by Reza Mehrad
The Ingredient Nobody Questions
Before the grind. Before the roast. Before the harvest and the processing, there is water. Colourless, tasteless, odorless water. The thing we take for granted and never think to question. And yet, water makes up roughly 98% of your espresso and over 99% of your filter coffee. According to the world’s most accomplished baristas, the water you use can make or break a coffee, no matter how extraordinary that coffee is.
This is the philosophy of water in coffee. And it starts with something deceptively simple: the difference between grams and milliliters.
The Question That Started It All
Here is a question that trips up a surprising number of people, even experienced ones: Is 1 milliliter of water the same as 1 gram of water?
The answer, for pure water at standard conditions (4°C), is yes. One milliliter of water weighs exactly one gram. The metric system was literally built around this relationship — the gram was originally defined as the mass of one cubic centimeter of water at its maximum density.
ρ (water at 4°C) = 1.000 g/mL
But temperature matters. At brewing temperatures — around 93°C — water’s density drops to roughly 0.963 g/mL. These differences are small, but in a world where we measure espresso doses to the tenth of a gram, small things matter.
What about espresso?
Espresso is not water. It’s a complex mixture of dissolved solids, emulsified oils, microscopic particles, and trapped gas. All of that adds mass without proportionally adding volume. The result is a liquid denser than water.
DENSITY REFERENCE
| Pure water (4°C) | 1.000 g/mL |
| Water at 93°C (brewing temp) | ~0.963 g/mL |
| Filter coffee | ~1.000 – 1.005 g/mL |
| Espresso (typical range) | ~1.010 – 1.060 g/mL |
| Espresso (high TDS, no crema) | up to ~1.080 g/mL |
In practical terms: if your recipe calls for a 1:2 ratio and you’re pulling 36 g from 18 g of coffee, measuring 36 g by weight is not the same as 36 mL by volume. The difference can be 1–3 grams depending on the shot. For most cafés, negligible. For competition-level precision, a real variable.
Always weigh your espresso. Volume is an illusion. Mass is the truth.
TDS: Total Dissolved Solids
Water is never truly pure. Every water source contains dissolved minerals, ions, gases, and compounds. The collective measurement of everything dissolved in water is called Total Dissolved Solids, or TDS.
TDS is measured in parts per million (ppm) or milligrams per litre (mg/L), functionally the same for water. A reading of 150 ppm means that in every million parts of water, 150 of those parts are something else: calcium, magnesium, bicarbonates, chlorides, sodium.
How TDS is measured
The most common method uses a TDS pen or conductivity meter. These pass a small electrical current through the water, and the more dissolved ions present, the better the water conducts, and the higher the reading. It’s fast and cheap, but it tells you how much is dissolved, not what.
For a full picture you need a proper water report — a lab analysis that breaks down individual components. Serious competition baristas and specialty roasters treat their water report the way a chef treats an ingredient list.
One important distinction: TDS meters measure ionic conductivity, not dissolved mass directly. Non-ionic compounds — like some organics — won’t register at all. This is why a water report is always more informative than a TDS number alone.
What’s Actually in Your Water
The specific minerals dissolved in your water have dramatically different effects on extraction. Understanding the key players is non-negotiable for any serious coffee professional.
Magnesium (Mg²⁺)
Widely considered the most important mineral in brewing water. Magnesium has a strong affinity for coffee’s flavour compounds, particularly the aromatic acids responsible for brightness and complexity. Water with adequate magnesium produces more vibrant, layered extractions.
Calcium (Ca²⁺)
Calcium also extracts coffee compounds, but with a rounder, heavier profile than magnesium. It’s also the primary cause of limescale buildup in espresso machines. High calcium means harder water, more scale, more maintenance, and eventually damaged equipment. The balance between magnesium and calcium is fundamental to both taste and machine health.
Bicarbonates (HCO₃⁻)
Bicarbonates are the alkalinity component of water. They buffer acidity in coffee by neutralizing brightness and rounding sharp flavours. In small amounts, useful. In excess, they create a flat, chalky, lifeless cup that mutes everything good about a specialty coffee. High bicarbonate water is one of the most common and most underdiagnosed problems in café quality.
Sodium (Na⁺)
In low quantities, sodium can enhance sweetness perception the same way a pinch of salt improves food. At high levels, it creates salty and metallic flavours. Most municipal sources are fine, but softened water using sodium ion exchange can have problematically high concentrations.
Chlorine and Chloramines
Added to municipal water as disinfectants and, without question, the enemy of good coffee. Chlorine is volatile, removable with activated carbon filtration. Chloramines are more stable and require more aggressive treatment. Both create medicinal, plastic off-flavours. Eliminate them entirely.
