I saw this post at knifeforums and thought some of you could use this information. The poster (yuzuha) is a well regarded sharpener. She frequently posts detailed information about various stones and their characteristics.
"According to the table in Norton's catalog their waterstones correspond to the following micron sizes:
220 grit = 65 microns
1,000 grit = 14 microns (this is 800 grit Japanese industrial standard)
4,000 grit = 6 microns (about 2,300 grit JIS)
8,000 grit = 3 microns (4,000 grit JIS)
For the grit sizes of imported Japanese stones look at the first two columns here: http://members.cox.net/~yuzuha/jisgrits.html
(this is the more recent Japanese industrial standard that uses electrical resistance to sort grit... the older standard used a sedimentation method and the numbers were slightly coarser for a given grit number... i.e. the old grit number 3,000 was 5.7 microns and the closest in the new grit table is 5.5 which is now 2,500 grit).
So, yes... Norton stones are considerably coarser than a Japanese stone with the same grit number. And, with more than one measurement method, two Japanese stones of the same grit might be different too but they should still be pretty close to each other.
Bear in mind that the cutting speed and surface finish produced are not only a function of the micron size of the abrasive grains but are also influenced by the binding material and how densely the grains are bonded together. I.e. two stones can use 5 micron abrasives... a porous soft bond will cut faster, a dense, tight bond will leave a finer finish. Then there are some stones with a slightly flexible resin binder that try to get the best of both worlds. Others, like the natural aoto/blue stone have grit that will break fairly easily... i.e. the stone itself is about 1,000 grit, but the grains come off the stone and form a sharpening paste... the grains in the paste then break between the stone and blade and become smaller, so you can get a 3,000 grit finish off a blue stone if you take advantage of the slurry."