A cup of filter coffee is more than 98% water. With bad water, no bean or recipe can save it. That’s why it pays to spend a little time understanding what water you use, at what temperature and in what amount.
You don’t need a lab. If you know where your water comes from and have a kettle that lets you control the temperature and a scale, you have everything you need to stop fighting with coffees that never quite taste the way you expected.
Hardness
Water needs some minerals to extract well, but not too many. The ideal water hardness is around 50 to 100 mg/l. Very hard water mutes the acidity and leaves limescale in the brewer; very soft water gives flat cups.
Why minerals matter
Magnesium and calcium help pull aromatic compounds out of the bean, while bicarbonates neutralize part of the acidity. Water with a lot of bicarbonate leaves a dull cup, however good the coffee. That’s why two waters with a similar total hardness can give very different results.
How to know what water you have
The company that runs your town’s water supply usually publishes its analyses by area. There are also cheap test strips that give you a rough idea of the hardness. If your brewer builds up limescale often or your kettle has a white crust, your water is almost certainly hard.
What to do if your water isn’t good
- Filter pitcher: it reduces chlorine and some of the limescale, and it’s the most convenient fix.
- Low-mineral bottled water: useful for comparing and for delicate coffees.
- Mixing tap water with very soft water: an easy way to bring the hardness down.
- Avoid distilled water on its own: without minerals, the extraction falls flat.
With a delicate coffee, like many Ethiopian coffees, the difference between hard water and balanced water shows right away. With the second, the acidity and floral notes come through; with the first, they stay buried under an earthy background.
Temperature
Between 90 and 96 °C. Hotter extracts more bitterness, and cooler falls short. If you don’t have a thermometer, let the water rest for 30 seconds after it boils.
With a French press it’s best to stay at the top of that range, around 94 °C, because the coffee spends four minutes in contact with the water.
How to measure it
A kettle with temperature control is convenient, but not essential. A kitchen thermometer works just as well, and with practice you’ll learn how long your kettle needs to rest to land in the right range. What matters is doing it the same way every time, so temperature stops being a guess.
Depending on the roast
A light roast likes the top end of the range, because the bean is denser and harder to extract. A dark roast, on the other hand, extracts easily and turns bitter if the water is too hot. With the V60 and a light coffee, starting at 93 or 94 °C is a good reference.
Ratio
The reference is 60 g of coffee per liter of water. From there, adjust to taste. Cold preparations like cold brew use a much more concentrated ratio.
- Filter: between 60 and 65 g per liter.
- Immersion: between 60 and 70 g per liter.
- Espresso: measured differently, by the ratio between ground coffee and the drink in the cup, often one to two.
Ratio and grind go hand in hand. If you add more coffee without touching the grind size, the cup gains intensity but can also turn more bitter. Adjust the ratio first until the coffee has the strength you like, and then fine-tune the flavor with the grinder.
Weighing the water instead of measuring it by volume gives you more precision. A milliliter of water weighs practically a gram, so the same scale works for the coffee and the water.
Habits that help
Use water fresh from the tap or the pitcher, not water that has been sitting in the kettle for hours or that you have already boiled several times, which keeps getting more concentrated. And descale your brewer regularly: built-up limescale changes the real temperature of the water coming out and the flow at which it passes through the coffee.
If you’re unsure, try a simple test: brew the same coffee twice, once with tap water and once with filtered or low-mineral water, and taste them side by side. Few comparisons teach you as much in so little time. And if you travel with your coffee, expect it to taste different in another city just because of what comes out of the tap.
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