Coffee Screen Sizes

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.

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

• "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 by screen

Four screen numbers carry most East African grade names. Each behaves differently commercially, which is why a buyer asking for one of them should know what it costs and what it leaves out.

Screen 18

Screen 18 is the size most often requested by name and the one with the largest price consequence. 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 producing it. In a small-bean season — drought during cherry development shifts the whole distribution down — it may not be fillable at the volume required. The differential over the same coffee at 15+ reflects scarcity of the size class, not a quality difference.

1. 1Roast uniformity on a tight profile — large, evenly sized beans absorb heat predictably.
2. 2Visual match in a blend — where components must look consistent in the bag or hopper.
3. 3A contractual requirement — some markets specify AA or screen 18 and the requirement is simply given.
4. 4Not a reason: the belief that a larger bean is a better bean. Where uniformity is the real need, a tight 16/17 band is often cheaper and roasts just as evenly.

Supply pages: Uganda Robusta Screen 18, Kenya AA, Tanzania AA.

Screen 17

Screen 17 (17/64 inch ≈ 6.75 mm) rarely travels alone. The step between consecutive screens is under 0.4 mm, so separating 17 from 18 produces two thin parcels rather than one useful one. Kenya resolves this by writing AA as a 17/18 band, which is why a Kenyan AA screen analysis shows beans across both. Tanzania keeps A (screen 17) nominally distinct from AA, though in practice A is often merged into AB. Ugandan Robusta trades Screen 17 as a grade in its own right.

• As a tight band with 16, a 16/17 specification gives most of the roast-uniformity benefit of an 18 at a lower differential.
• In Ugandan Robusta, 17 sits between the larger 18 and the volume 15 and can be the right compromise on price.
• As an agreed fallback when a screen 18 requirement cannot be filled from the current crop — better settled in the contract than discovered at shipment.

Screen 16

Screen 16 is 16/64 inch — exactly a quarter inch, 6.35 mm. Kenyan and Tanzanian AB combines the A and B fractions, roughly screens 15 and 16, and because AB is normally the largest block in an outturn, a lot of East African Arabica volume sits here. A buyer who wants the top of that distribution without paying an AA differential can ask for a 16+ separation from an AB preparation; it changes the price basis, because it leaves a smaller, lower-value remainder behind.

Rwandan and Burundian washing stations normally prepare to 15+, and 16+ is the standard tightening request there. Expect a reduced outturn and a differential that reflects it; on very small station lots it can take the parcel below a viable size. The cup is unchanged — if the aim is a better cup, tighten the station or the harvest week, not the screen.

Screen 15

Screen 15 (15/64 inch ≈ 5.95 mm) does more commercial work than any other number in the region. Ugandan Robusta Screen 15 is the most traded East African Robusta specification: there is enough of it in a normal crop to support multi-container programmes, it avoids the scarcity differential of 18, and Uganda’s two harvest cycles keep it available for most of the year. It is the usual input for roast-and-ground blends where Robusta brings body, and for much soluble production.

For washed Arabica from Rwanda, Burundi and the Kenyan AB block, 15 is where most preparations stop. That is a milling and marketing convention, not a quality threshold: beans below 15 are smaller, not defective, and a well-cupping small-screen separation from a high-altitude station can be good value if the roast profile suits the size.

Bean size

Bean size is measured by screen, and it is set on the tree. Processing changes what surrounds the seed, not its width. What drives it:

1. 1Variety. Different Arabica selections have characteristically different seed geometry.
2. 2Growing conditions during fill. Water and nutrition while the cherry develops set how much resource the seed gets.
3. 3Crop load. A heavily bearing tree spreads finite resources across more cherries, producing smaller seed; biennial bearing shows in the size distribution.
4. 4Altitude. Slower maturation at altitude tends to produce denser, often somewhat larger seed.
5. 5Stress events. Drought or hail during development compresses the whole season’s distribution downwards.

Every lot has a size distribution rather than a size, and a screen analysis — the percentage retained on each screen in the stack — is far more informative than the headline grade.

Uniformity is what matters in the roaster. Heat enters a bean through its surface; smaller beans have more surface per unit mass and develop faster. In a batch with a wide distribution, pushing the large beans to full development scorches the small ones, and protecting the small ones leaves the large ones underdeveloped. No profile fully resolves that.

Screen size and quality: the real relationship

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