TDS and EY Extraction Guide: Refractometer, Calculations, Golden Cup
Two numbers summarise the state of a coffee extraction: TDS (Total Dissolved Solids) and EY (Extraction Yield). Popularised by the SCA and practitioners like Scott Rao and Matt Perger, these metrics allow you to move from subjective tasting impressions to objective analysis. This guide explains what each value measures, how to calculate them, how to use a refractometer, and how to interpret the results to dial in your brewing.
TDS: What Are We Actually Measuring?
TDS (Total Dissolved Solids) expresses the proportion of dissolved matter in the cup relative to the total liquid mass. A TDS of 1.25% means that in 100 g of brewed coffee, 1.25 g is dissolved solids (sugars, acids, emulsified lipids, caffeine, aromatic compounds) and 98.75 g is water.
TDS measures the concentration of the cup, not the quality of the extraction. A high TDS simply means a stronger, more concentrated drink. A ristretto espresso might have a TDS of 8–12%, a filter coffee 1.2%, a cold brew concentrate 2–4%. Each style has its own norms.
For filter coffee, the SCA established a preference zone known as the "Gold Cup Standard": 1.15–1.35% TDS. This emerged from consumer preference studies in the 1950s–60s by E.E. Lockhart, refined since by the SCA. It's a statistical reference, not an absolute truth — many professionals and enthusiasts prefer 1.40–1.50% (stronger) or around 1.05% (lighter, more tea-like).
EY: The Extraction Yield Explained
EY (Extraction Yield) measures what proportion of the dry coffee mass has been extracted and ended up in the cup. An EY of 20% means that from 20 g of ground coffee, 4 g of soluble matter was dissolved into the liquid. The remaining 16 g stays in the puck or grounds (insoluble fibres, cellulose, unextracted proteins).
Why can't you extract 100%? Because a large portion of coffee is insoluble (cellulose, some proteins), and soluble compounds don't all dissolve at the same rate or temperature. Extraction is selective: acids extract first (within the first seconds), sugars next, bitter and astringent compounds last. Under-extraction (EY < 18%) produces sour, salty cups; over-extraction (EY > 22%) produces bitter, astringent ones.
The EY Calculation Formula
The standard formula used by the SCA and most brewing software:
EY (%) = (TDS% × brewed beverage weight in g) / (coffee dose in g × 100)
Practical worked example:
- Dose: 20 g ground coffee
- Brewed volume: 320 ml (weighed: 320 g, density ≈ 1)
- Measured TDS: 1.30%
- EY = (1.30 × 320) / (20 × 100) = 416 / 2000 = 20.8%
20.8% sits comfortably in the target window (18–22%). This cup should be balanced, with good sugar development and controlled acidity.
SCA Gold Cup Table by Brewing Method
| Method | TDS target (%) | EY target (%) | Typical coffee:water ratio |
|---|---|---|---|
| Filter (V60, Chemex, batch brew) | 1.15 – 1.35 | 18 – 22 | 1:15 to 1:17 |
| AeroPress (diluted recipe) | 1.15 – 1.35 | 18 – 22 | 1:13 to 1:16 |
| Espresso (classic double shot) | 8 – 12 | 18 – 22 | 1:2 to 1:2.5 |
| Moka pot | 2.0 – 3.5 | 15 – 20 | 1:7 to 1:10 |
| Cold brew concentrate | 2.0 – 4.0 | 12 – 18 | 1:4 to 1:8 (then diluted) |
| Lungo espresso (1:3) | 5 – 8 | 20 – 25 | 1:3 |
Using a Refractometer: Step-by-Step Protocol
A refractometer measures the refractive index of a liquid — how light bends as it passes through. The more concentrated the solution, the greater the bending. Coffee-calibrated refractometers convert this refractive index directly into TDS.
Recommended Devices
- Atago PAL-COFFEE (BX/RI): professional reference, ±0.01% Brix precision, compact and robust. Around €200–250. Ideal for professional baristas and serious enthusiasts.
- Difluid R2 Extract: newer, more affordable alternative (~€80–100), precision comparable to the Atago according to independent tests (Barista Hustle, Lance Hedrick). Bluetooth app for logging.
- VST Coffee Tools (VST LAB Coffee III): the World Barista Championship reference tool, exceptional precision (~€500). For professionals and competitors.
Measurement Protocol
- Let the sample cool: digital refractometers compensate for temperature, but for stable readings, wait until the coffee reaches 20–25°C. Pour a small amount into a glass or tablespoon.
- Calibrate with distilled water: place 2–3 drops of deionised water on the prism, verify the reading shows 0.00%. Use the calibration function if not.
- Apply the sample: 2–3 drops of cooled coffee on the prism. Avoid air bubbles.
- Read and record: note the displayed TDS%. For reliability, take 2–3 readings and average them.
- Clean between readings: wipe the prism with a clean, damp microfibre cloth.
Interpreting Results and Adjusting
Once you have TDS and EY calculated, four quadrants are possible:
- Low EY + Low TDS: under-extracted and under-concentrated. Grind finer, increase temperature, extend contact time, or increase coffee:water ratio.
- Low EY + High TDS: under-extracted but concentrated (rare). High dose, short extraction. Sour but intense. Adjust ratio or extraction time.
- High EY + Low TDS: over-extracted and diluted. Too much water for the dose. Reduce water volume or increase dose.
- High EY + High TDS: over-extracted and concentrated. Bitter, astringent, heavy. Grind coarser or reduce contact time.
The refractometer doesn't replace tasting — it complements it. Some coffees are delicious at 23% EY (light Ethiopian roasts), others become flat beyond 20%. Data guides, the palate decides.