A long while back when this confusion first arose I sent an email query to 3M who invented Trizact and manufactures it. This is their answer:
To extrapolate the micron to grit conversion from 3M:
A35 ~ 35 micron ~ 360 grit,
A10 ~ 10 micron ~ 1100 grit
A5 ~ 5 micron ~ 1600 grit
Trizact and micron(u) grading are two separate things. Trizact products
are graded on their own scale ranging from A6 - A160.
3M Superabrasives are micron graded, that is, measuring the average size
of an individual particle on that product.
Standard abrasive grading is based on the screen mesh used to separate
out the particles for a particle grade. In simplest terms, in a 1"
line of grade 24 product there are ~24 particles. In a 1" line of
grade 600 product there are ~600 particles.
In comparison, 9u=1200 grit, 15u=600, 20u=500, 30u=400, 40u=320
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