Understanding Extraction Yield & TDS
The science behind why some cups taste incredible and others don't — and the two numbers that explain the difference.
The Problem Extraction Yield Solves
You brew a perfect cup on Tuesday. On Wednesday, you try to repeat it — same beans, same method — and it's not as good. What changed? Without measurement, you're guessing. Extraction yield gives you a number that describes exactly how much flavour you pulled from your coffee.
Think of coffee grounds like a sponge full of flavour compounds. Some of those compounds taste great (sugars, acids, aromatics). Some taste terrible (bitter tannins, astringent phenols). Extraction yield tells you what percentage of the solubles you actually dissolved into the water.
The SCA defines the ideal extraction yield range as 18–22%. Below 18% you're under-extracting (sour, thin). Above 22% you're over-extracting (bitter, harsh). The magic happens in between.
Two Numbers That Tell You Everything
TDS measures the concentration of your brew — how strong the cup is. It's the percentage of dissolved coffee solids in the liquid. You measure it with a refractometer.
EY measures how much of the coffee you actually extracted. Out of all the solubles in your grounds, what percentage ended up in your cup? It's calculated from TDS, not measured directly.
The Formula
Extraction yield is calculated, not measured. Here's the formula:
What the Numbers Mean in Your Cup
Under-Extracted
Not enough flavour was pulled from the grounds. The pleasant sweet and bitter compounds are still locked inside. You're only getting the first wave — fast-dissolving acids.
Ideal Extraction
The balance point. Enough acid for brightness, enough sugar for sweetness, just enough bitterness for depth. This is where coffee sings.
Over-Extracted
Too much was pulled — including the harsh, late-dissolving compounds (tannins, phenols) that make coffee taste like you're chewing on a tea bag.
Do You Actually Need a Refractometer?
Refractometers (like the VST or Atago) cost $100–$700. You don't need one to brew great coffee. But if you're the type of person who loves data and wants to dial in objectively rather than by feel, it's a game-changer.
Without a refractometer, you can still use extraction principles by:
- Tasting deliberately. If it's sour, you're under-extracting. If it's bitter, you're over-extracting. Adjust one variable at a time.
- Tracking brew parameters. Log dose, water, grind, time, and taste rating. Over time, you'll see which combinations consistently produce the best cups.
- Using the Brewio calculator. The app's built-in extraction yield calculator lets you input TDS and dose and instantly see your EY% — no math required.
How Variables Affect Extraction
Every brew variable pushes extraction in a direction. Here's your cheat sheet:
Grind Size
Finer grind = more surface area = faster and higher extraction. The single most impactful variable.
Water Temperature
Hotter water extracts faster. 90–96°C is the standard range. Below 85°C, extraction drops significantly.
Brew Time
Longer contact time = more extraction. But after a point, you're only extracting the harsh compounds.
Agitation
Stirring and pouring agitation speed up extraction. A gentle swirl vs. aggressive stirring makes a noticeable difference.