How to Read Granulometric Curves of a Coffee Grinder

Granulometric curves might look intimidating at first – full of peaks, percentages, and microns – but they’re one of the most powerful tools for understanding how a grinder performs. In this guide, we’ll break down what these curves mean, why they matter, and how you can interpret them to get the most from your grinder.

We’ll also look at real examples from the Macap Supra 83, comparing results with a 100% Arabica coffee and a 70/30 blend, to see how different beans can affect distribution.

What is a Granulometric Curve?

A granulometric curve is simply a graph of particle size distribution. It shows how ground coffee is spread across different particle sizes.

• X-axis (horizontal): Particle size in micrometers (µm). Smaller numbers = finer particles, larger numbers = coarser particles.
• Y-axis (vertical): Volume percentage (%). This tells you how much of the coffee (by volume) falls into each particle size category.

The shape of the curve gives us insights into consistency, fines content and the overall grind quality.

Key Metrics to Look At

When reading a granulometric report, a few terms always come up:

• Mode (µm): The most common particle size – shown at the peak of the curve.
• D[4,3] (µm): The “volume-weighted mean,” or the average particle size by volume.
• % Below 200 µm (fines): The percentage of ultra-fine particles, which strongly affect flow and extraction.

Comparing the Supra 83 Results

Both tests below were run on the Supra 83, but with different coffees. They highlight how the same grinder can behave slightly differently depending on the beans used.

Supra 83 – 100% Arabica

• Mode: 434 µm
• Mean (D[4,3]): 320 µm
• Fines (<200 µm): 35.3%

This distribution shows a clean, central peak with a modest proportion of fines. It suggests an espresso-friendly grind with balance between body and clarity.

Supra 83 – 70/30 Blend

• Mode: 452 µm
• Mean (D[4,3]): 329 µm
• Fines (<200 µm): 35.4%

Here we see a slightly coarser mode and mean, with almost identical fines. This produces a grind with excellent consistency — a great match for blends where body and flow control are important.

The key point: Both Supra 83 results show strong uniformity, low fines compared to many grinders and a distribution shape that supports efficient, repeatable extraction.

Why It Matters for Coffee

• Narrow peaks mean consistent grind sizes, which lead to stable extractions. • High fines (<200 µm) can create body and crema, but too many may clog or cause over-extraction. • Wide spreads (boulders + fines together) can result in uneven brews.

How to Interpret a Curve at a Glance

1. Look at the peak – is it sharp or broad? Sharper means more consistent.
2. Check the fines percentage – <200 µm tells you how much “dust” you’re dealing with.
3. Compare mode and mean – if they’re close, the grind is evenly distributed. If they’re far apart, there may be too many boulders or fines pulling the average.