What is TDS in coffee?

TDS (Total Dissolved Solids) is the mass percentage of dissolved matter in the beverage: 1.15-1.45 % for espresso, 1.25-1.55 % for filter coffee. A digital refractometer (VST LAB Coffee III, Atago PAL-COFFEE) reads it in seconds. It is the objective gauge of perceived strength in the cup.

TDS is an optical density reading: the refractometer measures the beverage's refractive index and converts it to mass percentage through a calibration curve specific to coffee (not seawater, not beer). VST, the US company founded by Vince Fedele around 2008, published the first open coffee curves; Atago, a century-old Japanese maker, followed with the PAL-COFFEE around 2013. A pocket unit weighs 150 g, reads in three seconds, and costs €300-€700 depending on precision (±0.01 % vs ±0.05 %).

TDS alone is not enough: it describes concentration — the perceived strength — but says nothing about extraction yield (EY). The same coffee can hit TDS 1.35 % with EY 18 % (balanced) or with EY 23 % (over-extracted bitterness), depending on grind and ratio. That is why Scott Rao and the SCA recommend reading TDS and calculating EY together, then plotting the point on the coffee compass (horizontal EY, vertical TDS) popularised by Matt Perger at Barista Hustle.

Reference points: a traditional Italian espresso sits around 8-12 % TDS (tight 1:1.5 ratio), a 1:2 third-wave espresso drops to 8-10 %, a standard James Hoffmann V60 lands near 1.35 %, a diluted cold brew at 1.2-1.4 %, and an American diner cup at 0.8-1 %. The SCA Gold Cup places ideal filter between 1.15 and 1.35 % — Norway and Scandinavia aim higher, 1.40-1.55 %, the so-called 'Nordic strong' style.

In Belgium, TDS readings entered the Brussels specialty scene around 2014-2015, often through AEA (Association des Entreprises de Café) barista competitions. Ghent and Antwerp bars trained through Barista Hustle now dial espressos to TDS 9-10 % and EY 20-21 %, reset every Monday morning against a reference recipe, since tap water hardness varies across the Flemish grid (Ghent 15 °f, Antwerp 20 °f, Brussels 25 °f).

Typical TDS windows by brew method