SCA IDEAL BREWING WATER
| Total Dissolved Solids | 75 – 250 ppm (target: 150) |
| Calcium Hardness | 50 – 175 ppm (target: 68) |
| Total Alkalinity | 40 – 70 ppm (target: 40) |
| pH | 6.5 – 7.5 (target: 7.0) |
| Sodium | < 30 ppm |
| Chlorine/Chloramines | 0 ppm |
The Acid-Base Balance
pH measures acidity or alkalinity on a scale of 0 to 14. Pure water sits at a neutral 7.0. Coffee brewing water should stay between 6.5 and 7.5.
Too acidic and the water over-amplifies the natural acidity in coffee, creating sharp, sour extractions. Too alkaline (usually caused by high bicarbonates) and it flattens everything, suppressing the brightness that makes specialty coffee worth drinking.
High-TDS water isn’t automatically alkaline, and low-TDS water isn’t automatically acidic. This is why understanding the composition of your water matters far more than any single number.
TDS is not the goal. Mineral balance is the goal. TDS is just the opening conversation.
Water as a Solvent
At its most fundamental level, brewing coffee is a dissolution process. Water enters a coffee bed, solubilizes flavour compounds, and carries them out as a liquid solution. The water is the solvent, the coffee is the solute, and the cup is the result.
This is why water chemistry is not a secondary concern. The solvent determines what gets extracted, how quickly, and in what proportions.
The extraction equation
Extraction yield is expressed as a percentage: the mass of coffee that ended up in the cup, divided by the original dose weight.
Extraction Yield (%) = (Brewed Weight x Beverage TDS) ÷ Dose Weight x 100
Specialty coffee targets 18–22% extraction yield for both espresso and filter.
The same coffee, extracted to the same yield with two different water sources, will taste completely different. The yield tells you how much was extracted. The water tells you what.
Hard vs. soft water in extraction
Hard water extracts more aggressively. Fuller body, more intense flavour — but higher risk of over-extraction and guaranteed scale damage over time. Soft water under-extracts. Distilled or RO water is too pure — it literally lacks the minerals that act as extraction carriers, producing a flat, hollow cup. Water needs minerals to brew well.
Controlling Your Water
- Carbon filtration
The baseline. A carbon block removes chlorine, chloramines, and sediment while leaving the mineral profile intact. The minimum viable setup for any café. If your tap water has a reasonable mineral balance, carbon alone can produce excellent results. - Scale inhibition
Technologies like TAC or polyphosphate dosing change how minerals behave rather than removing them, preventing scale deposits without affecting taste. Protects equipment, doesn’t fix bad water. - Reverse Osmosis + remineralization
The most precise approach. RO strips water to near-pure levels. A remineralization stage then adds back controlled amounts of specific minerals — typically magnesium and a small amount of calcium bicarbonate. This is the method used by most competition baristas and serious specialty programs. - Third-party mineral products
Products like Third Wave Water offer pre-made mineral packets designed for distilled or RO water. Convenient and surprisingly effective for home brewers or small operations that want precision without a full filtration investment.
Water and Espresso — A Special Case
Everything above applies to all brewing methods. But espresso deserves its own section. The combination of high pressure, high temperature, short contact time, and extreme concentration makes espresso uniquely sensitive to water chemistry.
At 9 bars and 90–94°C, water passes through a compressed puck in 25–35 seconds. The resulting liquid has a TDS of 8–12% — compared to 1.2–1.5% for filter coffee. That’s an order of magnitude difference in concentration, and it amplifies every variable, including water flaws.
The same mineral imbalance that produces a mildly flat filter coffee will produce a dramatically harsh or sour espresso. There’s no hiding bad water in a shot.
Machine health
Espresso machines are also more vulnerable to poor water quality than any other brewing equipment. Boilers, group heads, heat exchangers, and solenoids are all susceptible to scale. A machine running on unfiltered hard water can develop serious deposits within months. A proper filtration system pays for itself in equipment protection alone, long before you count the cup quality.
The Last Ingredient
Here is the point all of this chemistry builds toward: water is an active participant in every cup of coffee ever made.
When we talk about terroir — the idea that a coffee tastes the way it does because of where it grew, how it was processed, what microclimate it came from — we are describing the accumulated influence of countless variables. Water is one of them. The rain that fed the plant’s roots is part of that story.
This is why the best baristas obsess over water. Not because they are pedantic, but because they understand their job is to honour the work of the farmer, the processor, and the roaster, including choosing the right water.
So the next time someone asks whether 1 milliliter equals 1 gram, you can answer yes. But then you can tell them the more interesting truth: in coffee, water is never just water. It is chemistry, philosophy, and intention, dissolved into a single transparent ingredient that most people never think to question.
Key Takeaways
| 1 mL water = 1 g water? | Yes at 4°C. Close enough in practice. |
| Espresso density | ~1.01–1.06 g/mL. Always weigh. |
| Ideal brewing TDS | 75 - 250 ppm. Target: 150. |
| Most important mineral | Magnesium |
| Biggest enemy | Chlorine + high bicarbonates |
| Distilled water for coffee? | No. Too pure to extract properly. |
| Best filtration approach | RO + remineralization |
