Extraction, TDS and the Brewing Control Chart
The two numbers that describe every cup of coffee — how much you dissolved, and how concentrated it is — and how they relate.
Brewing coffee is a solvent problem. Roasted coffee is roughly 28–30 % soluble by mass; the rest is cellulose and other structural material that will never dissolve in water. Every recipe is a strategy for pulling the right portion of that soluble fraction into the cup.
Two numbers describe the result of that strategy.
Extraction yield (EY)
Extraction yield is the percentage of the coffee dose that ended up dissolved in the beverage:
EY (%) = (TDS × beverage mass) / dose × 100
If you brew 20 g of coffee into 300 g of beverage at 1.35 % TDS, you dissolved 4.05 g of coffee, which is a 20.25 % extraction.
The Specialty Coffee Association's long-standing target is 18–22 %. Below 18 % the cup tends to taste sour, salty and thin, because acids and light aromatics dissolve first and sweetness hasn't caught up yet. Above 22 % the heavier, slower-dissolving compounds — bitter phenolics, chlorogenic acid lactones, drying tannin-like compounds — dominate. Modern light roasts on excellent grinders are often brewed happily at 22–24 %, so treat the range as a compass rather than a fence.
Total dissolved solids (TDS)
TDS is the concentration of dissolved coffee in the liquid you drink. It is measured in percent by mass and is what your tongue reads as strength. Filter coffee typically sits between 1.15 and 1.45 %; espresso runs 8–12 % because it uses far less water per gram of coffee.
TDS is measured with a refractometer: dissolved solids change how light bends through the liquid, and coffee-specific refractometers convert that refractive index to a TDS reading. A drop of filtered, cooled coffee is enough.
Strength versus extraction
These two axes are independent, and confusing them is the most common dialling-in mistake.
| Low extraction | High extraction | |
|---|---|---|
| Low TDS | weak and sour | weak and bitter |
| High TDS | strong and sour | strong and bitter |
- Ratio (water per gram of coffee) mostly moves strength.
- Grind, temperature, time and agitation mostly move extraction.
The brewing control chart plots TDS on the vertical axis against EY on the horizontal, with diagonal lines for each brew ratio. The ideal box sits at 18–22 % EY and 1.15–1.45 % TDS. Because a given ratio constrains you to a diagonal, changing ratio alone slides you along a line; to move perpendicular to it you have to change extraction.
Mass balance and retained water
Not all the water you pour makes it into the cup. Spent grounds retain roughly 2 g of water per gram of coffee in a filter brew (more in immersion, ~1.1 g/g under espresso pressure). So:
beverage ≈ water poured − dose × retention
This is why the calculator in this app reports both water and beverage mass, and why measuring beverage weight matters when you compute EY from a TDS reading.
What actually dissolves, and when
Extraction is not uniform over time. In rough order of how quickly they leave the grounds:
- Acids and salts — fast, small molecules. Dominant early; taste sour/salty in isolation.
- Sugars and Maillard products — the sweet, caramel and brown middle. Balance arrives here.
- Large phenolic compounds and lactones — slow; drying and bitter. Take over when everything else is spent.
Under-extracted coffee has too much of 1 and not enough of 2. Over-extracted coffee has too much of 3. Good extraction is capturing 1 and 2 while stopping before 3 takes over. Everything else in coffee brewing — grind, water, temperature, agitation — is a lever for controlling that curve.