Water hardness and the flat cup

March 8, 2026 · 2 min read

For three weeks in February every cup tasted flat. Same beans as before, same grinder, same recipe, same kettle, and the coffee was dull, slightly chalky and lacking the sweetness I had been getting in January. I blamed the beans, then the grinder, then myself. It was the water.

I only worked it out because I made coffee at a friend’s place with my own beans and grinder and it tasted right. The difference was their tap water. A cheap test strip at home read total hardness around 300 parts per million; at their place it read closer to 80. The city had apparently switched the supply to a different source for maintenance work, and the water coming out of my tap had changed without anything else changing.

Hard water is not simply bad for coffee. Some mineral content is needed, and water with almost none tastes hollow and extracts poorly. But very hard water does two things: the high bicarbonate buffers the acids in the coffee, flattening the brightness, and the calcium and magnesium at those levels change the extraction in a way that I can only describe as muddy. The chalkiness I was tasting was literally chalk.

What I tried, in order:

A jug filter. Hardness dropped to about 150 on the strip. Better, noticeably, but still not January.
Bottled water with a low mineral content, around 50 to 60 total dissolved solids on the label. Too soft; the cup was thin and sour.
Mixing one part tap to two parts the bottled water. Roughly 130 on the strip. This was close.
A small packet of brewing minerals added to distilled water at the suggested dose, which should land around 90 to 100. This was the best cup of the month and the one I have kept.

It is an annoying hobby that can be ruined by the water company, but I would rather know. I now keep a strip test in the drawer and check the tap once a month. The supply switched back in early March and the tap went to about 170, which is good enough with the jug filter alone, so I am back to that for daily use and saving the mineral packets for beans that deserve them.