Coffee is 98–99% water. Whatever the water carries, the coffee shows. Most home brewers spend hundreds on grinders and pennies on water — that's backwards.
What's in your water that matters
Two things, mostly:
- Total dissolved solids (TDS) — minerals overall. Drives extraction capacity. Distilled water (TDS 0) doesn't extract enough. Hard tap water (TDS 300+) extracts too much, badly.
- General hardness (GH) vs carbonate hardness (KH) — calcium and magnesium vs bicarbonate. GH pulls flavor compounds out; KH neutralizes acidity. The ratio between them shapes the cup.
The Specialty Coffee Association recommends ~150 mg/L TDS, with GH around 50–100 mg/L and KH around 40 mg/L. Most municipal tap water is wildly off these numbers in one direction or the other.
Telltale signs your water is off
Cup is flat, dull, no acidity even with a coffee that should sing → your KH is too high. Carbonates are buffering the acids the coffee is producing. Try a softer water.
Cup is harsh, edgy, even bitter at recipes that taste fine elsewhere → your TDS or GH is too high. Try cutting your tap with distilled or RO water.
Coffee tastes "watery" or hollow even at full strength → TDS too low. Distilled water and untreated RO water both do this.
Same recipe tastes different at home vs at a café using the same beans → almost always the water.
What to actually use
In rough order of cheap-to-fancy:
- Bottled water with a label that lists minerals. Look for soft to medium hardness (~50–100 mg/L hardness, ~150 mg/L TDS). Bottled water for babies or for tea is often a good range. Volvic is a popular workhorse.
- Brita / pitcher filter: removes some hardness and most chlorine. Useful if your tap is harsh, doesn't help if your tap is bland.
- Mineral packets (Third Wave Water, Lotus, Empirical Water): you start with distilled or RO and add a pre-measured packet. Reproducible, controllable, but adds a step.
- DIY remineralisation: distilled water plus measured magnesium sulfate and sodium bicarbonate. The barista nerd path. Works, but the gain over a good bottled water is small.
How to test before changing
Buy two 1-liter bottles of different waters. Brew the same coffee, same recipe, on each. Taste side by side. Differences are obvious. If your tap brew tastes the same as the better bottle, your tap is fine — keep your money.
If your tap is way harder or softer than what works, swap to the bottle for brewing only. You don't need to drink it.