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Just had to one-up us all, didn't ya?

:p

Far t many variables to consider for there to be any simple formula.

I love it! :rock:

As far as how a manufacturer determines what grit to call a product, its approximation. They'll either make their own abrasive particles or buy in bulk, adjust binder strength,...

No, it might as well be thought of as arbitrary.
1k grit shapton isn't the same as a 1k norton

1k grit sandpaper in the us isn't the same as 1k sandpaper in uk

Edit: there's no hard and fast...

It has to do with sedimentation rate, or at least that's how the finer grit particles are separated. Here's a table with the particle diameters of common synthetics, along with some natural stones,...