How to interpret a coffee screen size chart?

When you look at a coffee screen size chart, you are essentially looking at a map of physical uniformity. In the world of green coffee, size is not just a measurement; it is a proxy for density and development. The grading process uses metal screens with standardized holes, where each number corresponds to 1/64th of an inch. For instance, a Screen 18 means the holes are 18/64 of an inch in diameter, or approximately 7.1mm.

These grading tools are vital for quality control because they allow producers to separate beans into consistent lots. When beans are uniform in size, they behave predictably in the roaster. If a batch contains a mix of tiny and massive beans, the smaller ones will heat up and roast much faster than the larger ones, leading to an uneven roast where some beans are scorched while others remain underdeveloped.

For the reader, this means that a bag labeled with a specific screen size—like 'Screen 17/18'—is a signal of intentional processing. It suggests that the producer has taken the time to remove outliers, ensuring that the coffee you brew will have a more balanced flavor profile. While size does not always guarantee superior taste, it is a foundational element of professional sorting information that allows roasters to dial in their equipment with precision.

In practice, interpreting these charts helps you understand why certain coffees are marketed as 'AA' or 'Supremo.' These are simply shorthand for specific screen size ranges. By understanding that a higher screen number indicates a larger bean, you can better appreciate the effort that goes into the physical preparation of your coffee before it ever reaches the roaster's drum.