The Invisible Ingredient: Is Your Tap Water Good Enough for Coffee?

Your cup of filter coffee is roughly 98 to 99 percent water. You can buy a stunning bag of beans, dial in your grinder and weigh everything to the gram, and still end up with a cup that tastes flat. Sometimes the culprit is the thing you never think about.

The good news: this one is easy to check. Here is what the research says and how to test your own tap.

What the SCA Recommends

The Specialty Coffee Association (SCA) publishes a target for brewing water. A few terms first. TDS is the total amount of dissolved minerals. Hardness is mostly calcium and magnesium. Alkalinity is the water's buffer, its ability to neutralize acid.

• TDS: 150 mg/L (acceptable range 75 to 250).
• Calcium hardness: 68 mg/L as calcium carbonate (acceptable range 17 to 85).
• Alkalinity: around 40 mg/L.
• pH: 7.0 (acceptable range 6.5 to 7.5).
• Chlorine: 0 mg/L.

You do not need to hit these exactly. They show the direction good coffee water lies in: some minerals, little buffer and no chlorine taste.

Minerals Help, Buffer Hurts

Research by chemist Christopher Hendon and barista Maxwell Colonna-Dashwood, published in 2014 and later expanded into their book Water for Coffee, found that magnesium and calcium help pull flavor compounds out of coffee, with magnesium doing it most strongly. Part of that work was computer modeling, so treat it as a strong hint rather than the last word.

• Too few minerals (distilled or reverse osmosis water): little to grab the flavor, so the cup tends to taste flat and thin.
• Too much alkalinity: the buffer neutralizes the acidity in coffee and leaves the cup dull and chalky.
• Sodium-rich water, such as softened water: it did not improve flavor in that research.

What About Swedish Tap Water?

Swedish tap water is generally soft to moderately hard, and in most places, including Stockholm, it lands close to the SCA's target for minerals. For most people it is a good starting point. The exceptions are, for example, very soft water in parts of the north and hard or high-alkalinity water in parts of Uppland, Skåne and Gotland.

It depends on where you live, so check your water company's published analysis. For instance, in Stockholm the tap water is soft, at 4 to 6 °dH (German degrees of hardness), which is roughly 70 to 110 mg/L as calcium carbonate. That is in the same ballpark as the SCA target and far from hard-water territory. Stockholm Vatten also says its water leaves the plant at a pH of about 8.5 and that it adds a small amount of chloramine, a chlorine compound, to keep it stable on the way to your tap.

So a Stockholm tap is probably a reasonable starting point. The pH sits a bit above the SCA range, but pH alone does not tell you the alkalinity, and the buffer is what matters for acidity. A side-by-side taste test says more than any number on a report.

Test It, Don't Guess

• Look up your numbers. Search for your municipality's drinking water analysis and find hardness (°dH), pH and chlorine or chloramine.
• Brew the same coffee twice. Same beans, same recipe (our Pour-Over Quick Start Guide has an easy one), one cup with tap water and one with bottled spring water that lists 75 to 250 mg/L of dissolved minerals. Taste them side by side.
• Optional: a cheap TDS pen shows your total dissolved solids in seconds.

If the tap cup tastes good, you are done. Do not buy anything.

If You Need a Fix

• Hard or high-alkalinity water: a filter jug made for coffee reduces both. Descale your kettle and machine regularly, because hard water leaves scale.
• Very soft, distilled or reverse osmosis water: add mineral sachets made for coffee, or blend with a mineral-richer water.
• Chlorine or a pool-like smell: a carbon filter usually helps.
• Bottled water: choose spring water with moderate minerals and skip heavy mineral water.

The Verdict

Water is rarely the first thing to blame, but it is the easiest thing to rule out. Check your tap, run one side-by-side test, and spend your money on beans instead of gadgets unless the test tells you otherwise.

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Run the Test on Good Beans

A lighter filter roast tends to show differences in water clearly. Pick a bag from our filter beans and brew it with two different waters.