Sandpaper Grit, Mesh and Microns: What the Numbers Actually Mean

P400 and 400 grit are not the same grade. How FEPA, CAMI and mesh counts relate to microns, and why mixing the two abrasive standards ruins a finish.

Introduction

Abrasives and sieves are graded three different ways, and the numbers look interchangeable without being so. A sheet marked P400 and one marked 400 are not the same grade. A 200 mesh sieve and a 200 grit paper have nothing to do with each other. And the micron, the one unit that actually measures something, barely appears on the packaging.

The Micron Is the Micrometre

Start with the unit the other systems are trying to approximate. A micron is a micrometre: one millionth of a metre, one thousandth of a millimetre. The name was officially retired decades ago but survives throughout abrasives, filtration and machining, where it means exactly the same thing. Our guide to nanometres and micrometres covers that scale in more detail.

In imperial terms, 1 thou (0.001 inch) is 25.4 microns, so a micron is about 0.04 thou. A 100 micron particle is roughly four thousandths of an inch.

Two Grit Standards, One Number

This is where the money gets lost. Coated abrasives are graded under two systems:

FEPA, the European standard, writes grades with a P prefix — P120, P400
CAMI, the American standard, writes them bare — 120, 400

Below about 180 the two track each other closely enough to treat as equivalent. Above that they diverge, and in a direction that surprises people: at the same number, the FEPA grade is coarser.

P220 and 220 — both around 68 microns, effectively the same
P320 is about 46 microns; CAMI 320 is about 36
P400 is about 35 microns; CAMI 400 is about 23
P600 is about 26 microns; CAMI 600 is about 16
P1000 is about 18 microns; CAMI 1000 is about 9

So a P400 sheet cuts roughly like a CAMI 320, and a CAMI 400 is nearer a P600. Someone following an American finishing schedule with European paper is working a grade or two coarser than intended at every step, which is exactly why the final finish looks scratched.

These figures are nominal average particle sizes rather than guarantees. Both standards specify a distribution, and published micron figures vary slightly between sources, so treat them as indicating the relationship rather than defining it.

Mesh Is Not Grit

Mesh count measures something else entirely: the number of openings per linear inch of a sieve. It describes a screen, not an abrasive, and it reaches abrasives only because powders are graded by the screen that passes them.

Higher mesh means finer, as with grit, but the relationship to microns is geometric rather than arbitrary:

microns ≈ 15,000 ÷ mesh

Checked against the published ASTM sieve openings, that approximation holds to within about 6 percent from 45 mesh through 400:

60 mesh = 250 microns (the rule gives 250)
100 mesh = 149 microns (rule: 150)
200 mesh = 74 microns (rule: 75)
325 mesh = 44 microns (rule: 46)
400 mesh = 37 microns (rule: 37.5)

It is an approximation because the opening depends on the wire thickness as well as the count, and manufacturers wind screens differently. For anything where the exact cut-off matters, use the stated opening rather than the mesh number.

Reading a Finishing Schedule

The practical rule in sanding is not to skip more than one grade, because each pass has to remove the scratches left by the one before it. Jumping from 80 to 220 leaves 80-grit scratches that the 220 is too fine to cut out, and they only become visible under finish.

A typical progression for bare timber runs P80, P120, P180, P240, stopping around P180 to P240 for stain, since sanding finer closes the grain and the stain takes unevenly. Automotive paint goes much further, through P800, P1500 and P2000 before polishing, where the aim is to make the scratches small enough that a compound can remove them entirely.

Both of those schedules assume one standard throughout. Mixing P-graded and bare-graded sheets in the same sequence is how an apparently careful progression produces a poor finish.

Quick Reference Conversion Table

Approximate particle size, with the nearest mesh:

P60 / CAMI 60 ≈ 265 microns ≈ 60 mesh
P100 ≈ 162 microns; CAMI 100 ≈ 141 microns ≈ 100 mesh
P180 ≈ 82 microns; CAMI 180 ≈ 78 microns ≈ 180 mesh
P220 / CAMI 220 ≈ 68 microns ≈ 220 mesh
P320 ≈ 46 microns; CAMI 320 ≈ 36 microns
P400 ≈ 35 microns; CAMI 400 ≈ 23 microns
P600 ≈ 26 microns; CAMI 600 ≈ 16 microns
P1000 ≈ 18 microns; CAMI 1000 ≈ 9 microns
1 thou = 25.4 microns; 1 micron = 0.0394 thou

Common Mistakes Worth Avoiding

Treating P400 and 400 as the same grade

Above about 220 they are not. P400 cuts closer to a CAMI 320, and the gap widens as the grades get finer.

Reading mesh as a grit number

Mesh counts openings per inch in a screen. It happens to run the same direction as grit but describes a different thing.

Skipping more than one grade

Each grade exists to remove the previous one's scratches. Skipping leaves marks that only appear once finish goes on.

Expecting a micron figure to be exact

Grit standards specify a distribution of particle sizes, and mesh depends on wire thickness. Both micron equivalents are nominal.

Conclusion

A micron is a micrometre and the only one of the three that measures anything directly. Mesh is roughly 15,000 divided by the count. And the grade on a sheet of abrasive only means something once you know which standard printed it: above 220, a P number is coarser than the same bare number, which is the difference between a finish that polishes out and one that does not.

Try our micrometers to millimeters converter for quick and accurate conversions.