Water is 98% of your shot. Everyone says this and then no one tells you what to do about it, because the answer involves words like "bicarbonate" and pH charts. You don't need any of that to make better coffee — you need to know what's wrong with your tap water and one of three replacements.
What's actually in your tap water
Two things matter, and you can ignore the rest. The first is total hardness — calcium and magnesium dissolved in the water. These bind to the bitter and acidic compounds in coffee and pull them out of the grounds. Too little hardness, and your shot tastes hollow. Too much, and it tastes flat or chalky, and your machine scales up.
The second is bicarbonate — sometimes called alkalinity. Bicarbonate buffers acidity. If there's too much of it, your espresso loses its brightness; the fruit notes that should sit on top of the cup get smoothed into nothing. Most tap water has plenty of both, in the wrong ratio, plus chlorine for flavor.
The SCA target, in plain English
The SCA recommends water with around 150 ppm total dissolved solids, hardness around 68 ppm as CaCO3, and bicarbonate around 40 ppm. You don't need to memorize this. You need to know that those numbers describe water with moderate hardness and low alkalinity — soft enough to extract well, low enough to let acidity through.
Hit those numbers approximately and the cup gets better. You don't need to test, you don't need to mix your own. You just need water that lives somewhere near that profile.
Three options that actually work
Bottled water — the laziest path. Volvic, Crystal Geyser, and Aquafina all sit close enough to the SCA targets to work without thinking. Avoid anything labeled "mineral water" or "spring water" without checking the bottle; some of them are extremely hard and will brick your espresso machine over time.
A filter jug — Brita, BWT, or any pitcher with a real ion-exchange filter. This drops chlorine and reduces hardness modestly. It's not perfect, but it's better than the tap, and the running cost is roughly nothing. The BWT "Magnesium" filters are slightly better for coffee specifically.
Remineralized distilled — for people who want to go further. You buy a gallon of distilled water and add a measured pinch of Third Wave Water or a similar mineral packet. This gives you an exact, repeatable water profile and is the cheapest way to hit the SCA targets exactly. It's also the most fiddly. Worth it if you're tasting differences between shots and can't figure out why.
What I use at home
A BWT Magnesium-filter jug, refilled twice a day, decanted into a glass bottle in the fridge. That's it. I've used Third Wave Water for a few months — it's better, technically — but the jug is the option I actually keep doing, which is most of the criterion.
When friends visit and ask which bottled water to buy, I tell them Volvic. It's available everywhere, it's close enough, and they'll never have to think about it again.
Water is the cheapest upgrade your coffee will ever get. A bag of beans costs $18; a year of better water costs less. Most home brewers spend months tweaking grind and dose before they touch what's coming out of the tap. Skip ahead. Fix the water, taste the difference, then go back to worrying about everything else.