why hard drives get less space than label says

 

Overview

You really DO have the space you were expecting, minus a couple of MB for overhead, out of all that TB of space, plus some space for the filesystem itself.

it's not actually as much as you might think (unless you've got a really full disk and you have a lot of little files, then the filesystem ($MFT included) takes up 12%.

in the usual case, however, with a new disk, what you are seeing is a microsoft-ism. it's one of the two ways of looking at sizes of disk space. 1024 bytes for 1K, or 1000 bytes for 1K. microsoft chose 1024 bytes for 1K. but microsoft doesn't say it properly - it's properly said 1KiB=1024 bytes. so you see less disk space when you look at the disk size numbers it reports. at least it's consistent.

standards

IEC units as defined by NIST

SI/IEC units listed together side-by-side with their differing values

calculations

so, for a 750GB drive, you will see 698GB (in windows due to mislabeling) instead. here's how:

1MB=1e6B
1MiB=2^20B
1GB=1e9B
1GiB=2^30B
1GiB = 230B = 10243B = 1,048,576B×1,024B = 1,073,741,824B
1TB=1e12B
1TiB=2^40B

750GB=750e9B
750e9B÷10243 = 698.491930962GiB
750GB=750e9/2^30=698.491GiB (microsoft would show this as 698.491GB)

industry measurement usage

when you buy a hard drive, it is measured in GB. when you buy RAM, it is measured in GiB. Microsoft made a mistake in measuring disk space in MiB and GiB. and they won't quit either for some reason. people confuse IEC and SI units and think they are the same.

  • Hard Disk Industry: properly 1kB=1000 bytes using 512 bytes and (soon) larger sectors, but they say 1KB, 1GB, 1TB. SI units should properly be in lower case. but when used with computers, they are used in upper case for some reason. doesn't mean anything. so I guess it's a defacto standard.
  • Optical Disc Industry: properly 1KiB=1024 bytes using 2KiB sectors, but they say 1KB, 1MB, 1GB. this is really a "computer unit of measurement" or IEC units.
  • RAM Industry: properly 1KiB=1024 bytes, but they say 1KB, 1MB, 1GB. this is a "computer unit of measurement" or IEC units.
  • SSD Industry: typically IEC units, 1GiB=1024^3
  • flash memory/drive industry
  • hard disk sector sizes are in IEC units. the hard disk size itself is in SI units. this has been the cause of much debate and misunderstandings.
  • Microsoft products and Windows like the industry label in SI units when they mean IEC units (confused and improper)."1GB" according to microsoft is incorrectly 1024^3: very wrong, defacto standard bug and much debated.
software
calculators which are really useful
ttcalc, available from sourceforge.net $free
a calculator I use for doing this sort of calculation. for the above calculation, you put in 750e9/1024^3 and it gives you a result as you type (yay!). or 1024^3 or 750e9/2^30. it does hexadecimal, decimal, binary, octal, and can be set to do 99 digits precision. yahoo! and it's small and fast.