In Mac OS 10.6 disk and file sizes are measured in base 10 (one kilobyte is defined as 1000 bytes, one megabyte is defined as 1000 kilobytes, etc.) in Finder and Disk Utility. Unfortunately this has created a lot of inconsistency even when working with Apple’s own programs since most of them still define file sizes in binary units (one kilobyte is defined as 1024 bytes, one megabyte is defined as 1024 kilobytes, etc.). This utility will switch the measurement of disk and file sizes in both Finder and Disk Utility back to binary units (or if the measurement has already been switched to binary units it will restore the system back to using base 10 units).
For Mac OS X Lion and later: switchDiskSizeBase v2.0.1
For Mac OS X Snow Leopard: switchDiskSizeBase build 11