Your Water Is Most of Your Coffee
A brewed cup is about 98% water, and its minerals shape extraction. Here's how calcium, magnesium, and bicarbonate work, plus the SCA target numbers.
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A cup of brewed coffee is somewhere around 98% water. We obsess over beans, grinders, ratios, and temperatures, all of which act on the 2%, and then pour the whole thing through whatever comes out of the tap. It’s a little funny when you say it plainly.
The biggest ingredient in your coffee is the one most people never think about.
Water for coffee brewing isn’t a neutral carrier. What’s dissolved in it changes how the coffee extracts and how the cup tastes, sometimes more than a grind adjustment would. You don’t need a chemistry degree to use this, but a little understanding goes a long way.
Pure water is a bad idea
Here’s the first surprise. Distilled or fully deionized water, water with nothing in it, makes flat, hollow coffee. The minerals in water aren’t contaminants to remove. They’re active participants in extraction. Certain dissolved minerals actually grab onto and pull flavor compounds out of the grounds. Strip them out and the water can’t do its job, so the cup tastes thin and lifeless.
So the goal isn’t clean water. It’s water with the right things dissolved in it, in the right amounts.
The three players that matter
Most of it comes down to three dissolved minerals, each pulling the cup in its own direction.
| Mineral | Pulls the cup toward | The short version |
|---|---|---|
| Magnesium | Bright, fruity, delicate flavors; clarity and acidity | The flavor extractor. More of it usually means a livelier, more complex cup. |
| Calcium | Creamy, chocolatey, fuller body | Works alongside magnesium. The two together are what we call water hardness. |
| Bicarbonate | Smoother, rounder edges (in small amounts) | The buffer, and the one to watch. Too much and it flattens everything. |
That last one earns a closer look. Bicarbonate neutralizes acidity, both in the water and in the cup. A little is useful, since it softens harsh edges. Too much mutes the very acidity that makes a bright coffee sing, and it’s the main driver of limescale in your machine. High bicarbonate is usually the culprit when good beans taste dull and lifeless.
The numbers worth knowing
The Specialty Coffee Association (SCA) publishes a water standard. The full chart has a lot of parameters, but a handful of targets are enough to aim at:
| Parameter | Target | Acceptable range |
|---|---|---|
| Total dissolved solids (TDS) | ~150 ppm | 75–250 ppm |
| Hardness (calcium + magnesium) | — | ~50–175 ppm |
| Bicarbonate | keep it low | under ~40 ppm |
| pH | ~7 (neutral) | — |
Don’t treat these as pass-fail. They’re a window, not a single right answer, and there’s real room to tune within them:
- Lower hardness and bicarbonate favors bright, clean filter coffee.
- A bit more on the mineral side suits espresso, where body and sweetness matter more than sharp clarity.
Plenty of competition brewers tweak their water around these ranges to match the coffee in front of them, which tells you the “ideal” is partly a matter of taste.
Fixing your water for coffee brewing
You have a few practical paths, and you don’t have to go full lab.
- Your tap water already tastes good and isn’t very hard. You may be most of the way there. Many cities have perfectly decent brewing water. A cheap test strip or your local water report tells you roughly where you stand.
- Your water is hard or heavily chlorinated. A simple carbon filter pitcher removes chlorine and softens things. Often a clear improvement for almost no money, and the highest-value first step.
- You want real control. Start from low-mineral or distilled water and add a measured mineral mix to hit a target, either a commercial packet or a homemade concentrate. This is where you can genuinely tune the cup toward clarity or body. It’s more fuss than most people want, and it has also surprised more home brewers than almost any other change.
There’s a second reason to care that has nothing to do with flavor: your machine. Hard, high-bicarbonate water lays down limescale inside boilers and lines, which is exactly what descaling fights. Softer water means less scale and less maintenance. So if you run an espresso machine, getting the water right pays you back twice.
The cheapest upgrade you’re not making
Most people will spend real money chasing a better grinder or fresher beans, then brew with hard tap water that quietly caps how good the result can be. Sorting your water is often cheaper than any of that, and it changes every single cup you make, not just the next bag.
Start small. Taste your tap water on its own, the way you’d taste the coffee. If there’s a faint note of chlorine or chalk, run it through a carbon filter and brew the same beans again tomorrow. One side-by-side like that has converted more skeptics than any chart I could show you.
Other deeper science pieces, like what the bloom is telling you, are worth a read if you want to follow the thread.