This is an excerpt from a paper that was presented to the list not long ago titled SOME ASPECTS OF ESPRESSO EXTRACTION by Jim Schulman "coffee strength, solubles yield, and taste Everyone knows espresso is stronger than regular coffee; the same tablespoon of coffee brewed into six ounces of regular coffee is used to make one ounce of espresso. But this is not the whole story. Not all the ground coffee goes into the cup or shot. In properly brewed coffee, about 18% to 22% of the ground coffee is dissolved into the water, while the rest is spent grounds; the range in espresso shots is wider, running from 15% to 25%. This proportion of ground coffee that ends up in the cup is called the solubles yield; a brew with too low solubles yield is called under-extracted, while one with too high solubles yield is called over-extracted. It is important to understand that solubles yield and brew strength are two separate things. For instance, imagine that the six ounce cup of brewed coffee was extracted at 24%. Its strength would be 24% of the coffee measure divided by 6 ounces or roughly 4% per ounce. If the same measure in a shot of espresso was extracted at 16%, all of it would still be in one ounce of water, so the under-extracted espresso would still have four times the strength of the over-extracted brewed cup" I saved the page to file for myself.