Understanding Advertised vs. Actual Drive Storage Capacities
It comes down to Bits and Bytes
All computer data is stored in a binary
The common prefixes are as follows:
- Kilobyte (KB) = 1,024 Bytes
- MegaByte (MB) = 1,024 Kilobytes or 1,048,576 Bytes
- Gigabyte (GB) = 1,024 Megabytes or 1,073,741,824 Bytes
- Terabyte (TB) = 1,024 Gigabytes or 1,099,511,627,776 Bytes
This is very important information because when a computer operating system or program reports the available space on a drive, it is going to report the overall total of available bytes or reference them by one of the prefixes. So, an OS reporting a total space of 74.51 GB actually has around 80,004,503,306 Bytes of storage space.
Advertised vs. Actual
Since consumers don't think in base 2 mathematics, manufacturers decided to rate most drive capacities based on the standard base 10 numbers we are all familiar with. Therefore, one Megabyte equals one million bytes while one Gigabyte equals one billion bytes. This isn't too much of a problem with fairly small numbers such as a Kilobyte, but each level of increase in the prefix also increased the total discrepancy of the actual space compared to the advertised space.
Here is a quick reference to show the amount that the actual values differ compared to the advertised for each common referenced value:
- Megabyte Difference = 48,576 Bytes
- Gigabyte Difference = 73,741,824 Bytes
- Terabyte Difference = 99,511,627,776 Bytes
Based on this, for each Gigabyte that a drive manufacturer claims, they are over reporting the amount of disk space by 73,741,824 Bytes or roughly 70.3 MB of disk space. So, if a manufacturer advertises an 80 GB (80 billion bytes) hard drive, the actual disk space is around 74.5 GB of space, roughly 7% less than what they advertise.Please use the following Quick Capacity Calculator to calculate the Actual useable capacity of your hard drive: