brewed cup is about 98 percent water, so the minerals dissolved in that water do real work. The same beans, ground and brewed the same way, taste different in different water because the mineral makeup, not just the cleanliness, shapes the extraction.
This article is the expert layer under the practical answer. It names the targets in numbers, explains the two hardness measures that matter, then shows the concept behind mixing your own water from mineral salts.
Before you mix anything
Everything below assumes you already brew a clean cup and want to measure and tune the water itself. It requires a TDS meter or a hardness test kit, a scale that reads to 0.01 g, and distilled or reverse-osmosis water as a blank canvas. If that is more than you want to own, the Foundation answer stands on its own.
The two hardness numbers that matter
Water chemistry for coffee comes down to two measurements, not one. General hardness is the extracting minerals; carbonate hardness is the buffer that controls how those minerals read in the cup. The balance between them is what you tune.
General hardness (GH) is the calcium and magnesium content, the two ions that actually pull flavor from the grounds. They are not interchangeable. Magnesium tends to pull bright, fruity, high-clarity flavors, so water leaning on magnesium reads vivid and separated. Calcium leans toward body and a rounder, heavier cup. Many recipes blend the two to place the cup where you want it.
Carbonate hardness (KH), also called total alkalinity, is the bicarbonate content. Bicarbonate is a buffer: it neutralizes the acids that extraction releases. Too much alkalinity dulls and flattens the acidity, so a bright coffee turns muddy. Too little leaves the cup sharp and unbuffered, where it can read harsh or sour. The GH-to-KH balance is what you adjust, not GH alone.
- General hardness (GH)
- about 50 to 68 mg/L as CaCO3(calcium + magnesium; acceptable roughly 17 to 85)
- Carbonate hardness (KH)
- about 40 mg/L as CaCO3(total alkalinity, the buffer)
- TDS
- about 150 mg/L(acceptable roughly 75 to 250)
- pH
- 7.0(acceptable 6.5 to 7.5)
- Sodium
- about 10 mg/L(chlorine zero, odor-free)
SCA water targets. ppm and mg/L are the same unit.
A note on units. ppm means parts per million, and for dilute water it equals mg/L, so the two are written interchangeably above. Calcium and carbonate hardness are both expressed as mg/L "as CaCO3", a convention that puts different ions on one comparable scale.
Water TDS is not brew TDS
Two very different numbers both get called TDS, and conflating them is the most common mistake at this level. One describes the water before it touches coffee; the other describes the finished drink, at a value roughly a hundred times higher.
| Quantity | Typical value | What it is | Measured by |
|---|---|---|---|
| Water TDS | about 150 ppm | Minerals dissolved in the brewing water before brewing | Conductivity or TDS meter |
| Brew TDS | about 1.2 to 1.4 percent, near 12,000 to 14,000 ppm | Dissolved coffee solids in the finished cup | Refractometer |
Water TDS is the mineral content you tune in this article, read on a cheap conductivity meter. Brew TDS is the strength of the finished coffee, read on a refractometer, and it is the number that feeds extraction yield. A 150 ppm water and a 1.3 percent brew are both "TDS", but they answer different questions. Strength and extraction live in the extraction guide; mineral content lives here.
Mixing your own water
The mix-your-own approach starts from a blank canvas. You take distilled or reverse-osmosis water, which has almost no minerals, and add back exactly the calcium, magnesium, and bicarbonate you want. The standard method is to make concentrated mineral stock solutions, then dose a measured amount of each into the RO water by weight.
Start from RO or distilled water
Near-zero minerals, a clean base you fully control.
Dissolve each salt into its own concentrate
A strong stock solution of each mineral in a small bottle.
Dose the concentrates by weight
A few grams of each stock into the RO water, weighed on a fine scale.
Three household salts cover the three jobs. Epsom salt, which is magnesium sulfate (MgSO4), builds magnesium hardness for the bright, fruity side. Calcium chloride (CaCl2) builds calcium hardness for body. Baking soda, which is sodium bicarbonate (NaHCO3), builds alkalinity, the buffer. By choosing how much of each you dose, you set both the GH level and the GH-to-KH balance.
Common questions
- What is the difference between GH and KH in coffee water?
- GH, general hardness, is the calcium and magnesium that extract flavor from the grounds. KH, carbonate hardness or alkalinity, is the bicarbonate that buffers acidity. GH does the extracting; KH controls how bright or flat the acids read. You tune the balance between them, not GH alone.
- Is water TDS the same as the TDS on a coffee refractometer?
- No. Water TDS, read on a conductivity meter, is the mineral content of the water, around 150 ppm. Brew TDS, read on a refractometer, is the dissolved coffee in the finished cup, around 1.2 to 1.4 percent or 12,000 to 14,000 ppm. They share a name but differ by about a hundredfold.
- Which salts do I need to mix my own brewing water?
- Three. Epsom salt (magnesium sulfate) for magnesium hardness, calcium chloride for calcium hardness, and baking soda (sodium bicarbonate) for alkalinity. You dissolve each into a concentrated stock, then dose the stocks by weight into distilled or RO water.