A laser diffraction report ends with three numbers, usually in this order: D10, D50, D90. Everything a buyer needs to judge a powder sits in them, and everything that goes wrong in production shows up in them. This page explains what each value means, how to read the spread between them, and which one to check for which problem.
Particle Size Is Always an Equivalent
Real particles are not spheres. A needle, a plate, and a fractured block each have many possible “sizes”, and the number a report prints depends on the measurement principle behind it.
The convention is the equivalent diameter: the diameter of a sphere that behaves like the real particle under the measurement used. Laser diffraction measures how light scatters and reports a volume-based equivalent diameter. Sieving measures which particles pass an aperture. Image analysis counts projected areas. The same powder can carry different D50 values under different methods, and none of them is wrong.
This is why two suppliers arguing over a 3 micron difference in D50 should first confirm they measured the same way. Comparing a sieving result against a diffraction result compares two different definitions.
The Three Percentiles
D10, D50, and D90 are points on the cumulative distribution. The percentage says how much of the powder, by volume, falls below that size.
| Parameter | Definition | What it controls in production |
|---|---|---|
| D10 | 10% of the particles are smaller than this size | The fine end: dust, health handling, early dissolution, burning behavior in energetic materials |
| D50 | 50% smaller, 50% larger: the median | The typical size: the value most specs target, and the number buyers compare first |
| D90 | 90% of the particles are smaller than this size | The coarse end: screen residue, surface defects, sedimentation, packing behavior |
A worked example makes the reading mechanical. Suppose a report gives D10 = 3 µm, D50 = 10 µm, D90 = 12 µm:
- 10% of the powder volume is finer than 3 µm
- 50% of the powder volume is finer than 10 µm, and 50% is coarser
- 90% of the powder is finer than 12 µm, so only 10% is coarser
Some industries quote D95 or D97 instead of D90. The meaning is identical, only the percentile moves. A spec written on D97 is a stricter top-size promise than one written on D90, because it forces the last 3% of coarse particles down as well.
Span: One Number for the Spread
The three percentiles also describe how uniform the powder is. The common expression is the span:
span = (D90 − D10) / D50
For the example above, span = (12 − 3) / 10 = 0.9. A narrower span means a more uniform powder. Two powders with the same D50 can behave completely differently if one has a span of 0.9 and the other 2.5: the wider powder packs differently, dissolves unevenly, and carries a coarse tail the narrower one does not have.
When a customer complains about inconsistency between batches, the first thing to compare is the span, not the D50. A drifting D50 with a stable span is a different problem from a stable D50 with a drifting tail.
Why Production Cares About the Ends, Not the Middle
The D50 gets the attention in negotiations, but production problems concentrate at the two ends.
- Coarse-tail complaints trace to D90 and D97. Specks in a coating film, scratches on a paper wire, sedimentation in a suspension: the cause is the small fraction above spec, which the D50 never shows.
- Fines complaints trace to D10. Dust in handling, dosage drift in pharmaceutical blends, and reactivity that runs ahead of schedule all come from the fine fraction.
- The D50 sits between them as the volume anchor. It sets throughput calculations and dosing, and it is the number the mill is tuned to deliver.
Industries differ in which end they police hardest. Pharmaceutical actives watch the fine fraction closely because dissolution and side-effect profiles follow it; the pharmaceutical milling application covers that window. Abrasives and pigments watch the coarse end, because a single oversize particle ruins a surface.
How Grinding Holds the Distribution
A grinding circuit controls the distribution at two points. The energy input sets how much fine material the mill creates, and the classifier sets where the top size cuts off. Move the classifier wheel speed and the D90 moves; change the mill speed or feed rate and the D10 and D50 follow.
This is also why two mills with the same D50 claim can deliver different powders. A circuit that holds a tight D90 needs an integrated classifier and the right mill type for the material; the comparison between impact and jet milling depends on hardness, abrasiveness, and how strict the top-size promise is.
For a worked example of a spec that lives or dies on the distribution, see the aluminum hydroxide case, where fill behavior in artificial stone follows the spread as much as the median.
Check the Report in This Order
When a powder report lands, read it in four steps: confirm the method matches the previous report, check D50 against the target, check D90 against the top-size promise, then compute the span and compare it with the last batch. Four numbers and one subtraction catch most of what goes wrong between suppliers.
If you are setting a spec for a new material, or a current powder keeps failing on a percentile the datasheet never mentioned, send us the report and the target. We will tell you which end of the distribution the equipment has to hold, and what that requires from the mill and classifier.
Epic Powder
Epic Powder designs grinding and classifying systems for minerals, chemicals, food, and pharmaceuticals. With more than 20 years of powder processing experience, we match the right mill to your target particle size and capacity. Tell us about your material and we will recommend a tested solution.
Other Information
- Feed Moisture in Grinding: Why a Mill Loses Capacity in Humid Weather
- Grinding Aluminum Hydroxide for Artificial Stone and Solid Surface: Particle Size, Purity, and Equipment Selection
- Turbo Mill vs Pin Mill: Which Is Better for Medium-Fine Grinding?
- How to Choose the Right Grinding Equipment: A Complete Guide by Material Type
- Pin Mill vs Air Jet Mill: Which Is Right for Your Material?
“Thanks for reading. I hope my article helps. Please leave a comment down below. You may also contact EPIC Powder online customer representative Molly for any further inquiries.”
— Jason Wang, engineer