It would be easier to convert 8 1-4 inches in to cm, then add 3.
In fact, here. 8 and a quarter is 21cm, (Which is kinda small, XD) Oh well,
then add 3 sm, so 24 cm. Then just add a 0 to then end, 240 mm, there is your height (or width/depth, whatever you measured) THat translates to around 9 1/2 inches.