Go to the System → Localization → Length classes
Units of measure to use in product options and shipping by product dimensions OR add new lengths, for example, if you sell your products using meters yards.
Only one Base Length Class may have value 1 (1.000000)
Others classes should be set with respect to Base class. So when you add dimensions in any units to your product it will be re-calculated
Default length classes cannot be removed from the system. You can add your own classes specific to your country or business