Understanding Advertised vs. Actual Drive Storage Capacities

Sooner or later, most of us will notice that the new external hard
drive that we have just bought from our favorite online store, does
not have the amount of space available that we paid for. Our first
thought is that the hard drive is not as large as it was advertised.
In this article, we take a look at how the capacity of hard disk
drives is calculated, and why their actual size differs from the
advertised size.

It comes down to Bits and Bytes

All computer data is stored in a binary format as either a one or zero (called a bit). Eight of these bits together are most commonly called a byte. Storage capacity is measured by a prefix to the B (byte) to represent a specific amount, ie K (Kilobyte), M (Megabyte), G (Gigabyte), T (Terabyte) etc. Since all computers are based on the binary system, these prefixes represent base 2 amounts. Each level is an increment of 2 to the 10th power or 1,024.

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: