Reported Hard Drive Capacity

Are you getting your bytes?

Some people notice that their operating system reports that drives attached to their computer are smaller than advertised. An example is having the operating system reporting 148GB for a drive advertised as 160GB. Where did the 12GB of space go to? The difference is in the basic math used, which I'll try to explain.

Computers have counted memory space in powers of 2 from the start. Yours is no different. What you need to understand is how disk space is reported in the OS (operating system) and that drive manufacturers report it differently.

For those who forgot or haven't been introduced to the concept, what is meant by power is that a number is multiplied by it self a number of times, the power. When 2 is multiplied by it self 10 times (2 to the power of 10) the sum is 1024. When 10 is multiplied by it self 3 times the answer is 1000. In terms of disk space, each is referred to as a Kilobyte. Shocking I know.

The following table gives a bigger clue to what is happening. The same terminology is used but the values differ. As the columns show this is because of how the calculation is done.

The Same Terminology for Different Values
Measurement Operating
Kilobyte (KB) (2^10) is 1,024 (10^3) is 1,000
Megabyte (MB) (2^20) is 1,048,576 (10^6) is 1,000,000
Gigabyte (GB) (2^30) is 1,073,741,824 (10^9) is 1,000,000,000
Terabyte (TB) (2^40) is 1,099,511,627,776 (10^12) is 1,000,000,000,000

As you can see the sums differ and that is where the problem lies. When operating systems report a Kilobyte or Gigabyte, they are using the powers of 2 where a Kilobyte is just over 1000. The manufacturers use the powers of 10 where they can calculate exactly 1000, or 1,000,000,000, not some odd ball number the average Joe doesn't understand.

Apples' System Profiler tells me that my 160GB (160,041,885,696 bytes actually) drive has a capacity of only 149.05GB. Where did that nearly 11GB disappear to? Well 160,041,885,696 divided by 1,073,741,824 (the OS GB value) is 149.05. If the OS had used 1,000,000,000 to divide the number by, then it would have reported that I had a 160.04GB drive.

As you can see, the manufacturer gave me my 160 billion bytes plus a little extra. The OS is dividing that large byte count by its definition of a GB to arrive at what appears to be a smaller number.

The act of formatting a drive does take up a little bit of space. Also, operating systems also reserve some space for their needs to maintain the drive. You may need to keep this in mind when purchasing as you might have to buy the next size up.

The next time you buy a drive, keep in mind you are getting your capacity. The manufacturer has chosen to count it in the power of 10, like you learned in elementary school.


Here is a calculator to help you out:

Manufacturer Label GB
Operating System GB