Understanding Coffee Extraction: TDS & Extraction Percentage Explained
Extraction is one of the most talked-about — and most misunderstood — concepts in coffee. Terms like TDS, extraction percentage, and over- or under-extracted get thrown around constantly, but without proper context they’re easy to misinterpret.
In this guide, we’ll break down what Total Dissolved Solids (TDS) and Extraction Percentage actually mean, how they differ, and how to use them as practical tools to better understand your brewing — rather than rigid rules to blindly follow.
TDS and extraction are not goals on their own. They are reference points that help explain why a coffee tastes the way it does.
What is TDS (Total Dissolved Solids)?
TDS, or Total Dissolved Solids, is a measurement of how much “coffee” is dissolved in your cup. In simple terms, it tells you how strong your coffee is.
TDS is expressed as a percentage. For example, an espresso with a TDS of 9.4% means that 9.4% of the liquid in the cup consists of dissolved coffee compounds. Even though espresso tastes intense, it’s still more than 90% water.
For comparison, the average filter coffee is more than 98.5% water.
TDS describes strength, not quality. A coffee can sit comfortably within a “normal” TDS range and still taste sour, hollow, or unbalanced.
Typical TDS ranges by drink type
Different coffee drinks naturally fall into different TDS ranges:
- Espresso: roughly 8–12%
- Filter coffee: roughly 1.2–1.5%
These ranges are useful for categorising drinks and understanding concentration, but they don’t tell us whether the coffee is well brewed — only how concentrated it is.
How do we measure TDS?
TDS is measured using a refractometer.
A refractometer works by passing light through a liquid and measuring how much that light bends. The more dissolved material in the liquid, the more the light refracts. In coffee, that dissolved material consists of coffee compounds extracted into water.
Because the measurement is optical, technique matters. Calibration, ambient light, and temperature can all affect readings. Used correctly, however, a refractometer provides a fast and repeatable reference point for brew strength.
Why TDS alone isn’t enough
It’s tempting to think that if your coffee lands within a standard TDS range, it must be good. In reality, that’s only half the picture.
A TDS value doesn’t say much on its own unless you also know:
- Whether the drink is filter or espresso
- The coffee dose
- The final beverage weight (yield)
Two coffees can have the same TDS and taste completely different depending on how evenly and how deeply they were extracted.
What is Extraction Percentage?
Extraction Percentage tells us how much of the coffee’s extractable material has been dissolved into the brew.
Roasted coffee contains roughly 28–30% extractable material by mass. In theory, water can dissolve most of this fraction — but in practice, extraction reaches a clear upper limit.
As discussed in Espresso: The Science of Quality, even under extreme laboratory conditions, coffee extraction plateaus at around 30%.
Pushing toward this limit does not produce better coffee. Different compounds extract at different rates, and not all extractable material contributes positively to flavour.
A balanced cup of coffee typically falls between 18% and 22% extraction.
- Below ~18%: sour, sharp, underdeveloped
- Above ~22%: bitter, dry, hollow
There are exceptions, but this range serves as a reliable reference point when learning how extraction behaves.
TDS vs Extraction Percentage: how they work together
TDS and extraction percentage describe two different — but related — things:
- TDS tells you how strong the coffee is in the cup
- Extraction Percentage tells you how well the coffee has been brewed
Together, they give you a much clearer picture of what’s happening during brewing. This is why professional baristas and roasters often look at both values when diagnosing brews.
How do you calculate extraction percentage?
Once you’ve measured TDS, extraction percentage can be calculated using the following formula:
(Brewed coffee weight × TDS) ÷ dry coffee dose
In practice, most people use apps from tools like VST or DiFluid to handle the calculation. The maths itself is less important than understanding what the number represents.
Using extraction as a tool — not a rule
TDS and extraction percentage are not targets to chase blindly. They’re diagnostic tools — reference points that help explain why a coffee tastes the way it does.
They can help you identify under- or over-extraction, compare brews objectively, and understand how changes in grind size, brew ratio, or contact time affect flavour.
Better gear enables better extraction
How much you can extract — and at what point the coffee tastes balanced — depends on several factors working together. The quality of the green coffee, how it was roasted, the brew water, and the grinder all play a role.
One of the biggest limitations in extraction is often the grinder. A typical entry-level home grinder produces a wider spread of particle sizes, with more fines and uneven grounds. This limits how far you can push extraction before bitterness, dryness, or harsh flavours start to appear. In practice, this means you often have to stop extracting earlier — not because the coffee is “fully extracted,” but because pushing further would make it taste worse.
With a higher-quality grinder that produces a more even grind distribution, extraction becomes more efficient and more forgiving. You can extract more of the coffee’s soluble material while maintaining balance, clarity, and sweetness. The result is often a cup that tastes both richer and cleaner at the same time.
That said, better gear can’t fix poor raw material. If the coffee itself isn’t good to begin with — for example, if it’s under-roasted, baked, or made from under- or overripe cherries — higher extraction will usually highlight those flaws rather than hide them. In those cases, a lower TDS and gentler extraction often produce a more pleasant result.
In short: better equipment expands what’s possible, but the quality ceiling is always set by the coffee itself.
If a coffee tastes great but measures “wrong” — don’t change anything. Keep the recipe as it is. Taste always comes first.
