another_jim wrote:16 inches.
It would be close to impossible to use under an 18 inch cabinet even if you spoon the beans in; the clearances are too tight. There is no problem using it under a 24 inch cabinet either spooning, or replacing the monster hopper with a DIY job or the shorter hoppers from the K2 or K3.
Regarding the shorter hoppers intended for other Compak grinders, I cannot recommend their purchase for use on the Compak K10 WBC.
When I first became the owner of a K10 WBC, I fabricated a couple of "hoppers" from miscellaneous plastic plumbing parts from the Home Depot. I should have stopped there.
Later, when I saw a picture posted on the forums of a K10 WBC with one of the smaller hoppers intended for smaller Compak grinders, I contacted the new USA offices of Compak USA, and was kindly sold one of these smaller hoppers, which I installed in my grinder. Shortly thereafter I noticed a rather large crack that started out on the bottom of the hopper and then extended upwards. I attributed this to the adjustment screw/bolt which secures the hopper in place, which I had never adjusted at all, and which I thought was too tight. Subsequently, I backed this screw off completely so that it no longer contacted the hopper, and I contacted Compak USA about this problem.
The kind folks at Compak USA sent me another identical brand new micro-hopper, which I installed. The set screw has never been tightened down, however this second hopper has now cracked in an identical fashion to the first one that I received, for reasons I cannot explain. It reminds me, a little, of the problems that people had with an ancient Inova conical grinder that used to be recommended eons ago on alt.coffee, and that grinder had the unfortunate behavior of eating its plastic hoppers. I am not aware of any problems with the original large hoppers that normally ship with the WBC K10 grinder.
My suggestion would be to either live with the gargantuan hopper that ships with this grinder, or to invest in $5 or $6 worth of Home Depot parts plus some epoxy to glue those parts together to make your own micro hopper, which will probably live a lot longer than the smaller Compak hoppers will in the WBC K10, and if damaged will not cost hardly anything to replace.
ken