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When a Byte is not a Byte

Understanding Advertised vs. Actual Drive Storage Capacities

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Introduction

At some point in time, most users have come across a situation where a drive or disk being used run out of space even though there is thought to be plenty of space. A lot of times, this is the rude awakening for the consumer that the device they are attempting to store data on is not as large as it was advertised. In this article, we take a look at how manufacturers rate the capacity of storage devices such as hard drives, floppies, flash media and compact disks and their actual size.

Bits, Bytes and Prefixes.

All computer data is stored in a binary format as either a one or zero. Eight of these bits together for the most commonly referred to item in computing, the byte. The various amounts of storage capacity are referred by a prefix to represent a specific amount, similar to the metric prefixes. Since all computers are based on 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 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 70.4 GB actually has around 75,591,424,409 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.

Now, this isn't true for all the drives and storage media on the market. This 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 storage is based around the actual memory amounts. So a 512MB memory card has exactly 512 MB of data capacity, but this leads to the next area of reported space.

Formatted vs. Unformatted

In order 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. This is where formatting of a drive comes in. 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 of these formatting schemes, a portion of the storage space is allocated so that the data on the drive can be catalogued enabling the computer or other device to properly read and write the data to the drive.

This means that when a drive is formatted, the functional storage space of the drive will be less than its unformatted capacity. The amount by which the space is reduced will vary depending upon the type of formatting used for the drive and also the amount and size of the various files on the system. Since it does vary, it is impossible for the manufacturers to quote the formatted size. This problem is most frequently encountered with flash media storage over larger capacity hard drives.

Conclusions

Hopefully this look at how both manufacturers and computer devices see storage devices has been helpful to explain why the values will differ. It is important when purchase a computer, hard drive or even flash memory to know how to read the specifications properly. Typically manufacturers will have a footnote in the device specifications to show how it is rated. This can help the consumer to make a much more informed decision.

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