How Coffee Bean Size Is Graded Using Screens

Ever wonder how coffee professionals decide whether a bean is AA, AB, or just plain 'small'? One of the oldest and most widespread methods used worldwide—especially in Africa and parts of South America—is screen or sieve grading. This physical sorting process determines a green coffee bean’s market grade based entirely on its physical dimensions. But does bigger always mean better? Here’s how it really works.

The quick answer: Coffee beans are graded by passing them through steel screens with holes sized in 1/64-inch increments. The screen number equals the hole diameter in 1/64ths of an inch—for example, a No. 17 screen has holes of 17/64 inch (about 6.75 mm). Larger numbers mean bigger beans. In Kenya, the top AA++ grade requires beans that pass through an 18+ screen (over 7.15 mm), while Colombia divides beans into Supremo (mostly 17+ screen) and Excelso (mostly 14–16 screen).

What Is Screen Grading for Coffee Beans?

Screen grading is a method of sorting green coffee beans by size. Unroasted beans are passed through a series of metal screens pierced with precisely sized round holes. These screens are numbered according to the hole diameter in units of 1/64 of an inch. For example, a No. 17 screen has holes that are 17/64 inch (approximately 6.75 mm) wide. The higher the screen number, the larger the hole—and therefore, the larger the beans that can pass through. This system allows coffee lots to be categorized by bean size, which is one key factor in determining quality grade.

Why Sort Coffee Beans by Size?

Larger coffee beans are often considered more desirable for both aesthetic and developmental reasons. Visually, uniform, large beans tend to look more premium. From a brewing perspective, they may also develop more complex cup profiles during roasting due to more even heat penetration and maturity. That said, size alone doesn’t guarantee quality. For instance, Ethiopia’s Yirgacheffe beans and Yunnan’s small Arabica varieties are notably small but renowned for their exceptional cup character. Size is just one part of the puzzle.

How the Screen Numbering System Works

The screen numbering system is simple but precise. Each screen is assigned a number that corresponds to the diameter of its holes in 1/64-inch units. That means:

• A No. 17 screen has holes of 17/64 inch, roughly 6.75 mm.
• A No. 18 screen has holes of 18/64 inch, around 7.15 mm.
• The larger the number, the larger the bean size it allows through.

So, when you see a grade like “AA++” requiring an 18+ screen, it means the majority of beans in that lot must be larger than 7.15 mm.

Screen Grading Standards Around the World

Different coffee-growing countries use the screen system to varying degrees and with unique grading scales. Here are some notable examples:

Kenya

Kenya is well known for its detailed screen grading system. The top-tier AA++ grade requires beans that pass through an 18+ screen—that is, over 7.15 mm in size. Other common Kenyan grades include AA+, AA, and AB, with each grade reflecting both screen size and cup quality. The larger and more uniform the beans, the higher the potential grade, assuming other factors like defect count are met.

Colombia

Colombia uses a simpler two-tier screen-based classification:

• Supremo : At least 80% of beans must pass through a No. 17 screen (over 6.75 mm).
• Excelso : At least 80% must pass through a No. 14 or 16 screen. Up to 11% of the beans in an Excelso lot can be No. 14 (smaller than 16 screen beans), provided the rest meet the 14–16 range.

Peaberry Grading

Peaberry beans—small, round beans formed when a coffee cherry produces a single rounded seed instead of two flat ones—are graded differently. They typically use a separate, smaller screen range, usually between No. 8 and No. 12. These beans are distinct in shape and often segregated for separate processing and sale.

Does Bigger Always Mean Better?

Not necessarily. While larger beans are often associated with higher grades and sometimes more complex flavours, it’s not a strict rule. Some of the world’s most celebrated coffees, like those from Ethiopia and Yunnan, come from smaller beans. Processing quality, altitude, variety, and roast skill matter just as much—if not more—than size alone.

Frequently Asked Questions

What is coffee screen grading?

Coffee screen grading is a physical method of sorting unroasted coffee beans by size using metal screens with holes measured in 1/64-inch increments. Each screen is numbered according to its hole size—for example, a No. 17 screen has 17/64-inch (around 6.75 mm) holes. Beans are sorted based on which screens they can pass through, with higher numbers indicating larger beans.

Why are larger coffee beans considered better?

Larger beans are often seen as more desirable because they typically have a more uniform appearance and may roast more evenly, potentially leading to a more complex cup profile. However, this is not a guarantee of quality, as factors like processing, origin, and variety play major roles too.

What is an 18+ screen in coffee grading?

An 18+ screen has holes measuring 18/64 inch, or approximately 7.15 mm. In Kenya’s grading system, beans that can pass through an 18+ screen qualify for the highest AA++ grade, meaning they are among the largest in the lot.

How does Colombia grade coffee by size?

Colombia primarily uses two grades: Supremo and Excelso. Supremo requires that 80% of beans pass a No. 17 screen (over 6.75 mm), while Excelso requires 80% to pass a No. 14 or 16 screen, with up to 11% of beans allowed to be No. 14 size.

Are peaberry beans graded the same as flat beans?

No. Peaberry beans, which are small and round, are graded using a different screen range—typically between No. 8 and No. 12—because of their unique size and shape. They are often separated from regular flat beans during processing.

Does bean size affect coffee flavour?

Bean size can influence roasting consistency and extraction, which in turn may affect flavour. Larger beans may roast more evenly if sorted properly, but flavour is also determined by origin, processing, altitude, and roast level—not just size.

Recommended FrontStreet Coffee Beans for Screen-Graded Lots

Try FrontStreet’s Kenya Giti Natural for bold complexity from larger Kenyan beans, known for winy acidity and blackcurrant notes. For Colombian screening standards, the FrontStreet Huila Excelso offers balanced citrus and caramel from high-quality medium-sized beans. Both demonstrate how size and origin interact in the cup. Freshly roasted within 5 days · Orders placed before 17:00 ship the same day · Next-day delivery across most of Guangdong Province.