Why operating system shows less disk space than what I have?

Number of times I have been asked this question as why my Operation System shows less disk space than what I have? Few years back when the disk sizes were limited to only few hundred gigabytes, this wasn’t as big of concern as the lost disk space was few gigabytes. But nowadays with disk size all the way up to 2 TB and 3TB on horizon, the lost disk space can be few hundred Gigabytes. And to make it worst, if you are building disk array (RAID or JBOD), this loss could be even more than whole 1TB.

There are few explanations for lost disk space and are explained below:

Actual Disk Size and Marketing Disk Size

The way human thinks of the size and computer thinks is totally different. Computer use the power of 2 whereas humans think in terms of power of 10. Below chart shows what the size is interpreted in both the ways:

Size Human Readable Computer
Kilo- 1000 (103) 1024 (210)
Mega- 1,000,000 (106) 1,048,576 (220)
Giga- 1,000,000,000 (109) 1,073,741,824 (230)
Tera- 1,000,000,000,000 (1012) 1,099,511,627,776 (240)

Looking at the above table, a human readable size of 500GB disk drive would be 500,000,000,000 where as for computer  it would be 536,870,912,000. Now human readable size of 500,000,000,000 bytes will actually be around 465.66 GigaBytes. Now, disk drive manufacturers use the human readable sizes when manufacturing the disk drive but market them using computer readable disk size capacity (GigaByte, TeraByte etc).

So if you purchase a disk drive of 1TB, you expect the total size to be 1,099,511,627,776 and your computer would report the disk size 1TB if there really are 1,099,511,627,776 bytes on disk drive. But now as manufacturer used human readable Tera size, the actual number of bytes on disk drive are around 1,000,000,000,000 (It won’t be exactly the power of 10 considering the complexity of cylinders sizes and such) which when read by your computers comes out to be 931.32GB. Ooops… you lost 92.68GB but I am sure you as you are human, you are also doing calculation in terms of human readable sizes

1000 – 931.32 = 68.68

instead of

1024 – 931.32 – 92.68

On my computer, a 2TB Western Digital hard disk drive (Model WD2003FYYS) shows the disk size to be 1863GB (1907730MB). Wow… whole 185GB.. thats 10 times more than the disk size of my first laptop.

A simple rule which you can use to calculate expected disk space is that you will get around 93% of the marketed disk size.

Disk Space Reserved by Operating System

So to add more to physically lost disk space, you will end up loosing some more (on system disk drive) once you install the Operating System. Based on what operating system you are using, this “lost disk space” can be used for different reasons by Operating System. Below is the list of few areas where this disk space can be used…

  • system restore file
  • hidden files
  • swap file
  • hibernation file if the hibernation is enabled
  • recycle bin
  • recovery partition if you purchased computer with pre-installed OS
  • shadow copy storage

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