Coffee Screen Size: What Screen 18 Means

Read enough coffee bags and you will see “Screen 18,” “17/18” or “AA.” A coffee screen size is simply a measure of green bean size: screen 18 means the beans did not fall through a sieve with round holes 18/64 of an inch wide, about 7.14 mm. Producers and exporters use stacks of these sieves to sort beans into uniform sizes before they are sold.

Why should you care? Because uniform size helps beans roast evenly, and size grades are tied to the names you see on labels, such as Supremo, Excelso and AA. But bigger is not automatically better, and the details are worth knowing.

Key takeaways

• Screen numbers count 64ths of an inch: screen 18 = 18/64 inch ≈ 7.14 mm.
• Sieves usually run from screen 8 to 20; peaberries are sorted on screens with oblong slots.
• Screen 18 lines up with grades like Colombian Supremo and African AA; screen 16 with Excelso and AB.
• Size uniformity matters more for roasting than size itself; flavor depends on far more than bean size.

In this article

1. What is coffee screen size?
2. Screen size chart in inches and millimeters
3. Screen sizes and grade names
4. Does bean size affect flavor?
5. Reading labels like 17/18
6. FAQ

What is coffee screen size?

After processing and drying, green coffee is passed through a stack of perforated metal screens, each with slightly smaller holes than the one above. Beans drop until they reach a screen they cannot pass through. The number of that screen is the bean’s size. As coffee roaster Driftaway Coffee explains, screens are numbered by the width of their holes in 64ths of an inch and typically run from 8 to 20.

So “screen 18” describes beans retained on the 18/64-inch screen. In practice a lot is sold as a range, such as 17/18, with a small allowed tolerance for beans outside it; Driftaway notes the SCAA permitted a 5% variance.

Peaberries need different screens

A peaberry forms when only one seed develops inside the cherry, giving a small, round bean instead of two flat-sided halves. Because of its shape, it is sorted on screens with oblong slots rather than round holes. Learn more in our peaberry coffee guide.

Size is only the first sort

Screening is usually just one step at the dry mill. After sizing, beans are commonly sorted again by density, on vibrating tables that separate light, underdeveloped beans from heavy ones, and by color, by hand or by optical machines that reject black, sour or insect-damaged beans. A lot labeled screen 18 has therefore normally been cleaned of defects too, which is part of why carefully sized coffee tends to cost more.

Coffee screen size chart

To convert any screen number to millimeters, multiply it by 25.4 and divide by 64. Example: 18 × 25.4 ÷ 64 = 7.14 mm.

Fun detail: Driftaway notes that even-numbered screens were traditionally used for Arabica and odd-numbered screens for Robusta, though modern lots are often sold in mixed ranges like 17/18.

Screen sizes and grade names

Many producing countries name their export grades by size. The same physical size can carry very different names depending on where it was grown.

Important: some grading systems, especially in Central America, rank coffee by growing altitude or bean density instead (for example “Strictly Hard Bean”). Others combine size with defect counts. Our guide to coffee grades explained covers those systems.

Does coffee screen size affect flavor?

Bigger beans have a good reputation. Driftaway writes that, all other factors being equal, a larger bean tends to brew a higher-quality cup, and that roasters rarely buy below screen 16. Very large “elephant” beans are an exception: they can be fragile and break during roasting.

But all other factors are rarely equal. Variety, altitude, ripeness, processing and roasting shape flavor far more than size. Some celebrated coffees are naturally small-beaned, and many Ethiopian heirloom lots are smaller-beaned than a typical Supremo yet taste spectacular.

Why uniformity matters most

The practical reason screens matter is roasting. Larger beans roast more slowly than smaller ones. In a mixed lot, small beans can scorch while large ones stay underdeveloped, giving a cup that is both bitter and sour. Sorting to a narrow range lets the roaster apply one heat curve to every bean. Our article on the Maillard reaction in coffee roasting explains what happens inside the bean.

Reading labels like “Screen 17/18”

• “17/18”: most beans are retained on screens 17 and 18 (about 6.75–7.14 mm).
• “15+”: beans are screen 15 or larger.
• “AA”: an African or Indian size grade roughly equal to screen 18.
• “Supremo”: Colombia’s largest regular grade, roughly screen 17–18.
• “PB”: peaberry, sorted separately.

If a label gives the screen size, it usually signals a carefully sorted lot. If it doesn’t, that does not mean poor quality; many specialty roasters list origin, altitude and process instead, which tell you more about flavor. See how to read a coffee label.

Our single origins

Every Feel Espresso single origin is specialty grade and roasted 1–5 days before delivery. Our product pages focus on what shapes flavor most: region, variety, altitude and process. Compare Kenya Nyeri (SL28 and SL34, washed, 1,700–1,890 m) with Colombia Medellin from the mountains of Antioquia, or explore all single origins.

Coffee screen size FAQ

What does screen 18 mean in coffee?

The beans stay on a sieve with round holes 18/64 inch (about 7.14 mm) wide.

What screen size is Kenya AA?

AA corresponds roughly to screen 18 on the common size chart, though exact cut-offs are set by each country’s rules.

Is Supremo better than Excelso?

Supremo beans are larger (about screen 17–18) than Excelso (about 14–16). Size alone doesn’t guarantee a better cup.

How do I convert screen size to mm?

Multiply the screen number by 25.4 and divide by 64. Screen 16 = 6.35 mm.

Sources

1. Driftaway Coffee. Coffee Grades: How Coffee is Sorted by Size. driftaway.coffee.

Facts checked in October 2026.

Keep reading: Coffee grades explained · The coffee bean center line · Single Origins