A cup of coffee is more than 98% water, and the minerals dissolved in that water change both how the coffee brews and how it tastes. Very hard water can make coffee taste flat and chalky; very soft water can make it thin and sharp; chlorine makes it taste of a swimming pool.
That said, most tap water makes perfectly good coffee. This guide is about telling whether yours does, and what to do if it does not, without turning your kitchen into a chemistry lab.
Start with taste and smell
Pour a glass of cold tap water, let it sit until it reaches room temperature, and taste it. Warm water shows off smells that cold water hides.
- If it tastes clean and neutral, brew with it. You are done.
- If it smells of chlorine, like a swimming pool, a simple carbon filter will help.
- If it tastes metallic, salty, or of sulfur, try a filter or bottled water, and compare a cup brewed with each.
- If your kettle builds up a thick white crust, your water is hard. It may still taste fine, but it is worth reading on.
If your water does not taste good cold, it will not taste good as coffee.
What the minerals do
Two properties of water matter most for coffee, and they pull in different directions.
- Hardness is mostly calcium and magnesium. These minerals help water pull flavor out of the coffee. Too little and the cup can taste thin; too much and it tastes heavy and chalky, and scale builds up in your kettle and machine.
- Alkalinity is mostly bicarbonate, and it neutralizes acid. A little keeps coffee from tasting sour and sharp. Too much flattens the bright, fruity flavors out of the cup, which is why coffee brewed with very hard, alkaline water often tastes dull.
Two other things are simpler. Chlorine and chloramine, which utilities use to keep water safe, should be absent by the time water reaches your coffee: they taste of the pool. Sodium is fine in small amounts but can taste salty in large ones.
Why a TDS number is not enough
Total dissolved solids, or TDS, measures how much is dissolved in water, but not what. Two waters can show the same TDS and brew very differently: one rich in calcium and magnesium, the other in sodium and bicarbonate. A cheap TDS pen tells you whether water is very soft or very hard, but “use exactly 150 TDS” is not reliable advice on its own.
For reference, the Specialty Coffee Association’s water standard looks like this:
| Measure | Target | Acceptable |
|---|---|---|
| TDS | 150 mg/L | 75–250 mg/L |
| Calcium hardness | 68 mg/L | 17–85 mg/L |
| Alkalinity | 40 mg/L | At or near 40 mg/L |
| pH | 7.0 | 6.5–7.5 |
| Sodium | 10 mg/L | At or near 10 mg/L |
| Chlorine | None | None |
You do not need to hit these numbers. Plenty of water outside the range makes good coffee. They are most useful as a way to see how far from the middle your water is.
How to find out what is in your tap water
- Look up your water report. Water utilities publish an annual water quality report. In the US it is called a Consumer Confidence Report, and many list hardness, sodium, and whether the water is treated with chlorine or chloramine. Search for your utility’s name and “water quality report”.
- Use test strips. Inexpensive hardness strips, sold for aquariums and water softeners, give you a hardness reading in a minute. Many also read alkalinity.
- Check your kettle. A thick white crust that returns quickly after descaling is a good sign of hard water.
Water is classed by hardness, measured as calcium carbonate:
| Hardness | mg/L |
|---|---|
| Soft | 0–60 |
| Moderately hard | 61–120 |
| Hard | 121–180 |
| Very hard | Over 180 |
Soft to moderately hard water is usually the easiest to brew with.
What to do about it
| Your water | Simplest fix |
|---|---|
| Tastes clean | Use it as it is |
| Smells of chlorine | Use a carbon filter pitcher |
| Hard, with heavy kettle scale | Mix half and half with distilled water |
| Very soft, or distilled | Add coffee mineral packets, or use bottled spring water |
| From a salt-based softener | Use bottled water, or unsoftened water if you have it |
Carbon filter pitchers remove chlorine well and improve most off tastes, and some reduce hardness a little. They are not water softeners, so very hard water stays fairly hard. Chloramine, which many utilities now use instead of chlorine, is harder for a basic filter to remove and does not evaporate if you leave water standing, as chlorine does.
Mixing with distilled water is the cheapest fix for hard water: half and half halves the hardness and alkalinity. Adjust the mix to taste.
Distilled or reverse-osmosis water on its own usually makes flat, hollow coffee. It also cannot be used in some espresso machines, which rely on minerals in the water to sense how full the boiler is. If that is all you have, add minerals back with coffee mineral packets, which dissolve into distilled water to make a consistent brewing water.
Salt-based household softeners swap calcium and magnesium for sodium, but leave the alkalinity behind. The result is kind to pipes but often makes coffee taste flat.
Bottled water: choose still water with a clean taste and a moderate mineral content. Many labels list total dissolved solids; somewhere around 75–250 mg/L is sensible. Avoid sparkling water and heavily mineralized waters.
Protect your equipment
Hard water leaves scale in kettles and coffee makers. Scale slows the flow, stops machines heating properly, and eventually breaks them. Espresso machines are the most vulnerable, because scale inside a boiler is expensive to remove.
- Descale regularly, following the manufacturer’s instructions, rather than waiting for the flow or heating to fail. The drip guide gives a schedule.
- For an espresso machine, follow the manual’s advice on water. Many manufacturers recommend filtered water, and some sell their own filters.
- Never mix cleaning chemicals, and rinse the equipment thoroughly before brewing again.
Water is one of the two things to rule out before you change a recipe; the other is stale coffee. If both check out, the troubleshooting guide works through what is left.