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 |
How the SCA formula connects measurement to meaning
The SCA's extraction formula — EY (%) = (TDS% × brew weight in grams) / (dry coffee weight in grams) — is deceptively simple, but each term repays close examination. TDS% is what the refractometer measures directly. Brew weight is the actual weight of liquid in the cup after brewing — not the water poured in, because some water is absorbed by the coffee grounds and lost to evaporation (typically 1.5–2g per gram of dry coffee in filter brewing). Dry coffee weight is weighed before brewing on a scale accurate to 0.1g. The formula is a mass balance calculation: it tells you what fraction of the dry coffee's total mass ended up dissolved in the final cup.
The formula's output — EY in percentage — is a proxy for sensory quality but not a guarantee. A 20% EY from a well-sourced, freshly roasted specialty coffee tastes excellent. A 20% EY from a poorly processed, old commodity coffee tastes mediocre. The formula measures extraction completeness, not coffee quality. This is why specialty cafés combine EY measurement with cupping scores: EY tells you whether the extraction process worked as intended; cupping scores tell you whether the raw material and roast justified the effort. Both measures together give a complete picture that neither provides alone.
Going deeper
For practical home use, the formula enables proportional adjustments. If your current brew measures 18% EY (under-extracted) and you want to reach 20%, you can calculate the grind or time change required rather than guessing. If your brew ratio is 1:16 (16g water per 1g coffee) and TDS is 1.2%, EY = 1.2 × 16 / 100 × 100 = 19.2%. To reach 20% EY while keeping TDS at 1.2%, you would need to increase the brew weight to 16.67× the dry coffee weight — or alternatively keep the ratio and tighten the grind to push more dissolved solids into the cup. The formula, used this way, becomes a reverse-engineering tool for recipe adjustment.