If you've spent time in specialty coffee circles, you've probably heard that water for coffee should have a TDS of around 150 ppm. It sounds precise. It sounds scientific. It's also misleading. TDS (Total Dissolved Solids) is one of the most popular metrics people use to evaluate brewing water. But measuring it tells you very little about whether your water will make good coffee. Here's why.
What TDS actually measures
TDS measures the total concentration of everything dissolved in the water. Calcium. Magnesium. Sodium. Chloride. Sulfate. Bicarbonate. Heavy metals. Anything dissolved counts.
A TDS reading of 150 ppm could be:
- Water rich in calcium and magnesium bicarbonates: excellent for extraction
- Water dominated by sodium and chloride: potentially salty and flat
- Water with high bicarbonate and almost no hardness: will suppress coffee's acidity
Three very different water profiles. Same TDS reading.
TDS is a total. It says nothing about composition.
Where the 150 ppm number came from
The SCAA (now SCA) published water guidelines in 2011 that included a TDS target of 150 mg/L.
The 2018 SCA Water Quality Handbook shifted the focus explicitly to alkalinity and total hardness as the two core parameters that drive flavor and extraction.
The two parameters that actually matter
Alkalinity (ideal: 40-75 ppm) acts as an acid buffer in the cup.
Total hardness (ideal: 50-175 ppm) drives extraction.
The SCA handbook notes that total hardness should exceed alkalinity; when the buffer outweighs the extraction minerals, quality suffers.
TDS is a total. It says nothing about composition. And composition is everything.
When TDS is useful (and when it isn't)
TDS meters are good for one thing: a quick consistency check.
But as a standalone quality indicator for brewing water? It will mislead you more often than it guides you.
The honest position
TDS is not useless. But it's not a water quality indicator. It's a dissolved content counter.
If you're evaluating water for coffee, look at:
- Alkalinity: ideally 40-75 ppm
- Total hardness: ideally 50-175 ppm, and always higher than alkalinity
- pH: 6.5-7.5
- Chlorine: none detectable
Cafe com Agua evaluates water against SCA parameters, with alkalinity weighted at 50% of the score, hardness at 30%, and TDS at just 2%. Available on Google Play.
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