I have the sizes for the original zak slicks.

they are listed as

fronts 10.0/225x16
rears 12.5/27.5x19

I just need to work out the overall diameter of the tyre.

at first I thought the 27.5 number meant 27.5" total diameter but in that case wtf does the 225 number mean for the fronts ? if its section height in mm its 9" lol