You probably have a hard drive in your home that’s one terabyte or more. If you open Computer (Windows Key + E), you’ll see your terabyte hard drive might only show a capacity of around 930 GB. Where did all those extra gigabytes go? Aren’t there a thousand gigabytes in a terabyte? If you have questions like these, this guide answers them.
Terabyte vs. Tebibyte
Hard drive manufacturers use the International System of Units (SI) convention to when listing hard drive capacity. Kilo-, mega-, giga-, and tera- are some of the prefixes used in accordance with the SI and are base-10 (decimal.) “Tera” means trillion so a terabyte is one trillion, 1,000,000,000,000, or 1020 bytes.
When Windows shows the drive’s capacity, it lists it as “less” because it’s calculating the size using a base-2 (binary numeral) system. Windows is really listing the sizes in tebibytes not terabytes. A tebibyte is 1,099,511,627,776 or 240 bytes.
The words kilobyte, megabyte, gigabyte, and terabyte are used interchangeably with kibibyte (kilo binary byte), mebibyte (mega binary byte), gibibyte, and tebibyte respectively; however, there is a difference and this difference gets bigger at each order of magnitude.
In 1999, the International Electrotechnical Commission (IEC) came together to help clear up the ambiguity between the base-10 and base-2 systems by providing us with a different set of prefixes to identify base-2 byte prefixes in computing. This chart shows the percentage difference between a kilobyte and kibibyte etc. as we progress through orders of magnitude:
A 1 TB drive is 1000GB. A 1TB drive is also 931 gibibytes (GiB) or 0.91 tebibytes (TiB.)
It really doesn’t matter what unit of measurement is used as the hard drive will always have the same capacity regardless of the numbering convention used to quantify it. The fact is a “terabyte”, according to standards, is a trillion bytes or a thousand gigabytes. Although many may use 1 TB to mean 1024 GB, the standard (which helps alleviate confusion for all of us) states that only 1 TiB is 1024 GiB and that 1 TB is 1000 GB.
We’ve been using “kilo” to mean 1024 bytes for so long that it really is quite a struggle to think of it differently (or accept the prefix “kibi” at all.) Adoption of this terminology is slow but, correct use shows you’ve really done your homework and that you’ve paid attention to detail!
Hopefully this guide has cleared up some ambiguity for Windows users who are wondering where all that extra hard drive space went. If you’re using OS X Leopard or a newer release of many Linux versions, you may have noticed that space is reported in base-10, which was primarily done to clear up any confusion.