The application will bring up the System Administrator panel and upon authentication as Administrator will spawn a background process that runs as sudo to measure all files on a given Volume/Folder regardless of permissions. This is useful when measuring an entire Volume. The background process will quit when the measurements are completed or the main application quits.

We measure the Physical Size and Logical Size of the files/folders and display the Physical Size. This is actually a proper way to show how much disk space a file is taking. This is the same value you see when you perform the Get Info on a File/Folder in Finder.

This is the size of the file. As an example, a straight text file with one character will be 1B, one byte in Logical Size.

This is the size of that the file takes in the file system. Assuming the file system Block Size is 4KB a straight text file with one character will be 4KB, 4096 bytes in Physical Size.

This is the smallest chunk of information the operating system will use to store data. The default is usually 4KB, but this value can vary between 4KB to 256KB. As an example, if you create a plain text file and have one simple character in it, the operating system will still use one block to store this file. So this file’s Physical Size will be 4KB and the Logical Size will be 1B, one byte.

One Bit is just one tiny piece of information, pretty much useless.
One Byte is 8 Bits.
One KB (Kilo Byte) is 1024 Bytes.
One MB (Mega Byte) is 1024*1024 = 1048576 Bytes. (think 600MB for a CD Audio)
One GB (Giga Byte) is 1024*1024*1024 = 1073741824 Bytes. (think 4.5GB for one DVD, or 25GB for a Blue Ray disk)
One TB (Tera Byte) is 1024*1024*1024*1024 = 1099511627776 Bytes. (1TB is the biggest single hard drive currently)
One PB (Peta Byte) is 1024*1024*1024*1024*1024 = 1125899906842624 Bytes. (think huge, a milion DVDs or Large Hadron Collider)

So you license a copy of the app and go and measure your brand new empty Volume. The numbers don’t seem to match. You just bought and installed a 150GB Western Digital Raptor, Once formated the Volume will show only as 139GB or less available. Even though this Volume is empty, the Operating System and the File System took a nice 11GB for their data structures. This is normal. Actually the bigger the drive the more the File System will take.