print sizes from inch to cm

IF you click the custom layout in the print panel you can switch between inches and cm.

otherwise iPhoto should use the system default set set in System Preferences > Language and Region.

you are asking about printing photos yourself, right? You can no longer have any iPhoto Print products printed by Apple.

If you want to print from iPhone or iPad using:

different sizes of photos

or different sizes of paper

or multiple photos of differnet sizes on one page

or change from inches to cm

use an app called Print to Size.

I have just found it and it is simple and brilliant.