for calcium and magnesium, and avoid waters that are heavy in sodium or bicarbonate.
Reverse osmosis or distilled water should never be used straight for coffee. It is the very soft extreme described above, and it will make even excellent beans taste hollow.
Simple fixes without a lab kit
You do not need to measure TDS at home to get most of the benefit. A few practical steps cover almost every situation:
- If your tap water is hard (frequent limescale, white residue on kettles), start with a basic carbon filter jug. This alone solves the majority of hard water issues.
- If your water is already soft (little to no limescale, and coffee tastes flat even with more coffee or finer grind), try switching to a moderately mineral tap water, or add a small amount of a coffee specific mineral drop product to plain filtered or distilled water. These are designed to hit the SCA style target without any calculation on your part.
- If you brew with an espresso machine, filtering your water is not optional in the long run. It protects the machine as much as it improves the cup.
- When in doubt, taste test. Brew the same coffee with your usual water and with filtered water side by side. The difference is often more obvious than people expect, and it costs nothing to check.
Where Warawul fits in
We roast coffee to bring out balance, sweetness, and clarity in the cup, and none of that survives contact with the wrong water. In our own café, we filter our water specifically to land in that moderate hardness range the SCA recommends, because we would rather you taste what we intended when we cupped and roasted the coffee, not a version flattened or muddied by tap water.
If you already dial in your grind and your ratio carefully, water is very likely the next lever worth pulling. It is inexpensive to fix and it touches every single cup you brew.