This script automatically converts dimensions in inches to cm on the site thegamecrafter.com
No guarantees are made that the conversions are 100% correct. I'm pretty sure they are, I'm using this script myself too, but use at your own risk.
The original dimensions in inches will always be shown along with the converted sizes. If it's a matter of millimeters to make everything fit, be sure to use the original inch values and convert them yourself to prevent discrepancies caused by rounding the numbers.