What is the SCA extraction formula?
The SCA extraction formula is: Yield (%) = (TDS × beverage weight) / coffee weight × 100. It positions any coffee on the Brewing Control Chart (BCC), a two-dimensional graph linking TDS and yield to define the optimal quality zone (TDS 1.15-1.45%, yield 18-22% for filter). This theoretical framework, developed in the 1950s-60s by Dr Ernest Lockhart, remains the universal reference for coffee extraction chemistry.
The SCA formula is often presented as a simple equation, but its scope is significant: it connects three measurable parameters (TDS, beverage weight, coffee dose) to derive a fourth (yield) that quantifies extraction efficiency.
Historical context: in the 1950s, the Coffee Brewing Institute (CBI) in the United States commissioned Dr Ernest Lockhart, food chemist at MIT, to study brewing conditions that maximise consumer satisfaction. Lockhart conducted large-scale sensory studies and identified TDS and yield ranges associated with highest preference. These results were formalised in the 'Brewing Control Chart' published in the 1960s, adopted by the SCAA (SCA's predecessor) and became the theoretical foundation of all serious barista training since.
The formula in full form is: Yield (%) = [TDS (%) × M_beverage (g)] / M_coffee (g)
Where: — TDS is measured as a mass percentage (via refractometer) — M_beverage is the mass of beverage obtained (weighed on scale) — M_coffee is the mass of ground coffee used (dose)
The result is extraction yield — the percentage of coffee mass dissolved into the beverage.
The BCC is useful in multiple ways. It diagnoses an extraction: a point at lower left = under-extracted and weak; upper right = over-extracted and strong; lower right = well-extracted but diluted; upper left = under-extracted and concentrated (rare but possible with very tight espresso). It also explains how variable adjustments affect position on the chart: increasing dose shifts upward (stronger) without changing yield; coarser grind can reduce yield without changing concentration if time is adjusted.
The formula has important limitations. It treats coffee as a homogeneous whole, whereas extraction is actually differential: acids and sugars emerge first, bitter and astringent compounds later. A 20% yield reached quickly vs slowly, at high vs low temperature, with vs without agitation, is not identical. These nuances are what distinguishes extraction chemistry from mere number-reading — this is where the barista's sensory experience intervenes to interpret data within the context of a specific coffee.
Brewing Control Chart: zones and interpretations (filter)
| TDS (%) | Yield (%) | Diagnosis | Correction |
|---|---|---|---|
| < 1.15 | < 18 | Too weak and under-extracted | Finer grind, higher dose |
| < 1.15 | 18–22 | Well-extracted but diluted | Reduce water/coffee ratio |
| 1.15–1.45 | 18–22 | Ideal zone | Maintain |
| 1.15–1.45 | < 18 | Concentrated but under-extracted | Finer grind |
| 1.15–1.45 | > 22 | Well-concentrated but over-extracted | Coarser grind |
| > 1.45 | > 22 | Too strong and over-extracted | Reduce dose or open grind |