Buyer guide2 min read
Robusta screen sizes S13 to S18: what the numbers mean for your blend
How green coffee is graded by bean size and defects in Vietnam, and when it pays to buy S18 rather than S16 or S13.
Vietnam is the world's largest producer of Robusta, and most of it is sold by screen size and grade. The labels look technical, but they come down to two questions: how big are the beans, and how clean is the lot?
What a screen size is
Green beans are shaken over perforated sieves called screens. The screen number is the hole diameter in 64ths of an inch, so screen 18 has holes of 18/64" (about 7.1 mm) and screen 13 about 5.2 mm. "S18" means that most of the lot — typically 90% — stays on top of screen 18.
The common Vietnamese Robusta grades
- S18 (Grade 1): the largest beans. Even size gives an even roast, which matters for roasted whole-bean retail and espresso blends.
- S16 (Grade 1): slightly smaller, still very uniform — a popular balance of quality and price for blends.
- S13/S14 (Grade 2): smaller beans with a higher defect allowance, widely used for instant coffee and price-driven blends.
Grade is about defects, not only size
Alongside the screen, the specification sets limits for moisture (usually 12.5% max), foreign matter and black and broken beans. A "clean" lot has foreign matter around 0.1% max; Grade 1 typically allows about 2% black and broken, Grade 2 about 5%.
Wet polished or natural?
Most Vietnamese Robusta is dry-processed (natural). Wet polished beans go through an extra polishing step that removes the silverskin, giving a cleaner look and a slightly cleaner cup — worth it when appearance matters to your customers.
How to choose
Match the grade to the product: S18 for premium whole-bean and espresso, S16 for most roast-and-ground blends, S13/S14 for instant and value lines. The best check is always a pre-shipment sample roasted on your own profile.
