Your tap water is doing more to your coffee than your machine

A cup of coffee is about 98% water by mass. I knew that number for years without it changing anything I did, because water is the free part and equipment is the interesting part.

Then I made the same bag of beans in two flats three streets apart, and one of them was noticeably better. Same grinder, same kettle, same scale, same person. The only variable was what came out of the tap.

Two different problems, often confused

Tap water goes wrong in two ways that have nothing to do with each other, and people tend to fix the wrong one.

Hardness is dissolved calcium and magnesium. It is what makes scale. Hard water buffers the acids in coffee, which sounds harmless and is not: the brightness and fruit that make a coffee interesting are acids, and hard water rounds them off. The cup tastes flat, slightly chalky, and dull at the finish. It is not bad exactly. It is just less.

Chlorine is what the water company added. It is there for a good reason and it is volatile, so it leaves the water if you let it stand. In coffee it reads as a medicinal note, most obvious in the aftertaste, and it is the thing people describe as "the coffee tastes like the tap".

You can have both, one, or neither. The kettle tells you about the first; your nose tells you about the second.

The kettle test

Fill the kettle, boil it, pour it out, look inside once it has dried.

A white chalky ring at the water line, or flakes at the bottom: hard water. This is the common case in most of southern England, much of the US Midwest, and anywhere with chalk or limestone geology.

Nothing at all after a year: soft water. Your problem, if you have one, is chlorine or the pipes, not minerals.

Blue-green staining: copper pipes, and worth a conversation with your water supplier rather than a coffee article.

This is not a precise test, and it does not need to be. It tells you which of the two fixes to try first.

What the numbers mean, briefly

Coffee people talk about water in terms of total dissolved solids and a target range that usually lands around 75 to 150 mg/L, with some alkalinity target below that. You do not need to measure any of this to make a better cup, but it explains why the advice sounds fussy.

Too little mineral — distilled or heavily reverse-osmosed water — and extraction is poor. The cup tastes thin, metallic, and hollow. This is a real failure mode and it surprises people who assume pure water must be best.

Too much — very hard tap water — and the cup is dull and chalky, and your kettle scales, and your machine eventually dies.

In the middle is where coffee tastes like coffee. A filter jug gets most hard tap water into the middle. It will not get very hard water there, and it will not help soft water at all.

The cheap fixes, ranked

A filter jug. Thirty pounds, a cartridge a month, keep it in the fridge. It removes chlorine reliably and reduces hardness substantially. This is the fix I would try first, because it is cheap, reversible, and the difference is usually obvious within one cup.

Let the water stand. If your only problem is chlorine, filling a jug and leaving it uncovered in the fridge overnight does most of the same job for nothing. It is slower and it takes up shelf space, and it works.

Descale the kettle. A scaled element takes longer to boil, which throws off every timing rule you have learned — including the ninety seconds in water temperature without a thermometer. Citric acid, two or three times a year, ten minutes.

An under-sink filter. Better flow, no counter space, and a bigger job to install. Worth it if you drink as much water as coffee; not worth it for a rental you are leaving in a year.

Bottled water. I would not. It is expensive per litre, the mineral profile is a lottery, and most of it is not better than filtered tap for this purpose.

What not to do

Do not brew with distilled water. It is the single most common mistake people make when they start reading about water. Pure water has no minerals to bind to coffee compounds, extraction is uneven, and the cup tastes metallic and thin. If you want to build water from scratch, you add minerals back — that is a hobby with scales and salts, not a shortcut.

Do not buy an expensive water for coffee without trying it first. Buy one bottle, brew one cup, compare it to your filtered tap. Most people cannot tell, and the ones who can usually prefer the filtered tap because they are used to it.

Do not chase the numbers. A $200 water testing kit will tell you things that do not change what you do next. The kettle test tells you enough.

Two cups, one variable

If you want to know whether your water is the problem without buying a test kit, do this once. It takes ten minutes and it settles the question permanently.

Grind two doses of the same beans, the same weight. Brew one with your tap water, exactly as you normally would. Brew the other with filtered water from a jug, or with the cheapest still bottled water you can find. Same grind, same temperature, same timing, same cup.

Then taste them next to each other, and — this is the part people skip — smell them next to each other. Water problems show up in the aroma before they show up in the flavour, because the compounds that make coffee smell like coffee are the ones most easily bound up by minerals.

Most people find the difference obvious. Some find nothing, and that is a useful result too: it means your water is fine and you can stop thinking about it and spend the money on beans. What almost nobody finds is that the tap version is better.

The one thing to avoid is doing this with two different coffees on two different days. Memory for taste is short and unreliable, and a comparison across days is a comparison of moods.

The order I would spend money in

Beans, then water, then grinder, then scale, then temperature, then machine. That is not the order anyone wants to hear, because only two of those are fun to buy.

The water is the one that gets skipped because it is invisible and cheap, which is exactly why it is worth doing first. It costs thirty pounds, it takes one cup to hear the difference, and unlike a grinder it improves every other drink in the house at the same time.

I have made coffee in six kitchens now. The best cup was not in the one with the best equipment. It was in the one where somebody had put a filter jug in the fridge and stopped thinking about it.