Coffee screen size is a grading measure that sorts green coffee beans by their physical width using a stack of perforated metal screens. Each screen carries a number equal to the diameter of its round holes in sixty-fourths of an inch, so a screen 18 has holes 18/64 of an inch across. Beans that rest on top of a given screen are counted as that size, giving graders a fast, repeatable way to describe how big the beans in a lot are and to build consistent commercial grades.
What the screen number actually means
The screen number is a unit of measurement, not a quality score. A screen 18 has perforations measuring 18/64 inch (about 7.1 mm); a screen 14 has holes of 14/64 inch (roughly 5.6 mm). Because the numbering climbs in increments of one sixty-fourth of an inch, each step up the ladder means a bean that is fractionally wider. Most commercial green coffee falls somewhere between screen 13 and screen 20, with the bulk of Arabica lots landing between screens 14 and 18.
Graders talk about screens because width is the dimension that most reliably passes through or catches on a round hole. Length and thickness vary too, but a round perforation keys on the widest cross-section, which makes the screen an honest, low-tech gauge of overall bean size. The system traces back to older sieve standards and has become shared vocabulary across the trade, even in countries that also describe beans in millimetres.
How beans are sieved and sorted
Sizing happens after milling and hulling, and it sits within the wider chain of coffee processing methods. Dried parchment or naturals are hulled to expose the green bean, then poured over a sizing machine. In practice this is usually a vibrating screen grader: a stack of interchangeable perforated plates, arranged largest holes at the bottom and smallest at the top, shaken continuously so beans travel across each surface.
Beans small enough to fall through a screen drop to the next plate down; beans too wide to pass are retained. The result is a set of fractions, each labelled with the screen that caught it. A lot described as "screen 17/18" is one whose beans mostly passed a slightly larger screen but were held on screen 17. Rotary drum sizers do the same job with cylindrical mesh sections. Screening is nearly always paired with density and defect sorting, so size grading is one filter among several rather than the whole story.
- Screen 20: ~20/64 inch — very large, elephant-grade beans.
- Screen 17-18: large, the premium size band for many origins.
- Screen 15-16: medium, the workhorse commercial sizes.
- Screen 13-14: smaller flat beans.
- Peaberry: round single seeds, graded on their own dedicated screens.
Mapping size grades across origins
Different producing countries wrap the same screen measurements in their own grade names, which is why a shelf of green coffee carries so many labels. Understanding the screen behind each name makes them easy to compare, and it complements the broader picture in our overview of coffee grading.
Colombia: Supremo and Excelso
Colombia's two familiar export grades are defined almost entirely by size. Supremo is the largest standard grade, generally screen 17 and above (often quoted as 17/18). Excelso is smaller, typically spanning screens 14 to 16. The names sound like quality tiers, and Supremo does command a premium, but both are drawn from the same harvests; the difference on the label is bean width, not an inherent cup ranking.
Kenya, Tanzania and the AA system
Much of East Africa uses a letter system layered on top of screen sizes. In Kenya and Tanzania the headline grade AA covers the largest flat beans, roughly screen 17 to 18. AB combines the A (screen 16) and B (screen 15) fractions into a single widely traded grade. Below that, C denotes smaller beans, and E stands for elephant beans, unusually large seeds that are often malformed. PB is the peaberry grade. Because these letters map directly onto screens, an AA lot and a Colombian Supremo lot describe a similar physical bean size even though the names look unrelated.
Peaberry as its own category
Peaberry, known as caracol (snail) in Spanish-speaking origins, is a natural variation in which a coffee cherry develops a single round seed instead of the usual two flat-faced halves. Because its shape is rounder and often smaller, it will not sort cleanly on the flat-bean screens, so it is pulled out and graded separately, frequently on a distinct set of round-holed peaberry screens. Many origins market it as a premium curiosity. If you want the full botany of why cherries produce these round seeds, see our guide to peaberry coffee.
Why size matters for roasting
The strongest practical reason to sort by screen size is roast uniformity. Heat penetrates a bean from the outside in, so larger beans need marginally more energy and time to reach a target development than smaller ones. When a batch mixes screen 14 and screen 18 beans, the small beans can scorch or over-develop before the large ones finish, producing a muddled, uneven roast. A tightly sized lot behaves predictably: the beans reach first crack together and develop at a similar rate, giving the roaster a cleaner, more repeatable curve.
Uniform sizing also improves how beans tumble and flow through a roaster's drum and cooling tray, and it makes defect sorting easier because outliers stand out. This is the real value screen grading adds for buyers and roasters, and it is why size appears on nearly every green coffee spec sheet.
Size is not the same as quality
It is tempting to read a big screen number as a promise of a better cup, but size and quality are only loosely linked. Screen grading measures a physical dimension; it says nothing directly about sweetness, acidity, aroma, or the absence of defects. Large beans can come from high-yielding trees with unremarkable flavour, and some celebrated varieties and high-altitude lots produce comparatively small beans that cup beautifully.
Where size and quality do correlate, it is often indirect: careful farms that invest in sorting also tend to invest in picking ripe cherries and processing well, so their large-bean grades happen to taste good. Screen size is best understood as one useful descriptor for consistency and presentation, sitting alongside density grading, defect counts, and cup scoring rather than replacing them. A serious buyer reads all of these together.
Frequently asked questions
What does coffee screen size mean?
Coffee screen size is the width of green coffee beans measured against perforated screens whose holes are numbered in sixty-fourths of an inch. A screen 18 bean is one that rests on a screen with 18/64-inch holes, so it is about 7.1 mm wide. The number is a size measurement, letting buyers and roasters describe how large the beans in a lot are and build consistent commercial grades.
How are coffee beans sorted by screen size?
After hulling, green beans are poured onto a vibrating screen grader: a shaking stack of perforated plates arranged with the smallest holes on top and largest at the bottom. Beans fall through screens they are small enough to pass and are caught on those they are too wide for. Each retained fraction is labelled by the screen that held it, producing grades such as screen 15, 17, or 18.
Is a bigger coffee screen size better?
Not necessarily. Screen size measures bean width, not flavour, sweetness, or acidity. Large beans mainly help a lot roast evenly and look uniform, and premium grades like Supremo or AA are defined by size. But small, dense, high-altitude beans can cup superbly, so size is only one descriptor. Judge quality using cup scores, density, and defect counts alongside the screen grade rather than by size alone.
How do Supremo, AA, and peaberry relate to screen size?
They are all size-based labels from different origins. Colombia's Supremo is roughly screen 17/18 and Excelso screens 14 to 16. Kenya and Tanzania use AA (about screen 17/18), AB, C, and E for flat beans of decreasing or unusual size. Peaberry, or caracol, is the round single-seed form graded on its own screens. Once you know the screen behind each name, the labels are easy to compare across countries.
Why does screen size affect roasting?
Heat moves from a bean's surface inward, so larger beans need slightly more time and energy to develop than smaller ones. A batch mixing very different sizes will roast unevenly, with small beans scorching before large ones finish. A tightly screened lot reaches first crack together and develops at a similar pace, giving the roaster a cleaner, more repeatable roast and making defect sorting easier.