Coffee is mostly water, so the minerals in that water have a major influence on what ends up in your cup. But good brewing water isn't simply about having a certain TDS number. The balance between hardness and alkalinity matters much more.
Three measurements are especially useful when evaluating brewing water: GH (general hardness), KH (carbonate hardness or alkalinity), and TDS (total dissolved solids). Understanding what each one tells you makes choosing, filtering, or building your brewing water much easier.
General hardness, or GH, measures the amount of dissolved calcium and magnesium in your water. These minerals play a major role in coffee extraction because they interact with flavor compounds as the water passes through the coffee grounds.
Magnesium tends to pull out brighter, fruitier, and more acidic flavors, while calcium can contribute to sweetness, body, and balance. You don't need extremely hard water to make great coffee—the goal is to have enough mineral content to support extraction without creating excessive scale or overpowering the coffee.
Think of GH as your water's extraction power. Too little hardness can leave coffee tasting weak or flat. Too much can contribute to harsh flavors and mineral buildup in your equipment.
KH represents your water’s alkalinity—its ability to neutralize acids. In coffee brewing, alkalinity acts like a buffer that influences how much of the coffee’s natural acidity you actually taste.
If alkalinity is too high, it can neutralize too much acidity and leave a bright, complex coffee tasting dull or flat. If it is too low, acidity can become sharp and the brew may taste unbalanced.
Think of KH as your water’s acid control. The goal isn’t to eliminate acidity—it’s to have enough buffering to keep the cup balanced while still allowing the coffee’s character to come through.
TDS, or total dissolved solids, estimates the overall amount of dissolved material in your water. A digital TDS meter makes this measurement quick and easy, which makes it useful for comparing water sources and checking whether your filtration system is changing the water.
But TDS does not tell you what those dissolved solids actually are. Two samples of water can have the same TDS reading while containing very different amounts of calcium, magnesium, alkalinity, sodium, and other dissolved substances—and they can brew very different cups of coffee.
Think of TDS as a snapshot, not a complete diagnosis. Use it alongside GH and KH testing rather than relying on the TDS number alone.
The easiest way to understand brewing water is to give each measurement a different job. GH tells you about the calcium and magnesium available to support extraction. KH tells you how strongly the water buffers the coffee’s natural acids. TDS gives you a quick estimate of the total amount of dissolved material in the water.
None of these numbers should be judged in isolation. A TDS reading can look reasonable while the mineral balance is poorly suited for coffee, and two waters with similar TDS can produce noticeably different cups.
GH = Extraction
KH = Acid control
TDS = Quick overall snapshot
Together, these measurements give you a much clearer picture of what your brewing water is doing—and what, if anything, you should change.
You don’t need a laboratory to get a useful picture of your brewing water. A few simple tools can give you enough information to decide whether your water is ready to brew with or needs adjustment.
Start with a TDS meter for a quick baseline reading. Then use a GH/KH drop test kit to measure hardness and alkalinity separately. Together, these tests tell you much more than a TDS reading alone.
Test the water you actually plan to brew with—whether that’s straight from the tap, filtered water, bottled water, or water you’ve remineralized. Write down your results so you can compare them with how your coffee tastes and track changes over time.
Once you know your GH, KH, and TDS, the next step isn't automatically to change your water. If your coffee tastes good, your measurements are reasonable, and you're not seeing excessive scale buildup, you may not need to do anything at all.
If your water is very hard or has high alkalinity, filtration may help reduce unwanted minerals and protect your brewing equipment. If you're starting with distilled or reverse-osmosis water, you'll usually need to add minerals back before brewing.
The goal isn't to chase a perfect number. It's to create water that produces good-tasting, repeatable coffee while being appropriate for your brewing equipment.
Good water? → Brew with it.
Problematic tap water? → Filter it.
RO or distilled water? → Remineralize it.
There isn't one perfect water recipe for every coffee, but these ranges provide a practical starting point for home brewing.
GH (General Hardness): 50–100 ppm
Provides calcium and magnesium to support extraction.
KH (Alkalinity): 40–80 ppm
Helps buffer acidity without flattening the coffee's flavor.
TDS (Total Dissolved Solids): 75–200 ppm
Useful as an overall reference, but remember that TDS alone doesn't tell you which minerals are present.
These are starting ranges, not absolute rules. Your coffee, brewing method, equipment, and taste preferences can all influence where your ideal water falls.
You don't need to become a water chemist to improve your coffee. Start by understanding three measurements: GH, KH, and TDS. Test the water you're already using, pay attention to how your coffee tastes, and make adjustments only when they're needed.
Once you understand your starting water, the next question becomes simple: Should you use it, filter it, or build better brewing water from scratch?
That's where the next guides come in.