Coffee Grinder Burr Geometry: Conical vs Flat — Why It Matters

Summary

A burr grinder crushes coffee between two hard, ridged surfaces called burrs. The two main burr shapes are conical, a cone turning inside a ring, and flat, two parallel rings facing each other. Measurements with laser particle analysers show that every burr grinder produces a range of particle sizes, with a main peak and a smaller peak of very fine particles called fines. Tests disagree on whether one burr shape always produces fewer fines, so the choice is less clear-cut than often claimed.

How Burr Grinders Work

Coffee Review, which tested several home grinders, describes conical and flat as the two main burr shapes. The speed at which the burrs turn is set by the manufacturer, depending on burr size and geometry and the torque of the motor. According to Coffee Review, conical burrs are cheaper to manufacture and pull beans into their teeth efficiently, while flat burrs have a greater potential to produce a consistent particle size.

Looking down into a coffee grinder: a conical inner burr inside a toothed outer ring, held by a wire clip

View of a conical burr grinder used for coffee preparation. Photo: Fletcher / Wikimedia Commons, CC BY 4.0.

What Measurements Show

The particle sizes in ground coffee are measured today mainly by laser diffraction, which has largely replaced sieving. A 2022 study published in the Philippines, using a laser diffraction analyser, found that the particle size distribution of coffee grounds was always bimodal: a large main peak and a small peak of fines. It found that flat burrs produced fewer fines than conical burrs.

An independent test reported on the Home-Barista forum, carried out with a laser diffraction analyser at the South Dakota School of Mines and Technology, compared two conical grinders and one flat grinder. Each showed a main peak at about 400 to 500 micrometres at the setting used, and a fines peak at about 40 micrometres. In that test the flat grinder produced the fewest fines.

A test at a grinder maker's laboratory, reported by Barista Hustle, compared several grinders for espresso. In that test the grinder with the fewest fines was a large conical model. So the general rule that flat burrs always give more even grounds is not supported by every measurement.

Barista Hustle also notes that a bimodal grind was long considered desirable for espresso, on the theory that fines slow the flow of water through the coffee; its findings question that theory.

Watch: ESPRESSO ANATOMY - Flat Vs Conical Burrs — The Real Sprometheus

Burr Wear

Burrs wear with use. A study from Swansea University, which measured ground coffee by both precision sieving and laser diffraction, found that wear and damage to the burrs reduce the precision of the particle size distribution, and that monitoring the distribution can show when burrs need replacing.

Flavour

It is often said that conical burrs give coffee more body and flat burrs give more clarity. This encyclopedia found no controlled tasting study that establishes this as a rule.

Key Facts

Two main burr shapes: conical and flat
Measurement method: mainly laser diffraction
Typical result: a bimodal distribution, a main peak plus a peak of fines
Flat versus conical: two tests found fewer fines with flat burrs; another found a conical grinder had the fewest
Burr wear: reduces the precision of the grind

Frequently Asked Questions

Are flat burrs always more consistent than conical burrs? Not always. Two tests found fewer fines with flat burrs, but a laboratory test reported by Barista Hustle found a conical grinder produced the fewest fines.

What are fines? Very small particles, often around 40 micrometres, that form a second peak in the particle size distribution of ground coffee.

Do burrs wear out? Yes. A Swansea University study found that worn or damaged burrs make the grind less precise.

Sources

Coffee Review, test of four home burr grinders with laser diffraction analysis. Supports: the two main burr shapes; burr speed set by size, geometry and motor torque; conical burrs cheaper to manufacture; flat burrs' potential for consistency.
Malvern Panalytical, application note "Measuring the particle size distribution of coffee grounds using laser diffraction". Supports: laser diffraction largely replacing sieving.
"Effect of Embryo Surface Morphology and Grinder Type on the Particle Size Distribution of Coffee", Philippine e-journals, vol. 8, no. 1 (2022). Supports: the always-bimodal distribution; fewer fines from flat burrs.
Home-Barista, "Titan Grinder Project: Particle size distributions of ground coffee". Supports: the test of two conical and one flat grinder; the 400–500 and 40 micrometre peaks; fewer fines from the flat grinder.
Barista Hustle, report on a grinder test at the Mahlkönig laboratory. Supports: a conical grinder with the fewest fines in that test; the theory that a bimodal grind helps espresso flow.
Griffiths, C., and others, "Wear Monitoring of Coffee Grind-On-Demand Burrs Using Precision Sieving and Laser Diffraction", Swansea University (2021). Supports: burr wear reducing the precision of the grind.

Claims this encyclopedia could not source

The following statements appear in published material about grinder burrs. This encyclopedia could not trace them to a reliable source:

That conical burrs turn at 300–500 rpm and flat burrs at 700–1,500 rpm.
That flat burrs always produce a unimodal distribution and conical burrs always a bimodal one.
That one burr shape always generates more heat or retains more coffee than the other.
That conical burrs always give more body and flat burrs more clarity.
That burr geometry is the single most consequential choice in coffee equipment.
That Puerto Rican beans are especially dense or taste of stone fruit, and so need a particular burr.
Recommendations of particular grinder models and their prices.

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