Ex-data centre bod here....from my experience, in the DC's I've worked in (in the UK and Europe), the racks used were most often 600mm(w) x 800mm(d) 42U, or 800

Ex-data centre bod here....from my experience, in the DC's I've worked in (in the UK and Europe), the racks used were most often 600mm(w) x 800mm(d) 42U, or 800x800 racks if the rack was densely cabled (i.e. there being a gap on either side of the 19" frame to bring in cable from the sides). These would then sit on 600x600 raised floor tiles, which was a convenient thing, geometry-wise. But that was back in the late 90's/early 2000's, so things might have changed.