Screen sizes
Coffee Screen Sizes
Quick answer
A coffee screen number is the diameter of the sizing screen’s perforation in 64ths of an inch. Screen 18 is an 18/64 inch hole, about 7.14 mm. Multiply the screen number by 0.396875 to get millimetres. "Screen 15+" means the parcel is retained on a screen 15 sieve, subject to a contractually agreed percentage allowed to pass.
What a screen number measures
A sizing screen is a metal plate perforated with holes of a uniform diameter. The screen number is that diameter expressed in 64ths of an inch. Screen 18 has 18/64 inch holes; screen 15 has 15/64 inch holes. Coffee is passed over a stack of screens, and each screen retains the beans too wide to fall through it.
The conversion
- Inches
- screen number ÷ 64
- Millimetres
- screen number × 0.396875
- Worked example
- Screen 18 → 18 ÷ 64 = 0.28125 in → × 25.4 = 7.14 mm
- 124.76 mm
- 135.16 mm
- 145.56 mm
- 155.95 mm
- 166.35 mm
- 176.75 mm
- 187.14 mm
- 197.54 mm
Perforation diameters drawn to relative scale. Screen number ÷ 64 = diameter in inches; × 0.396875 = diameter in millimetres.
| Screen | Inches | Millimetres | Common grades |
|---|---|---|---|
| 20 | 20/64" | 7.94 | Kenya E, Tanzania E |
| 19 | 19/64" | 7.54 | Upper bound of AA separations |
| 18 | 18/64" | 7.14 | Kenya AA, Tanzania AA, Uganda Robusta Screen 18 |
| 17 | 17/64" | 6.75 | Kenya AA (17/18), Tanzania A |
| 16 | 16/64" | 6.35 | Kenya AB (15/16), Tanzania B |
| 15 | 15/64" | 5.95 | Uganda Robusta Screen 15, Rwanda Grade 1, Burundi Grade A |
| 14 | 14/64" | 5.56 | Kenya C, Tanzania C |
| 13 | 13/64" | 5.16 | Peaberry separations, small Robusta |
| 12 | 12/64" | 4.76 | Uganda Robusta Screen 12 |
How sizing is done
At the dry mill, hulled and pre-cleaned coffee passes over a vibrating or rotating stack of screens arranged from largest hole to smallest. The coffee that stays on top of the screen 18 plate is the screen 18 fraction; what falls through moves to the next plate, and so on. It is a purely mechanical separation on bean width.
Two consequences follow. First, sizing says nothing about density, defects or cup — those are separate steps, done on a gravity table, by hand and on the cupping bench respectively. Second, sizing is imperfect: beans present themselves to the holes at varying orientations, and a proportion of nominally undersized beans will always be retained.
Tolerances and how to write them
A screen specification without a tolerance is incomplete
Screen tolerances are contractual, not universal. The permitted percentage passing the nominated screen is negotiated per parcel and varies by origin, grade and mill. Write the figure into the contract.
- "Screen 15+" is a retention statement: the parcel is retained on a screen 15 sieve. It does not mean every bean measures 5.95 mm or more.
- Contracts normally allow a stated percentage passing the nominated screen. Agree the number, and agree it per parcel.
- Screening is performed on a sample drawn to an agreed method. Two labs screening different draws from the same lot will not report identical figures — build that into the tolerance.
- Handling and transport cause a small amount of chipping, so a re-screen at destination will typically read slightly softer than the origin figure.
- Peaberry runs on slotted screens, not round ones. A PB 15 is not comparable to a flat-bean screen 15.
Screen size and quality: the real relationship
What bean size does and does not tell you
Bean size is weakly associated with growing conditions — well-nourished trees at suitable altitude tend to produce larger seed — but it is not a measure of cup quality. A large bean from poorly fermented cherry is a large bean from poorly fermented cherry. Screen belongs in a specification as a physical deliverable, next to a cup approval, not instead of one.
Where screen size genuinely matters to a buyer is in the roaster. A uniform size distribution roasts more evenly, because heat penetrates similarly across the batch. Roasters running tight single-origin profiles often value a narrow screen band for that reason alone, independent of any quality claim. Blenders matching a visual appearance across components have a similar, purely practical, interest.
And tightening a screen has a cost. Asking for screen 16+ from a lot prepared for screen 15+ removes volume from the parcel and changes the price basis. That trade-off should be explicit at the quoting stage.
Screen conventions by origin
| Origin | Convention |
|---|---|
| Kenya | Screens map onto letter grades: AA at 17/18, AB at 15/16, C at 14 and below. |
| Tanzania | Same logic as Kenya: AA at 18+, A at 17, B at 16, C at 14–15. |
| Uganda (Robusta) | The screen number *is* the grade — Screen 18, 17, 15, 14, 12 — plus unsized FAQ. |
| Ethiopia | Grade is defect and cup based; screen is quoted separately when a buyer needs it. |
| Rwanda & Burundi | Grade plus an explicit screen request, commonly 15+ or 16+. |
Need a specific screen band?
Tell us the screen, the tolerance and the volume. We will confirm what the crop and the mill preparation realistically support before quoting.
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Screen size explorer
A screen number is the diameter of the sizing perforation in 64ths of an inch. Select one to see what that means in millimetres and in practice.
Perforation diameters, drawn to true relative scale.
7.14mm
- Screen
- 18
- Fraction
- 18/64"
- Calculation
- 18 ÷ 64 in · × 25.4
- Grades
- Kenya AA, Tanzania AA, Uganda Robusta Screen 18, Bugisu AA
The reference large-bean screen across East Africa. Defines AA in Kenya and Tanzania and the largest routinely traded Ugandan Robusta parcel.
Screen 18 in detail →A screen specification is a retention statement, not a guarantee that every bean measures the stated width. Contracts allow a percentage passing the nominated screen, negotiated per parcel — and outside Ethiopia’s classification, screen size does not predict cup quality.
Frequently asked questions
What does screen 18 mean in coffee?
How do I convert a coffee screen size to millimetres?
Does a bigger screen size mean better coffee?
Why does my destination screen analysis differ from the origin figure?
Tell us the coffee you need
Lots can be specified by origin, region, process, grade, screen, moisture, defect tolerance, crop year and packaging. Send what you know and we will confirm what each origin realistically supports.
Keep reading
Related guides
- Screen 18 CoffeeScreen 18 coffee explained — 18/64 inch, approximately 7.14 mm. Which East African grades use it, what it costs in outturn, and how to write a screen 18 specification.
- Screen 17 CoffeeScreen 17 coffee — 17/64 inch, about 6.75 mm. Why it is usually contracted as part of a 17/18 AA band rather than alone, and where it is a grade in its own right.
- Screen 16 CoffeeScreen 16 coffee — 16/64 inch, exactly 6.35 mm. Where it sits in the Kenyan and Tanzanian AB block, and why 16+ is the usual tightening request.
- Screen 15 CoffeeScreen 15 coffee — 15/64 inch, about 5.95 mm. The volume workhorse of Ugandan Robusta and the practical screen floor for most East African specialty preparations.
- Coffee Bean SizeWhat determines green coffee bean size, how size distribution is measured, why uniformity matters more than maximum size, and where size does and does not affect quality.
- Green Coffee Grades ExplainedWhat coffee grades actually encode — bean size, density, defect count and preparation — and how the systems used across six East African origins differ.