Screen sizes

Screen 18 Coffee

Screen 18 is the size specification most often requested by name, and the one with the largest price consequence. This page covers what it delivers, what it costs in yield, and when specifying it is genuinely worth the differential.

Last reviewed July 20263 min read

Quick answer

Screen 18 is an 18/64 inch perforation, approximately 7.14 mm. It is the reference large-bean screen across East Africa: it defines AA in Kenya and Tanzania and identifies the largest routinely traded Ugandan Robusta parcel. It is a size class only and carries no cup guarantee.

The measurement

Screen number: 18
Perforation: 18/64 inch = 0.28125 inch
Millimetres: 7.14 mm
Screen shape: Round hole (flat beans). Peaberry uses slotted screens.

A parcel described as "screen 18" or "screen 18 up" is retained on a screen with 18/64 inch holes. In practice most contracts are written as "18+" with a stated percentage permitted to pass — see coffee screen sizes for the tolerance conventions.

Where screen 18 appears in East African grades

Grades built on screen 18
Origin	Grade	Notes
Kenya	AA	Usually specified as 17/18, so screen 18 is the upper half of the AA block.
Tanzania	AA	Screen 18 and above; the headline Tanzanian export grade.
Uganda	Robusta Screen 18	The screen number is the grade. The largest routinely traded Ugandan Robusta.
Uganda	Bugisu AA	Washed Mount Elgon Arabica at the AA size block.
Rwanda / Burundi	Screen 18 requests	Uncommon — 15+ and 16+ are the normal bands. Screen 18 volumes are small.

What screen 18 costs in yield

This is the part most often left out of the conversation. Screen 18 is the top of the size distribution, which means it is a minority of any outturn. Pulling a screen 18 parcel takes volume out of the AB or screen 15 block that would otherwise have absorbed it, and the smaller the resulting parcel, the higher the per-kilo cost of having produced it.

A screen 18 request from an origin whose crop ran small that season may simply not be fillable at the volume required.
Season matters: drought during cherry development shifts the whole size distribution downward.
The differential over the same coffee at screen 15+ reflects scarcity of the size class, not a quality difference.
Where uniformity rather than maximum size is the real requirement, a tight 16/17 band is often cheaper and roasts just as evenly.

When specifying screen 18 is worth it

There are three defensible reasons to pay for screen 18, and one common non-reason.

1Roast uniformity on a tight profile. Large, evenly sized beans absorb heat predictably, which matters when the roast window is narrow.
2Visual match in a blend. Where components must look consistent in the bag or the hopper, size uniformity is a real specification.
3Customer expectation. Some markets and private-label programmes specify AA or screen 18 contractually, and the requirement is simply given.
4Not a reason: the belief that a larger bean is a better bean. It is not — see the coffee bean size discussion.

Screen 18 is not a quality claim

A screen 18 parcel from carelessly fermented cherry will cup worse than a screen 15 parcel from well-managed cherry at the same origin. The screen tells you the width of the bean. Approve the cup separately.

Writing the specification

A complete screen 18 clause

Screen: Retained on screen 18, maximum X% passing
Sample basis: Screen analysis on an agreed sample draw and sample size
Defects: Maximum full defects per 300 g, stated separately
Moisture: Target range plus measurement method
Species and process: Arabica or Robusta; washed or natural
Cup: Approved pre-shipment sample

Screen 18 enquiries

Tell us the origin, volume and shipment window. We will confirm whether the current crop supports the size class at that quantity before quoting.

Request Current Crop Offer

Frequently asked questions

How many millimetres is screen 18 coffee?

Approximately 7.14 mm. Screen 18 means an 18/64 inch perforation; 18 × 0.396875 = 7.14 mm.

Is screen 18 the same as AA?

Closely related but not identical. Tanzanian AA is screen 18 and above. Kenyan AA is normally specified as 17/18, so screen 18 is the larger half of it. Ugandan Robusta Screen 18 is its own grade with no letter. Always read the screen alongside the origin’s grade ladder.

Can any lot be prepared to screen 18?

Only to the extent the crop contains beans that size. Screen 18 is the top of the distribution, so the available volume depends on the season, the region and the variety. In a small-bean year the size class may not be fillable at the quantity required, at any price.

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.