Water Is 98% of Your Cup

When you brew a cup of filter coffee, roughly 98–98.5% of the liquid in that cup is water. The remaining 1.5–2% is dissolved coffee solids — acids, sugars, lipids, melanoidins, and aromatic compounds extracted from the grounds. This arithmetic has an uncomfortable implication: the quality of your water matters enormously, possibly more than any other variable outside the coffee itself.

This is not an esoteric concern for competition baristas. The minerals dissolved in your water — and the ones absent from it — directly determine which flavour compounds are extracted, how much of them ends up in the cup, and whether the coffee’s natural acidity reads as bright and vibrant or flat and dull. Understanding the physics of extraction is the first step; understanding what is dissolved in your water is the second.

TDS: Total Dissolved Solids

Total Dissolved Solids (TDS) is the aggregate measure of all minerals, ions, and compounds dissolved in water, expressed in parts per million (ppm) or milligrams per litre (mg/L). These are functionally the same unit.

The Specialty Coffee Association (SCA) defines the ideal brew water TDS range as 75–250 ppm, with a target of 150 ppm. This range is not arbitrary — it reflects decades of sensory research correlating mineral content with extraction quality and cup character.

• Below 75 ppm (soft/distilled water): Almost nothing to dissolve compounds into. Extraction is weak, flavour is thin and hollow.
• 75–250 ppm (ideal range): Sufficient mineral content to carry and extract flavour compounds effectively. Most specialty coffee water targets the middle of this range.
• Above 300 ppm (very hard water): Over-mineralised water can produce harsh, chalky flavours and leaves significant scale in equipment. Extraction chemistry becomes unpredictable.

Magnesium: The Flavour Enhancer

Not all dissolved minerals are equal. Magnesium (Mg²⁺) is widely regarded as the most flavour-positive ion in brew water, and for good reason: magnesium ions have a particularly strong affinity for the organic acid molecules responsible for coffee’s fruity, floral, and sweet aromatic character.

Research by Hendon et al. (published in Food Chemistry, 2014) demonstrated that magnesium-rich water extracts more flavour-active compounds than calcium-rich water at equivalent TDS levels. Specifically, magnesium appears to preferentially bind to and extract aromatic esters and chlorogenic acid derivatives — the compounds associated with perceived sweetness and fruit notes.

In practical terms: brewing an Ethiopian pour-over with magnesium-forward water (such as Volvic, which has a favourable Mg:Ca ratio) tends to produce a cup with more pronounced floral and fruit character than the same coffee brewed with calcium-dominant hard water at the same TDS. The total dissolved solids are similar, but the character of what is extracted differs.

Calcium: Body and Structure

Where magnesium emphasises aromatic brightness, calcium (Ca²⁺) contributes body, viscosity, and structural weight to the cup. Calcium ions extract a different profile of coffee compounds — particularly larger-molecular-weight polysaccharides and proteins that contribute to mouthfeel.

Hard water areas where calcium dominates (much of the UK, central Europe) often produce espresso with exceptional body and density, even if aromatic complexity is somewhat muted. Many traditional Italian espresso blends were developed specifically for high-calcium water — the heavy, creamy body of a Roman espresso is partly a product of the local water chemistry as much as the roast style.

For espresso, a moderate calcium level (30–70 ppm Ca) contributes positively to extraction and crema stability. Very high calcium levels cause accelerated scale buildup in boilers and group heads — the most common cause of espresso machine damage in hard water regions.

Bicarbonate: The Acidity Buffer

Bicarbonate (HCO₃⁻) — sometimes called alkalinity or carbonate hardness — is the most misunderstood variable in brew water. Its primary function is buffering: bicarbonate neutralises acids in solution, keeping pH stable.

In coffee brewing, this matters because bicarbonate reacts directly with the organic acids extracted from coffee (citric, malic, phosphoric, quinic acid), neutralising them and raising the pH of the brew. A cup brewed with high-bicarbonate water tastes flat, dull, and low-acid — not because those acids were not extracted, but because they were chemically destroyed before you could taste them.

