Google sketchup = very large prints (17 replies)

I'm using google sketchup to create my 3d items. The problem I'm having though is that all my items come out larger then I drew them in google sketchup. I draw them using inches and then export an stl in mm. I then use slicer and print the object.

I created a floor drain that was supposed to be 6 1/4" wide and it was 6 3/4". The larger the prints the further off the sizes are. Can anyone explain to me why this is happening and how to fix it? Is it a program error or is it maybe a machine calibration error?

Thanks