Explainer · grinders

Coffee grinders explained: burr vs blade, flat vs conical, fines

A grinder does not make one particle size. It makes a spread of sizes, and the shape of that spread — how wide it is and how much fine dust it contains — changes how fast water flows and how evenly coffee extracts.

Updated 2026-10-18 · sources listed at the end

The grinder types at a glance

The table combines the encyclopedic description of grinding methods with a roaster's guide to burr shapes; the fines percentages are literature ranges explained in the flat-vs-conical section below.

How a burr grinder sets the size

A burr grinder crushes beans between two hard grinding surfaces, one of them turning. The gap between the burrs decides the grind: set them far apart and the grounds come out coarser, move them closer and they come out finer. Because that separation determines particle size, burrs produce a fairly uniform grind, and the setting can be changed and returned to.

Electric burr grinders keep the burrs turning slowly in one of two ways. Lower-cost designs use a small motor with reduction gears; others drive the burrs directly from a larger motor and belt. The encyclopedia notes that the gear-reduction versions are noisier and usually do not last as long.

Why blade grinders are inconsistent

A blade grinder uses small, blender-style blades that chop the beans inconsistently. Reference sources describe the output as particles of widely varying sizes, which makes consistent grinding from batch to batch difficult. Blade grinders also create "coffee dust" that can clog the sieves of espresso machines and French presses. Drip coffee makers tolerate them best.

Heat is a point where the sources disagree. Wikipedia's burr mill article says burr mills heat the grounds less by friction than blade grinders, while its coffee preparation article says a blade grinder does not cause frictional heat buildup unless it grinds very large amounts. Either way, the main documented drawback of blades is the uneven particle size.

Flat vs. conical burrs

Conical burrs take their name from the inner burr, which tapers upward to a point and is turned by the motor's drive shaft inside a ring-shaped outer burr. Flat burrs are two discs, one fixed to the grinder body and the other on a moving carrier. In both, one burr moves to change the spacing while the other spins to grind. Most manual coffee grinders use conical burrs.

The difference that matters in the cup is the share of fines. Counter Culture Coffee's guide puts it as a general rule: conical burrs produce more fines, flat burrs a more even distribution. Slides from the Coffee Excellence Center at ZHAW (Zurich University of Applied Sciences), citing earlier work by Eiermann, Smrke and colleagues, give literature ranges for the fines share at espresso fineness:

The ranges overlap: a particular conical grinder can make fewer fines than a particular flat one. The ZHAW slides describe the fines share as a "signature" of each grinder, and the burr pattern itself also shapes the distribution. Counter Culture links the extra fines of conical burrs to fuller-bodied cups and the evenness of flat burrs to lower astringency; treat that as a roaster's tasting description rather than a measured result.

Particle size distribution and fines

The mix of particle sizes a grinder produces is its particle size distribution. Espresso grounds have a characteristically bimodal distribution, according to the 2024 Scientific Reports study by Smrke, Eiermann and Yeretzian: one large peak of main particles and a second, smaller peak of very fine ones. Counter Culture notes that fines are produced every time coffee fractures, so every grinder makes some.

Definitions differ. The ZHAW study counts particles smaller than 100 µm as fines; Counter Culture's guide uses "less than 200 microns". Compare fines figures only when they use the same cut-off.

What fines do is better established. In the ZHAW study, adding sieved fines to espresso grounds showed that a higher share of fines lowers the permeability of the coffee bed, reduces flow and lengthens extraction time. The group's 2024 slides report that raising the fines share from 10% to 15% went with about 10 s longer extraction, while median particle size was not correlated with extraction time. The slides also note that adding fines did not drastically change extraction dynamics, and that the tasting optimum came with a relatively high fines share. Fines are not simply bad; they are a variable to keep constant.

When finer stops helping

For espresso, a 2020 paper in Matter by Cameron, Hendon, Foster and colleagues found a peak in extraction yield against grind setting: past a certain fineness, yield fell. The authors attribute this to inhomogeneous flow at fine settings — water finds uneven paths through the bed — which causes poor reproducibility and wasted coffee. More on what that means for shot recipes is on the espresso ratio page.

Full-immersion brewing behaves differently. UC Davis researchers Liang, Chan and Ristenpart tested median particle sizes from 579 to 1,311 µm and found that grind size has only a minor effect on final strength and extraction: finer grounds gave slightly higher values, but the differences were negligible next to the brew ratio, and extraction stayed near 21%. Grind mainly controls how quickly an immersion brew approaches equilibrium. For a French press or cupping, then, grind sets speed and sediment; the ratio sets strength.

Bean temperature, static and burr wear

• Bean temperature. Uman and colleagues ground beans at temperatures from room temperature down to −196 °C. Grinding cold gave a narrower distribution and a smaller mean particle size, and the distribution did not depend on the beans' origin or processing. Christopher Hendon, quoted in the University of Bath's release on the study, summarized the aim: "a grind that has the smallest difference between the smallest and largest particle."
• Static. Méndez Harper and Hendon report that the finer the grind, the more static builds up, causing clumping and grounds left in the grinder. Moisture inside the beans predicted charging better than roast color. Adding 20 µL of water per gram before grinding halved the charge, and 10 µL/g cut retention for a dark roast from over 10% to about 2.5%. In their espresso tests, shots took nearly 50% longer and strength rose from 8.2% to 8.7% TDS, which they attribute to a denser, more uniform bed. They caution that too much water may cause caking or corrosion inside the grinder.
• Burr wear. Burr edges dull with use, which reduces grind consistency, and manufacturers may give replacement intervals by pounds of coffee ground. A 2018 Swansea University study found that wear and damage reduce the precision of the particle size distribution, and that tracking the distribution can help predict when burrs need replacing.

Checklist: getting an even grind

• Use a burr grinder for espresso and for metal-filter brewers; blade "coffee dust" clogs their screens.
• Choose a burr grinder with an adjustable gap, so the setting decides the grind size.
• Start from the grind size chart, then change one step at a time.
• If a brew suddenly runs slower at the same setting, suspect fines or clumping before changing the recipe.
• For immersion brews, fix strength with the ratio rather than by grinding finer.
• Replace dull burrs at the manufacturer's recommended interval.

Grinder settings are not universal: "setting 15" on one grinder says nothing about another. Write down your grinder's setting next to dose, water and time, and adjust from your own notes.