How Coffee Bean Size Is Graded Using Screens

Ever wonder why some specialty coffee bags proudly label their beans as 'AA' or list a screen size like '18'? The answer lies not in marketing fluff—but in how those beans were physically sorted before roasting. One of the most fundamental steps in green coffee processing, especially for countries aiming for export-grade quality, is sorting beans by size using a simple yet precise tool: metal screens with precisely drilled holes.

In short: coffee beans are graded by passing them through iron screens punched with 1/64-inch holes. The screen’s number tells you how big those holes are—like '17' meaning 17/64-inch, or roughly 6.75mm. Bigger numbers mean bigger beans. That’s how coffee gets classified by size before it even hits the roaster.

What Does Sieve Grading Actually Mean?

Sieve grading—also called screen grading—is the method of separating green coffee beans based on physical size. Rather than relying solely on visual inspection or weight, producers run beans through a series of circular, perforated metal plates (the sieves), each drilled with holes of a standardized diameter. Each sieve is assigned a number corresponding to the hole size in 1/64-inch increments. For example, a No. 17 sieve has holes that are 17/64 of an inch (approximately 6.75mm) in diameter. The larger the sieve number, the larger the hole—and thus, the larger the coffee bean that can pass through.

Why Size Matters in Coffee Grading

Larger coffee beans are often considered more desirable—not necessarily because they taste better in every case, but because they typically have more even internal development, better roasting consistency, and an aesthetically pleasing appearance. Uniform size also helps during roasting, ensuring even heat transfer and development. That said, bean size alone doesn’t determine quality. For example, Ethiopia’s Yirgacheffe beans and Yunnan’s small Arabica beans are notably smaller but are celebrated for their complex and memorable cup profiles. Size is just one part of the puzzle.

How Countries Use Screen Sizes in Grading

Different coffee-growing countries adopt sieve grading with varying levels of detail. Kenya is among the most meticulous users of this system. Its top tier, AA++, requires beans to pass through a sieve size of 18 or higher—that translates to beans larger than 7.15mm. The Kenyan system includes multiple grades such as AA+, AA, and AB, each with specific sieve requirements. Colombia, by contrast, uses a simpler two-tiered system: Supremo (the highest grade) and Excelso. For Supremo, at least 80% of the beans must pass through a No. 17 sieve or larger. Excelso allows 80% of beans to pass a No. 14 or 16 sieve, with up to 11% of those allowed to be as small as No. 14. These standards help ensure consistency and marketability of the exported coffee.

It’s important to note: the sieve system described here applies to flat beans (the typical coffee bean shape). Peaberry beans, the small round beans formed when a coffee cherry produces a single rounded seed instead of two flat ones, follow a different sizing standard. For peaberries, sieve sizes generally range from No. 8 to No. 12.

Understanding the Sieve Numbering System

The numbering of coffee grading screens directly reflects the diameter of the holes in 1/64-inch units. This means:

• No. 17 sieve = 17/64 inch ≈ 6.75mm
• No. 18 sieve = 18/64 inch ≈ 7.15mm
• No. 14 sieve = 14/64 inch ≈ 5.56mm

The larger the number, the larger the hole—and the larger the bean that can pass through it. This system provides an objective, measurable way to categorize beans by size, which is crucial for maintaining consistency in specialty coffee production and trade.

Frequently Asked Questions

What is sieve grading in coffee?

Sieve grading is a method of sorting green coffee beans by passing them through metal screens with holes of a specific diameter, measured in 1/64-inch increments. The screen number indicates the hole size—for example, a No. 17 sieve has 17/64-inch (around 6.75mm) holes. Beans that can pass through a given sieve are graded accordingly.

Does a larger sieve number mean bigger beans?

Yes. The sieve number refers to the size of the holes in 1/64-inch units. A No. 18 sieve (with 18/64-inch holes, ~7.15mm) is larger than a No. 17 sieve (17/64-inch, ~6.75mm), so beans that pass through No. 18 are larger than those that only pass through No. 17.

Which countries use sieve grading for coffee?

Countries including Kenya, Tanzania, and Colombia use sieve grading as a primary or secondary method to classify green coffee beans by size. Kenya is especially detailed, with grades like AA++, AA+, AA, and AB based on sieve size. Colombia uses Supremo and Excelso grades with specific sieve requirements.

Are larger coffee beans always better?

Not necessarily. While larger beans are often easier to roast evenly and may have more consistent development, smaller beans—like those from Ethiopia or Yunnan—can still produce outstanding, complex cups. Size is just one factor in coffee quality.

Do peaberry beans use the same sieve sizes as flat beans?

No. Peaberry beans, which are small and round, are graded using a different sieve scale, typically ranging from No. 8 to No. 12, reflecting their smaller and more uniform size compared to regular flat beans.

FrontStreet Coffee Beans Sorted by Size & Quality

For coffee enthusiasts interested in how bean size affects flavor, try FrontStreet’s Ethiopia Yirgacheffe for bright floral notes and a silky body—known for its small but vibrant beans. Their Kenya AA offers bold blackcurrant and citrus from carefully sized beans graded under strict Kenyan standards. Also consider Colombia Supremo for balanced caramel and nut tones, sourced from beans meeting Supremo’s 17+ sieve size minimum. All showcase how sizing plays into profile. Freshly roasted within 5 days · Orders placed before 17:00 ship the same day · Next-day delivery across most of Guangdong Province.