What is screen size in green coffee grading?

Screen size is a measure of the physical dimensions of green coffee beans, determined by passing the beans over a series of metal screens with round holes of standardised diameters. A bean is retained by a screen if it is too large to pass through the hole - so a bean retained on Screen 18 is at least 18/64 of an inch (approximately 7.1mm) in diameter. Screen size is recorded as the smallest screen that retains the bean.

Screen sizing is one of the primary classification variables in green coffee grading, used alongside defect count, moisture content, and cup quality. Larger screen sizes are generally associated with denser, better-developed beans from high-altitude origins - though this correlation is not absolute. Colombian grades are formally defined by screen size: Supremo (Screen 17+) and Excelso (Screen 14-16) are the two principal export grades. In East African coffees, screen size forms part of the grade designation - Kenya AA is Screen 17-18, AB is Screen 15-16.

For buyers, screen size on an offer sheet provides a useful quality reference and helps predict roasting behaviour. Larger, more uniform beans tend to roast more evenly than a mixed-size lot, since different bean sizes absorb heat at different rates. Uniform screen sizing - where beans are tightly clustered around a single screen size - is a marker of careful preparation at the dry mill.