Water Chemistry for Coffee: SCA Standards, Hardness and Alkalinity Explained

Most brewing advice is about grind, ratio and temperature. Water gets less attention, even though it is the solvent doing all the work. This guide explains what the numbers on a water report mean for coffee, what the SCA standard actually says, and how to get close to it at home, whether you filter your tap water or build your own from distilled water. For the extraction side of the story, read our coffee extraction science guide first.

Why Water Changes How Coffee Tastes

Drinking water is never just H₂O. As the SCA’s factsheet with BRITA puts it, minerals and other substances from the environment are dissolved in it, and these change how the water behaves as a solvent and therefore how the coffee tastes. The most important naturally occurring minerals are calcium, magnesium, sodium and potassium, along with chloride, sulfate and hydrogen carbonate (bicarbonate) (SCA, “Water and the Taste of Coffee”).

Two groups matter most for flavour:

• Hardness: the dissolved calcium and magnesium. These positively charged ions interact with compounds in the coffee and influence what ends up in the cup.
• Alkalinity: mainly bicarbonate, which acts as a buffer. It neutralises acids, including the acids that give coffee its brightness. In most natural waters, alkalinity and carbonate hardness are effectively the same thing, so you will see both terms used.

A third group is anything that should not be there at all: chlorine and its by-products from water treatment, and organic off-flavours such as geosmin, which tastes earthy and musty even at tiny levels. The SCA factsheet notes that if your water already smells of chlorine, the coffee will very probably carry a chlorine-like aftertaste.

The SCA Water Standard, Explained

The Specialty Coffee Association’s published standard, “Water for Brewing Specialty Coffee” (SCAA, revised November 2009), sets a target and an acceptable range for each characteristic. The target is the most desirable point; anything inside the range meets the standard (SCAA Water Standard).

Two things are worth noticing. First, the standard allows a wide band of hardness but asks for alkalinity to sit at or near one value. That reflects how strongly bicarbonate affects taste. Second, the SCA later published the more detailed SCA Water Quality Handbook (2018), which works with zones on a hardness-versus-alkalinity chart rather than single targets, because brew method and equipment shift what is ideal (SCA Digital Products). The 2009 table is still the simplest reference for home brewers.

Hardness: Calcium and Magnesium

Calcium and magnesium are the minerals most often discussed for extraction. A computational study by Hendon, Colonna-Dashwood and Colonna-Dashwood modelled how dissolved ions bind to several coffee flavour compounds and found that magnesium bound more strongly than calcium or sodium, suggesting magnesium-rich water could extract those compounds more effectively (Hendon et al., J. Agric. Food Chem., 2014).

Treat that as a hypothesis rather than a rule. The SCA/BRITA factsheet reports that, compared with overall mineralisation and alkalinity, minerals such as sodium, magnesium and calcium play a more minor role in taste, and that a theoretically better extraction from extra magnesium went unnoticed by testers in a blind taste test. The popular shorthand that “magnesium brings fruit, calcium brings body” is not something the published research has established, so we no longer present it here as fact.

The practical cost of hardness is clearer: calcium carbonate scale. If you use an espresso machine or a kettle you care about, keep calcium within the SCA range and follow the manufacturer’s water advice.

Alkalinity: The Number That Matters Most

Alkalinity is the water’s capacity to neutralise acid. A little is useful: it takes the sharp edge off very acidic coffees and gives the water stability. Too much and it neutralises the acids that make specialty coffee taste lively. The SCA factsheet describes the result of high bicarbonate or very hard water as coffee that tastes “unbalanced, bitter and flat”, and says there is solid evidence that most people prefer coffee made with soft water.

That is why the standard pins alkalinity near 40 mg/L. If your coffee tastes dull and chalky despite a good grind and ratio, check alkalinity on your local water report before you change anything else. If it tastes sharply sour even when extraction looks right (see our brew ratio calculator to rule out a weak recipe), very low alkalinity, such as pure RO water, may be part of the reason.

