Extraction yield and total dissolved solids (TDS) are the two numbers that tell you whether a brew is objectively good before you taste it. Most baristas learn these terms but few apply them consistently at the bar. This article explains what each metric means, how to measure it, and what to do when results fall outside the target range.
1. What Extraction Yield Actually Means
Extraction yield is the percentage of dry coffee mass that dissolves into the brewing water. A 20 % extraction yield from an 18-gram dose means 3.6 grams of soluble material ended up in the cup. The remaining 14.4 grams are wet grounds you discard.
The Specialty Coffee Association (SCA) defines the optimal extraction window as 18–22 %. Below 18 %, the brew is under-extracted — sharp, sour, and hollow. Above 22 %, over-extraction introduces harsh bitterness and drying astringency.
2. How TDS Relates to Strength
TDS measures the concentration of dissolved solids in the finished cup, expressed as a percentage. A 1.35 % TDS means 1.35 grams of coffee solids per 100 grams of liquid. Higher TDS equals a stronger-tasting brew.
The SCA's Brewing Control Chart maps the relationship between TDS and extraction yield. The "ideal" zone sits at roughly 1.15–1.45 % TDS with 18–22 % extraction. Brewing outside this zone does not make a coffee undrinkable — but it does mean you are not extracting the full potential of the beans.
- Under-extracted (low yield, low TDS): sour, thin, lacking sweetness
- Under-extracted (low yield, high TDS): strong but sour — too coarse a grind with too much coffee
- Optimal: balanced acidity, sweetness, and body within target ranges
- Over-extracted (high yield, moderate TDS): bitter, dry finish
- Over-extracted (high yield, high TDS): very bitter and harsh — grind too fine or contact time too long
3. Measuring TDS Without a Refractometer
A digital refractometer is the standard tool for measuring TDS. The VST RefractometerIII and Atago PAL-COFFEE are the instruments used in professional cupping labs. Both are accurate to ±0.01 % when the sample is at room temperature and the lens is clean.
If you do not have a refractometer, you can estimate TDS from the brew ratio and a typical yield assumption. The CoffeeMap brew ratio calculator does exactly this — it uses a yield model calibrated against 1,847 refractometer readings contributed by testers across 12 countries.
The estimate is accurate within ±0.08 % for medium-roast arabica at a standard dose. It is less reliable for very light roasts or aged blends, where solubility behaviour differs.
4. Practical Adjustments
When extraction falls short of 18 %, the most effective lever is grind size — finer grinding increases surface area and slows flow, giving water more time to extract solubles. When extraction exceeds 22 %, coarsening the grind is usually the first step.
Water temperature is a secondary lever. For every 1 °C increase above 90 °C, extraction yield rises by approximately 0.3–0.5 percentage points, depending on roast level. Use this adjustment after grind size has been optimised.