is measured as a percentage based upon weight of your brew and its total dissolved solids (TDS) measurement which is an indicator of how concentrated the coffee brew is
extraction percentage = brewed coffee weight x TDS% / ground coffee weight = Brew Ratio x TDS%
TDS% is measured using a TDS refractometer which has been set to zero by measuring distilled water, and then the coffee is measured after it is passed through a paper filter to remove solid particles
TDS % is g of TDS per 100g brew
most brews have an extraction of 20-25% but vary greatly in their concentration or TDS
different coffees of the same brew type will taste best with different levels of TDS
TDS may be altered by many factors such as differences in harvesting, blending and roasting of the coffee beans, grind size, starting water volume and temperature in the case of a Moka
Turkish coffees are the most concentrated
Italian Espresso coffees are very concentrated as they use very high pressures to push water through very finely ground coffee and generally have a TDS of 8-12 and an extraction of 22-25%, plus the high pressures used create the characteristic crema on the surface
Moka coffees use a more coarse grind and at lower pressures and are about half as concentrated as expresso and have TDS of around 3.5-4.5
AeroPress Prismo higher pressure brews have a TDS similar to Moka of 3.5-5%
AeroPress and French Press coffees have a TDS of around 1.4-1.7
Pour Over brews use the most coarse grinds and just gravity as pressure and have a TDS of around 1.2-1.5