[PatG]

2 lbr = 40mm
3 lbr = 47mm (Tank gun only)
6 lbr = 57mm
13 lbr = 76mm (pre WWI piece some used as AA early WWII)
17 lbr = 76.2mm
18 lbr = 84mm (Field gun - some converted to 25lbr)
20 lbr = 84 0r 83.4mm (post WWII)
25 lbr = 87.5mm

There were others used in WWI but well out of service by WWII.

Keep in mind that other factors such as barrel length, type of round and charge used need to be considered when assessing the relative power of guns. While of equal calibre, the 25 lbr was much shorter than the German 88 and performance was much less as well. Similarily, the 17 lbr was much longer (and I believe had a heavier charge) than its Russian brothers so had considerably better performance.

Note that while often looked down on, for its time the 2 lbr was a kick *** AT gun.