Your Options at Home

Start by reading your water supplier’s annual quality report. It normally lists hardness and alkalinity (sometimes in mg/L as CaCO₃, sometimes in German degrees or grains). If the numbers already sit near the SCA targets, a simple carbon filter for chlorine may be all you need.

A Worked Example: Building Water From Distilled

If you want to make your own water, two common food-grade ingredients do most of the job: sodium bicarbonate (baking soda) for alkalinity and magnesium sulfate heptahydrate (Epsom salt) for hardness. The figures below are calculated from the chemistry, not a competition recipe, and aim at the SCA alkalinity target and a moderate total hardness:

1. Buffer concentrate: dissolve 1.68 g baking soda in 1 litre of distilled water.
2. Hardness concentrate: dissolve 2.45 g Epsom salt in 1 litre of distilled water.
3. Brewing water: combine 40 g buffer concentrate, 70 g hardness concentrate and 890 g distilled water (1 litre total).

That gives roughly 40 mg/L alkalinity and about 70 mg/L total hardness (both as CaCO₃). Two honest caveats. The hardness here comes from magnesium, while the SCA table specifies calcium hardness, so this recipe meets the spirit rather than the letter of the standard. And baking soda adds about 18 mg/L of sodium, above the 10 mg/L target; swapping it for 2.0 g of potassium bicarbonate per litre of concentrate gives the same alkalinity with no sodium. Taste, adjust the hardness concentrate up or down in 10 g steps, and change one thing at a time.

Brewing method matters too. Immersion brewers like the French press and percolation brewers respond differently to the same water, and espresso machines add scale risk, so use any recipe as a starting point. Our home brewing guide covers each method.

Frequently Asked Questions

What is the best water for brewing coffee?

Clean, odour-free water with no chlorine, around 150 mg/L total dissolved solids, calcium hardness near 68 mg/L, alkalinity near 40 mg/L and a neutral pH. Those are the targets in the SCA (SCAA) water standard; anything inside its acceptable ranges meets the standard.

Can I brew coffee with distilled water?

You can, but it is not ideal on its own. Distilled water has no hardness minerals or bicarbonate buffer, so it falls below the SCA standard’s acceptable range for dissolved solids. Many home brewers use distilled water as a base and add minerals back.

Is tap water okay for coffee?

Often, yes, if it is free of chlorine taste and its hardness and alkalinity are moderate. Check your supplier’s water quality report against the SCA targets, and use a carbon filter to remove chlorine and odours.

Why does my coffee taste flat even though my recipe is right?

High alkalinity is a common cause. Bicarbonate neutralises coffee’s acids, and the SCA’s factsheet describes coffee brewed with too much of it, or with very hard water, as unbalanced, bitter and flat. Softer, lower-alkalinity water usually helps.

Does magnesium make coffee taste fruitier than calcium?

That idea is popular but not established. A 2014 computational study found magnesium binds more strongly than calcium to some coffee compounds, but a blind taste test reported by the SCA did not detect a difference from extra magnesium. Overall mineral content and alkalinity have the clearer effect.

Keep Learning

• Coffee extraction science: TDS, extraction yield and the control chart
• Brew ratio calculator
• Espresso extraction workflow: a practical shot-by-shot method
• Chemex filter paper physics

Sources

• Specialty Coffee Association of America. SCAA Standard: Water for Brewing Specialty Coffee (revised 21 November 2009).
• Specialty Coffee Association / BRITA. Water and the Taste of Coffee. SCA News (sponsored factsheet).
• Hendon, C. H., Colonna-Dashwood, L. & Colonna-Dashwood, M. (2014). The Role of Dissolved Cations in Coffee Extraction. Journal of Agricultural and Food Chemistry 62, 4947–4950.
• Wellinger, M., Smrke, S. & Yeretzian, C. The SCA Water Quality Handbook (2018). Specialty Coffee Association.