The SCA target for bicarbonate is below 40 ppm. Above approximately 80–100 ppm, the buffering effect becomes strong enough to noticeably flatten bright, acidic coffees. This is why highland Ethiopian or Kenyan coffees — celebrated for their vibrant fruit acidity — often taste disappointing in areas with highly alkaline tap water.

Hard water and alkaline water are not the same thing, though they often coincide. A water can be soft (low TDS) but alkaline (high bicarbonate). A water can be hard but low in bicarbonate. The SCA water standard addresses them separately.

Hardness vs Alkalinity: Different Things, Different Effects

This confusion is widespread. Hardness refers to the total concentration of divalent cations — primarily calcium and magnesium — and affects extraction character, body, and equipment scaling. Alkalinity refers specifically to the buffering capacity of the water, dominated by bicarbonate and carbonate ions, and affects the perceived acidity of the final cup.

A water with high hardness and low alkalinity (high Mg + Ca, low HCO₃⁻) is often excellent brew water — it provides the minerals needed for good extraction while preserving the coffee’s natural acids. This is the profile that products like Third Wave Water aim to recreate.

A water with high alkalinity and moderate hardness is the brewer’s enemy — it may have enough mineral content to extract, but it neutralises the flavour compounds it extracts.

Distilled Water: The Wrong Answer

The intuitive solution to bad tap water is distilled or reverse-osmosis (RO) water — remove everything, start clean. The problem is that pure water with near-zero TDS extracts coffee poorly and produces thin, hollow-tasting brews.

Distilled water also has no buffering capacity, meaning pH swings dramatically during brewing. And without dissolved CO₂ and minerals, water’s surface tension is subtly different, affecting how it wets and penetrates coffee grounds.

More practically: distilled water is corrosive. Demineralised water actively leaches metals from brewing equipment, including boilers, group heads, and copper pipes. Running pure RO water through an espresso machine without remineralisation is a fast route to equipment damage.

The correct approach with RO water is remineralisation — adding precise quantities of magnesium sulfate and sodium bicarbonate (or a premixed product) to achieve the target mineral profile before brewing.

How to Fix Your Water

Scenario 1: Soft, low-TDS tap water (rural areas, Scandinavian regions) Your water is close to the ideal starting point. Check bicarbonate levels — if below 40 ppm, you may be able to brew directly. Consider adding a small amount of Third Wave Water Classic or a similar mineraliser to boost Mg content.

Scenario 2: Hard, high-calcium tap water (much of the UK, Germany, central Europe) Scale will damage your equipment. Use a Brita or similar ion-exchange filter to reduce hardness, or a water softener. Note that many Brita filters increase relative sodium content without improving the Mg:Ca ratio — for serious brewing, a partial RO + remineralisation setup gives more control.

Scenario 3: High-bicarbonate alkaline water This is the most flavour-damaging scenario. Bicarbonate cannot be removed by Brita filters (they soften hardness, not alkalinity). Options: partial RO blending, Third Wave Water, or a dedicated bicarbonate-reduction filter. Simply buying low-alkalinity bottled water (Volvic, Fiji) for your brew water is a practical, low-cost solution.

Espresso vs Filter: Different Ideal Profiles

The ideal water profile differs slightly between espresso and filter brewing. Espresso uses much less water volume in contact with coffee for much less time, but at pressure — it is more tolerant of higher TDS because the extraction window is so brief. Many espresso-focused roasters and cafés target 150–200 ppm TDS with moderate calcium for body and good crema stability.

Filter brewing (pour-over, French press, AeroPress) uses more water, lower temperature, and longer contact times. The buffering effect of bicarbonate has more time to act, so alkalinity control is more critical. The SCA standard of 40–80 ppm bicarbonate is especially important for filter brewing where bright acids are part of the cup character.

Further Reading

• Hendon, C.H., Colonna-Dashwood, L., Colonna-Dashwood, M. (2014). “The role of dissolved cations in coffee extraction.” Journal of Agricultural and Food Chemistry.
• Specialty Coffee Association. Water Quality Handbook — full methodology and sensory research behind the SCA water standard.
• Third Wave Water — premix mineral packets for both espresso and filter profiles, designed for use with distilled or RO water.
• Hoffmann, J. (2014). The World Atlas of Coffee — Chapter 3: water quality and its effect on brewing.