Sandpaper Grit Converter

Sandpaper grit numbers indicate the size of abrasive particles.

Higher grit = finer abrasive = smoother finish.

There are two grit scales in circulation and they are not the same scale. CAMI is the American one (ANSI B74.18), FEPA is the European one, and a FEPA grade is written with a P in front of the number. They agree closely down to about 220 and then split apart, which is the part that catches people out.

Grit to particle size, both standards side by side:

Look at the 400 row. A CAMI 400 paper cuts with 23.6 micron grain and a P400 cuts with 35 micron grain, so the European sheet is about 48% coarser despite carrying the same number. By 600 the gap is wider still. That is why a finish sanded to “400” on one continent does not match a finish sanded to “400” on the other, and why the micron figure is the only unambiguous way to specify an abrasive. Anything sold in the US as 1000, 1500 or 2000 grit wet-or-dry is almost certainly a P-grade, whatever the packaging implies.

Is there a formula? Not a good one. Both scales were defined by sieve and sedimentation tests rather than by an equation, so a table is the honest answer. Between P180 and P800 the rule of thumb microns = 15,000 / P-number lands within roughly 15%, and it falls apart outside that window.

The grit number is not a count of particles. It comes from the sorting screen: how many openings per linear inch the mesh had that the grain passed through. More openings per inch means a finer mesh, which is why a bigger number means smaller grit.

The practical rule when sanding is not to skip grits. Jumping from 80 straight to 220 leaves scratches the finer paper cannot erase, because each grade is only really capable of removing the marks left by the one before it. Step through the sequence (80, 120, 180, 220) and the surface comes out clean. Patience through the grades beats brute force every time.