Tap water carries dissolved minerals picked up from the ground. Two properties of those minerals matter for coffee, and they are different things that often rise and fall together.
Hardness is the amount of dissolved calcium and magnesium. These positively charged ions are involved in pulling flavour compounds out of the coffee. Alkalinity is the water's capacity to neutralise acid, and in drinking water it comes almost entirely from bicarbonate. It acts as a buffer: it reacts with the acids in the brewed coffee and cancels some of them.
pH, the figure most people look for, is the least useful of the three. It says how acidic the water is right now and nothing about how strongly it will resist the coffee's acids. Two waters of the same pH can behave completely differently.
Alkalinity has the most obvious effect. With a lot of bicarbonate, the coffee's acidity is neutralised and a lively coffee tastes flat, chalky and dull. With very little, nothing restrains the acids and the cup can taste sharp and sour. This is the commonest reason the same coffee tastes different in two cities.
Hardness is subtler. A 2014 study in the Journal of Agricultural and Food Chemistry used computational chemistry to compare how strongly magnesium, calcium and sodium ions bind to several coffee acids and aroma compounds. It concluded that magnesium binds more strongly than calcium, and that the balance between these ions and bicarbonate shapes flavour — so that 'hard water is bad for coffee' is too simple, because what kind of hardness matters. That work is a model of binding, not a tasting trial; later practical experience broadly supports the direction, and the size of the effect in the cup is still argued over.
The Specialty Coffee Association's long-standing water standard gives targets rather than a single recipe: total dissolved solids around 150 mg/L within a wide acceptable range, moderate calcium hardness, alkalinity near 40 mg/L, a roughly neutral pH, and no chlorine or odour. It was written with equipment protection in mind as well as flavour.
Water matters to machines as well as to flavour. When hard, alkaline water is heated, calcium carbonate comes out of solution as limescale. Temporary hardness — the calcium and magnesium that are paired with bicarbonate — is the part that scales. A kettle shows it as a white crust; a boiler hides it.
The opposite extreme is also a problem. Water with almost nothing dissolved in it can be corrosive to metal, extracts coffee poorly and can confuse the level sensors in some machines. Good brewing water sits between: enough minerals to extract and to protect the machine, not enough bicarbonate to flatten the cup or scale the boiler.
A carbon filter, the main component of most jug filters, removes chlorine and odours and little else. Jug filters that also contain ion-exchange resin reduce temporary hardness, which lowers both scale and alkalinity, and in doing so may add sodium or hydrogen ions. A water softener swaps calcium and magnesium for sodium: it prevents scale but leaves alkalinity alone, so it does not fix a flat cup. Reverse osmosis removes nearly everything, and its output is normally blended back with some unfiltered water or remineralised.
Remineralising means starting from very pure water and adding measured amounts of mineral salts, typically a magnesium salt for hardness and a bicarbonate for alkalinity, to reach a chosen composition. It gives full control and takes care. Many recipes circulate and none is authoritative; they are starting points to taste against.
Before any of this, find out whether you have a problem. Look up your supplier's water quality report, brew the same coffee with tap water and with a moderate bottled water, and taste them side by side. If you cannot tell them apart, your water is fine.
Your task. Find the hardness and alkalinity for your home in your supplier's water report and enter them. Then enter the figures from the label of a bottled water you can buy. Compare what the helper says about scale and about taste for each.
What to notice. The helper treats scale risk and flavour as separate questions, because hardness and alkalinity are separate properties. A water can be gentle on a machine and still flatten a coffee.
Exercise: Same coffee, different water
Objective. Find out whether your water is a limiting factor before spending anything on it.
You will need
One coffee, your tap water, and one or two alternatives: filter-jug water and a low-mineral bottled water
Cupping bowls or an immersion brewer, a scale
The mineral analysis from each bottle label, and your water supplier's report for the tap
Keep the same
Same dose, grind, water amount, temperature and steep time
Heat each water in a clean kettle, rinsed between waters
Procedure
Write down the hardness and alkalinity (or bicarbonate) of each water where you can find it.
Brew the same recipe with each water.
Taste side by side, shuffled, at the same temperature.
Taste the plain waters too.
What to notice
Does one cup taste flatter or chalkier? Sharper or thinner?
Does the water with the highest bicarbonate make the most muted cup?
Does any water taste of something on its own?
Reading what you found
High alkalinity tends to mute acidity and can make a lively coffee taste flat; very soft, low-mineral water tends to taste sharper and thinner. If your tap water makes a cup you like as much as the others, water is not your problem. If the differences are large, the water-suitability tool can help you decide what to change.
What this cannot show
Bottled waters differ in several minerals at once, so you cannot attribute a difference to one of them.
Never use distilled or pure reverse-osmosis water in an espresso machine without checking the manufacturer's guidance.
Supplier reports are averages for an area and season.
A protocol for you to run. CoffeeHQ has not run it and reports no result from it.
Work it through: Great at the roastery, flat at home
A coffee tasted vivid and fruity at the roaster's shop. At home, with the same recipe and a good grinder, it tastes muted and slightly chalky. The home is in a hard-water area and the kettle scales quickly.
Reasoning it out
The coffee, the recipe and the grind are the same; the water is the obvious difference.
A kettle that scales fast means high temporary hardness, which means high bicarbonate.
High alkalinity neutralises the acids that made the coffee taste fruity.
A plain carbon filter would not change that; it removes chlorine, not bicarbonate.
What to do
Test by brewing once with a moderate-mineral bottled water. If the fruit returns, alkalinity is the cause, and the options are a jug filter with ion-exchange resin, blending tap with low-mineral water, or building water from scratch.
SCA water standard for brewing specialty coffee — Specialty Coffee Association
The role of dissolved cations in coffee extraction, C. H. Hendon, L. Colonna-Dashwood and M. Colonna-Dashwood — Journal of Agricultural and Food Chemistry, 62(21)
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