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The short answer
Water is typically 98 to 99 percent of a cup of coffee by weight, and its mineral content — specifically calcium and magnesium — directly determines how much flavor gets pulled out of the grounds during brewing, which means two identical beans brewed with different water can taste meaningfully different from each other.
Why minerals matter more than purity
Magnesium and calcium are what extract flavor
Extraction isn't just water dissolving coffee compounds passively — magnesium and calcium ions actively bond with and pull certain flavor compounds out of the coffee, particularly the ones associated with sweetness and brightness. Water with too little mineral content, like distilled or heavily filtered water, has a reduced ability to do this, which is why coffee brewed with pure distilled water often tastes flat and underextracted even with an otherwise correct ratio and grind.
Too much mineral content causes its own problems
Water that's too hard — high in dissolved calcium and magnesium — can extract too aggressively or too unevenly, and often carries its own mineral taste that competes with the coffee's flavor rather than supporting it. Very hard water is also the main cause of limescale buildup inside espresso machines and kettles, a maintenance problem separate from but related to the taste issue.
The numbers that actually define good brewing water
TDS — total dissolved solids
Measured in parts per million (ppm), TDS is the broadest single measure of how mineral-rich water is. The Specialty Coffee Association's published target range is 75 to 250 ppm, with 150 ppm cited as an ideal center point — enough mineral content to extract well without overwhelming the coffee's own flavor.
Hardness — specifically calcium
The SCA's target range for calcium hardness sits around 17 to 85 mg/L (roughly 1 to 5 grains), with about 68 mg/L (4 grains) as a common target. This is the mineral most directly responsible for pulling flavor compounds out during extraction, which is why it's tracked separately from overall TDS.
Alkalinity and pH
Alkalinity (commonly measured as bicarbonate) buffers water's pH and affects how acidity in coffee comes through in the cup — too much alkalinity can flatten the perceived brightness of a coffee, even a naturally acidic one. The SCA's target range is roughly 0 to 40 mg/L, with a target pH close to neutral, around 7.0.
Sodium
Sodium is tracked separately and kept low, generally under about 10 mg/L in SCA guidance, since excess sodium can introduce a subtle salty or off note that most people register as "something's a little off" without being able to name it.
What Third Wave Water and similar products actually solve
Municipal tap water varies enormously by location — some cities have water that lands close to the SCA's ideal range without any adjustment, while others are meaningfully too hard, too soft, or too alkaline for good extraction. Rather than trying to correct unpredictable tap water, products like Third Wave Water start from distilled water, which has close to zero mineral content, and add a calibrated mineral packet to build up to a specific, known target profile. That approach trades the unpredictability of "whatever comes out of your tap" for a repeatable, known starting point every time.
The practical takeaway
If your coffee has tasted inconsistent across different locations or after a move, water is a genuinely underrated variable to check before blaming your grinder or beans — a water hardness test strip (cheap and available at most hardware stores) is a reasonable first step to see roughly where your tap water falls. If it's far outside the general 75-250 ppm TDS range, either a mineral-additive product built from distilled water or a filtration system designed to hit a coffee-specific target is a more reliable fix than continuing to guess at grind and ratio adjustments to compensate for water that's working against you.
Frequently asked questions
Can I just use distilled water to make coffee?
Not by itself — pure distilled water has essentially no mineral content, and minerals like magnesium and calcium are what actually pull flavor compounds out of coffee during extraction. Distilled water alone tends to produce flat, underextracted-tasting coffee even at a normal brew ratio, which is exactly the problem products like Third Wave Water are built to solve.
Does tap water work fine for coffee?
It depends entirely on your local water — some tap water sits close to the ideal mineral range and works great, while other tap water is too hard, too soft, or too alkaline and noticeably affects taste. Without testing your specific water, there's no way to know which category you're in from the tap alone.
What is TDS and why does it matter for coffee?
TDS, total dissolved solids, measures how much mineral content is in your water, in parts per million. The Specialty Coffee Association's target range is 75 to 250 ppm with 150 ppm as the ideal center point — too low and water can't extract enough flavor, too high and it can extract too aggressively or taste mineral-heavy on its own.
What does Third Wave Water actually do?
It's a mineral packet you dissolve into distilled water to hit a specific target mineral profile, essentially building ideal brewing water from a blank slate rather than trying to correct unpredictable tap water. Different packet blends target different profiles, including ones marketed specifically for espresso versus filter coffee.