Why Advertised Storage Does Not Match Real Data Capacity

Understanding advertised vs. actual drive storage capacities

At some point, you may come across a situation where the capacity of a drive or disc isn't as large as advertised. This article examines how manufacturers rate the capacity of storage devices such as hard drives, solid-state drives, DVDs, and Blu-ray Discs compared to their actual size. It also explains why hard drives are smaller than advertised.

Bits, Bytes, and Prefixes

All computers store data in a binary format as either a one or zero. Eight of these bits together form a byte. The various amounts of storage capacity are defined by a prefix that represents a specific amount, similar to the metric prefixes. Since all computers use binary math, 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 information is vital because when a computer operating system or program reports the available space on a drive, it displays the overall total of available bytes or references them by one of the prefixes. So, an OS that indicates a total space of 70.4 GB actually has around 75,591,424,409 bytes of storage space.

Since consumers don't think in base-2 mathematics, manufacturers decided to rate most drive capacities based on the standard base-10 numbers with which we're all familiar. Therefore, one gigabyte equals one billion bytes, while one terabyte equals one trillion bytes. This approximation was not much of a problem back when we used the kilobyte. Still, each level of increase in the prefix also increases 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, it is over-reporting the amount of disk space by 73,741,824 bytes or roughly 70.3 MB of disk space. Suppose a manufacturer advertises an 80 GB (80 billion bytes) hard drive. In this case, the actual disk space is around 74.5 GB of space, roughly 7 percent less than advertised.

This practice isn't correct for all the drives and storage media on the market, which is where consumers have to be careful. Most hard drives are reported based on the advertised values, where a gigabyte is one billion bytes. On the other hand, most flash media bases storage on actual memory amounts. So a 512 MB memory card has exactly 512 MB of data capacity. The industry has been changing on this as well. For instance, an SSD may be listed as a 256 GB model but have 240 GB of space. SSD makers set aside extra room for dead cells and binary vs. decimal difference.

Formatted vs. Unformatted

For any type of storage device to be functional, there must be some method for the computer to know which bits stored on it relate to the specific files. Drive formatting helps with this. The types of drive formats can vary depending on the computer, but some of the more common ones are FAT16, FAT32, and NTFS. In each formatting scheme, a portion of the storage space is allocated to catalog the data on the drive. This process enables the computer or another device to read and write the data to the drive correctly.

When a drive is formatted, the functional storage space of the drive is less than its unformatted capacity. The amount by which formatting reduces space varies depending upon the type of formatting used and the amount and size of the various files on the system. Since it varies, manufacturers can't quote the formatted capacity. Users encounter this problem more frequently with flash media storage than larger-capacity hard drives.