Water Chemistry — Hardness and Alkalinity
The single most useful idea in coffee water chemistry is that hardness and alkalinity are two different things, even though everyday language tangles them together. Hardness — measured as GH, general hardness — is the amount of calcium and magnesium dissolved in the water. Alkalinity — measured as KH, carbonate hardness — is the water's buffering capacity, almost entirely from bicarbonate (HCO₃⁻). They are measured in similar units, they often rise together in tap water, and they do completely different jobs in your cup. Untangling them is the key to understanding water, as Why Water Matters argues.
#GH: The Extraction Engine 🔋
General hardness is the mineral content that actively drives extraction. Magnesium is the more aggressive, flavor-binding ion — it tends to lift bright, juicy, acidic and fruity character. Calcium is gentler and associated with body, weight, and a rounder mouthfeel. Together they set how efficiently water pulls flavor from the bed: too little GH and the water extracts poorly, leaving a thin, hollow cup; too much and you risk over-extraction plus limescale on your kettle and equipment.
#KH: The Acidity Buffer 🛡️
Alkalinity is where most "good beans, dull cup" mysteries hide. Bicarbonate is a buffer: it neutralizes acids. Coffee is full of desirable organic acids — citric, malic, and others that read as brightness and fruit. The higher your water's KH, the more of those acids get chemically mopped up before they reach your palate, so a high-alkalinity brew tastes flat, chalky, or muted even when extraction is otherwise fine. Low KH lets acidity sing, but go too low and the brew can taste sharp, sour, or harsh, and the water becomes corrosive to equipment.
GH controls how much flavor comes out; KH controls how much of the coffee's acidity survives. Bright, fruity coffees want lower alkalinity to shine; balanced or chocolatey roasts tolerate, and sometimes prefer, a little more.
#Putting Numbers On It
Hardness and alkalinity are usually quoted in ppm as CaCO₃ (or in German degrees, °dH; 1 °dH ≈ 17.8 ppm). A rough orientation:
| Measure | What it is | Drives | A workable target |
|---|---|---|---|
| GH (general hardness) | Ca²⁺ + Mg²⁺ | Extraction, body, brightness | ~50–175 ppm; ~68 ppm ideal |
| KH (alkalinity) | Mostly HCO₃⁻ | Acid buffering, scale | ~40–75 ppm; ~40 ppm ideal |
| Ratio of the two | GH relative to KH | Overall balance | GH at or above KH |
These align with The SCA Water Standard and feed directly into total mineral content. The headline ratio matters: many specialty recipes aim for GH at least equal to KH, so the extracting minerals outweigh the buffering ones.
#A Contested Point, Flagged ⚠️
The widely cited "magic" figure of ~68 ppm GH and ~40 ppm KH comes from the SCAA/SCA work and the book Water for Coffee. It is an excellent starting point, not a universal optimum — the best water depends on the coffee, the roast, and your taste. Treat published numbers as a launchpad, then taste and adjust.
To act on all this — softening, hardening, or rebuilding water to hit a chosen GH and KH — see Building Water — Recipes and Remineralization and the everyday-source guidance in Filtered, Bottled, and Tap Water.
#Continue Reading
- TDS and Mineral Content — how GH and KH roll up into total dissolved solids
- The SCA Water Standard — the official window for hardness and alkalinity
- Building Water — Recipes and Remineralization — dial GH and KH to a recipe
- Acidity in Coffee — what the buffer is muting or letting through
- Water for Coffee — back to the water hub