What screen numbers mean, why bean size matters to roasters, and how to specify a Ugandan Robusta lot.
Green coffee is graded by passing beans through a stack of perforated screens. A screen number refers to the diameter of the holes in sixty-fourths of an inch, so a bean retained on a screen 18 sieve is at least 18/64" (about 7.1mm) across.
Larger, more uniform beans roast more evenly. That consistency is why bigger screens usually attract a premium, even though cup character is shaped by many other factors — processing, drying, storage and defects among them.
The three most traded Ugandan Robusta screens
Uganda's natural (dry-processed) Robusta is commonly offered in three screen grades:
- Screen 18 — the largest commercial screen; favoured for espresso blends and premium commercial roasting.
- Screen 15 — the workhorse grade, balancing quality and price for commercial roasting and blending.
- Screen 12 — smaller beans at an economical price point, widely used for soluble coffee and value blends.
What else to put in your specification
Screen size is only one parameter. A clear contract will also state maximum moisture (commonly around 12.5%), defect count or percentage, foreign matter, packing (typically 60kg jute bags, with or without hermetic liners) and whether pre-shipment samples are required.
If you are unsure which screen suits your blend, request samples of more than one grade — it is the most reliable way to decide.