Coffee Bean Sizes: Washed, Natural, Flat, and Peaberry Explained

If you’ve ever wondered why some coffee tastes cleaner, brighter, or more uniform than others, the answer often starts before roasting—with how the beans were processed, sorted, and even their physical shape. From the way coffee cherries are turned into beans to the numbers stamped on green coffee sacks, these details affect not just appearance but cup quality. Here’s how coffee bean specifications really work.

In short: Washed (or wet-processed) coffee beans are more uniform in quality because they’re mechanically cleaned in water; natural (dry-processed) beans rely on sun drying and can vary more. Flat beans (the common, two-seeded form) and peaberries (single rounded seeds) are sorted by mesh screen sizes—like No. 18 for large flat beans or No. 10 for average-sized ones—and graded by growing altitude, with high-elevation beans typically scoring higher in cup quality.

Washed vs. Natural Processing

Coffee beans are the seeds inside coffee cherries, and how those cherries are processed impacts bean quality. Washed (or wet-processed) coffee is made by removing the fruit’s flesh and mucilage using water tanks and friction tools, then drying the beans. This method results in more consistent, uniform beans. In contrast, natural (or dry-processed) coffee is dried in the sun with the fruit still on the seed, then hulled by machine. This approach is less predictable and can lead to greater variation in quality.

Flat and Peaberry Beans

A typical coffee cherry holds two oval seeds that face each other, forming a flat side where they touch—these are flat beans (also called flatberries). Rarely, a cherry produces a single round seed instead of two; this is a peaberry (also known as caracol). Peaberries occur naturally in about 5% of coffee cherries and are often separated during sorting. Though their shape differs, their taste is not inherently different from flat beans.

Coffee Bean Size by Screen Mesh

Coffee beans are sorted by size using numbered mesh screens. The higher the number, the larger the bean. For flat beans, the standard size categories are: No. 20–19 (very large), No. 18 (large), No. 17 (medium-large), No. 16 (standard), No. 15 (medium), No. 14 (small), and No. 13–12 (very small). For peaberries, the sizing is slightly different: No. 13–12 (large), No. 11 (medium-large), No. 10 (standard), No. 9 (medium), and No. 8 (small).

Altitude-Based Coffee Grading

Coffee is also graded by the elevation at which it’s grown. Higher altitudes generally produce denser, higher-quality beans due to slower maturation and more complex flavors. The typical grading system includes: Special Grade (1,500+ meters), First Grade (1,200–1,500 meters), Second Grade (1,000–1,200 meters), Special Washed Grade (900–1,000 meters), First Washed Grade (760–900 meters), Premium Washed Grade (610–760 meters), and Standard Washed Grade (up to 610 meters).

Quality Assessment via Defect Counting

Coffee quality is objectively measured by counting defects—foreign matter, insect damage, or imperfect beans—in a sample. This statistical method calculates a percentage called the defect count. The lower the defect count, the higher the coffee’s grade and overall quality. This system helps ensure consistency, especially in specialty coffee trading.

Flavor Testing Before Export

In countries like Brazil, Haiti, Kenya, and Uganda, coffee often undergoes sensory evaluation (taste testing) before export. These tests assess cup quality and help ensure that only coffees meeting certain flavor standards make it to market. While methods vary by country, the goal is the same: to verify taste quality alongside physical grading.

Frequently Asked Questions

What’s the difference between washed and natural coffee processing?

Washed coffee is processed by removing the coffee cherry’s fruit and mucilage using water and friction, resulting in cleaner, more uniform beans. Natural coffee is dried with the fruit still attached, then hulled, producing beans with more variable quality due to reliance on sun drying and longer fermentation.

What are flat beans and peaberries?

Flat beans are the standard coffee beans, formed when two oval seeds develop side-by-side in the coffee cherry, creating a flat surface. Peaberries are rare single round seeds that form when only one seed develops inside the cherry instead of two. They are usually sorted separately and may roast differently.

How are coffee beans sorted by size?

Coffee beans are sorted by size using mesh screens with numbered holes. Flat beans use sizes like No. 18 (large) or No. 16 (standard), while peaberries use sizes like No. 10 (standard) or No. 8 (small). The higher the number, the larger the bean.

Why does coffee altitude matter?

Coffee grown at higher altitudes tends to be denser and have more complex flavors due to slower growth and ripening. Grading systems classify beans by elevation, with higher-altitude beans (e.g., 1,500+ meters) considered premium.

How is coffee bean quality determined?

Quality is assessed by counting physical defects in a sample, expressed as a defect percentage. Lower defect counts indicate higher quality. This method provides an objective way to grade coffee, especially for specialty trade.

Do peaberries taste different?

Peaberries do not inherently taste different from flat beans, though their unique shape sometimes leads to slightly different roasting profiles. Their flavor depends more on origin, processing, and roast level than on their shape alone.

Recommended FrontStreet Coffee Beans for Size & Altitude Study

Explore FrontStreet Coffee’s Ethiopia Yirgacheffe for a bright, floral washed profile (typically sorted as size 15–16 flat beans from high altitudes), and their Kenya AA (often peaberry or large flat beans, graded at 1,700–1,800m), known for intense berry acidity. Both showcase how processing and altitude shape cup character. Freshly roasted within 5 days · Orders placed before 17:00 ship the same day · Next-day delivery across most of Guangdong Province.