Hard Drives and their "true" sizes

A lot of people do not realise that hard drive manufacturers "falsely" rate the size of their drives, well, I say falsely as they quote the drive size when unformatted, not formatted - which the drive has to be for you to use it. One thing though, is that seeing as all of them do it, you can compare the various size drives without fear of unknowingly getting a "raw deal".

Why is this? Well, drive manufacturers quote their drives as having 1000 kilobytes per megabyte when in fact there are 1024 kilobytes in a megabyte (Mb). This means that once you have bought your shiny new 60Gb hard drive, installed and formatted it, it will "only" have about 56Gb of actual storage space available.

Before I explain further, I think I should clarify hard drive and file sizes, and abbreviations, etc.

Abbreviations:

Kb = Kilobyte(s)
Mb = Megabyte(s)
Gb = Gigabyte(s)

The smallest unit of size used is the "bit", this in computer language is either "0" or "1", eight of these bits make up one byte - which is equal to one character in a plain text document. You can try this for yourself, open notepad and type in just one letter or number, nothing else, not even a carriage return and save the file. Navigate to the file using Windows explorer or similar, right click on the file and choose "Properties". It should report the size as "1 bytes".

Anyway on to sizes:

1 byte = 8 bits
1Kb = 1024 bytes
1Mb = 1024 Kb (1024 x 1024 bytes = 1,048,576 bytes or 1024²)
1Gb = 1024Mb (1024 x 1,048,576 bytes (1024Mb) = 1,073,741,824 or 1024³)

That is basically how formatted hard drive sizes are defined, which is fine - most of the time you don't have to worry about what means what. The problem is that hard drive manufacturers quote drive sizes with 1Kb as being 1000 bytes and not 1024.

Now, 24 bytes may not seem much and it isn't, but the time you multiply that up into the Gigabyte range, it adds up to a lot. This would not have been so noticeable 10 or so years ago when the average drive size was 20Mb and not 100Gb as they are today.

So then, I shall use an imaginary hard drive of 60Gb in size as an example. Using the unformatted 1000 bytes to 1 kilobyte as quoted by manufacturers:

1Mb is 1000² = 1,000,000 bytes
1Gb is 1000³ = 1,000,000,000 bytes
60Gb is 1000³ x 60 = 60,000,000,000 bytes

Now with the formatted size of 1024 bytes to the kilobyte:

1Mb is 1024² = 1,048,576 bytes
1Gb is 1024³ = 1,073,741,824 bytes
60Gb s 1024³ x 60 = 64,424,509,440 bytes

Hang on, 64,424,509,440 bytes is larger than the 60,000,000,000 bytes size of the drive, oops. So the drive is formatted to allow the maximum storage space where multiples of 1024 will fit in, this means that the actual storage space on a formatted drive will end up being less.

So, how do we find out the actual storage space on our "60Gb" hard drive? We simply divide the total number of bytes of an unformatted hard drive, by the number of bytes in 1Gb for a formatted drive, i.e:

60,000,000,000 bytes ÷ 1,073,741,824 bytes = 55,879,354,476 bytes

There is also a "remainder" that we need to add, so:

60,000,000,000 bytes - 55,879,354,476 bytes = 4,120,645,524 bytes

55,879,354,476 bytes ÷ 1024 = 54,569,682Kb
54,569,682Kb ÷ 1024 = 53,290Mb
53,290Mb ÷ 1024 = 52.04Gb

4,120,645,524 bytes ÷ 1024 = 40,240,679Kb

40,240,679Kb ÷ 1024 = 39,298Mb
39,298Mb ÷ 1024 = 3.84Gb

52.04Gb + 3.84Gb = 55.88Gb

This then means our formatted capacity of our 60Gb hard drive is in fact 55.88Gb, a discrepancy of 4.12Gb.

I bet you're asking how they get away with it, well, I dunno, but I expect the fact they do also provide (but not often advertise) the formatted capacity for their drives, helps.

Caveat emptor:

Your mileage may vary - every hard drive is slightly different and the file system the drive is formatted with will also affect the amount of formatted space available, so use the stuff above as a guide only.

This document is Copyright © 2003 Andrew Mills and may not be reproduced without prior permission.