{"id":52,"date":"2006-05-31T10:25:44","date_gmt":"2006-05-31T10:25:44","guid":{"rendered":"https:\/\/www.garfnet.org.uk\/cms\/metric-versus-binary-computer-prefixes\/"},"modified":"2026-09-02T16:43:20","modified_gmt":"2026-09-02T16:43:20","slug":"metric-versus-binary-computer-prefixes","status":"publish","type":"page","link":"https:\/\/garfnet.org.uk\/cms\/tables\/general\/metric-versus-binary-computer-prefixes\/","title":{"rendered":"Metric versus binary (computer) prefixes"},"content":{"rendered":"

Why does my 120 gigabyte hard disk only store 111 gigabytes?<\/h2>\n

The size discrepancy comes from the fact that manufacturers always use figures which show their product in the best way. Some would argue that it also stems from a cultural issue in the sense that many in the the US-led computing industry appear reluctant to join the rest of the planet in adopting & understanding the metric system.<\/p>\n

Normally, the metric prefix ‘kilo<\/strong>‘ means 1000 or 10^3. But in computing parlance ‘kilo<\/strong>‘ has been taken to mean 1024 or 2^10. The metric prefix ‘mega<\/strong>‘ means 1,000,000= 10^6. However in computing terms ‘mega<\/strong>‘ means 1024×1024 = 1,048,576 = 2^20.<\/p>\n

A new ISO standard attempts to work around this problem by adopting a new set of computing-specific prefixes – where 1024 bytes should now be referred to as 1 kibibyte<\/strong> (or 1.024 kilobytes). The table below details these prefixes…<\/p>\n

Metric Prefix<\/span><\/a><\/div><\/th>
Metric Abbreviation<\/span><\/a><\/div><\/th>
Metric Power of Ten<\/span><\/a><\/div><\/th>
Metric Multiplier<\/span><\/a><\/div><\/th>
Binary Prefix<\/span><\/a><\/div><\/th>
Binary Abbreviation<\/span><\/a><\/div><\/th>
Binary Power of Two<\/span><\/a><\/div><\/th>
Binary Multiplier<\/span><\/a><\/div><\/th><\/tr><\/thead>
Kilo<\/div><\/td>
k<\/div><\/td>
10^3<\/div><\/td>
1000<\/div><\/td>
Kibi<\/div><\/td>
Ki<\/div><\/td>
2^10<\/div><\/td>
1024<\/div><\/td><\/tr>
Mega<\/div><\/td>
M<\/div><\/td>
10^6<\/div><\/td>
1000000<\/div><\/td>
Mebi<\/div><\/td>
Mi<\/div><\/td>
2^20<\/div><\/td>
1048576<\/div><\/td><\/tr>
Giga<\/div><\/td>
G<\/div><\/td>
10^9<\/div><\/td>
1 \u00d7 109<\/sup><\/div><\/td>
Gibi<\/div><\/td>
Gi<\/div><\/td>
2^30<\/div><\/td>
1.074 \u00d7 109<\/sup><\/div><\/td><\/tr>
Tera<\/div><\/td>
T<\/div><\/td>
10^12<\/div><\/td>
1 \u00d7 1012<\/sup><\/div><\/td>
Tebi<\/div><\/td>
Ti<\/div><\/td>
2^40<\/div><\/td>
1.1 \u00d7 1012<\/sup><\/div><\/td><\/tr>
Peta<\/div><\/td>
P<\/div><\/td>
10^15<\/div><\/td>
1 \u00d7 1015<\/sup><\/div><\/td>
Pebi<\/div><\/td>
Pi<\/div><\/td>
2^50<\/div><\/td>
1.126 \u00d7 1015<\/sup><\/div><\/td><\/tr><\/tbody><\/table><\/div>\n

With small numbers, this difference is unimportant. But as the size of storage media has increased, the discrepancy has become quite significant. For example the difference between 1 terabyte<\/strong> (1,000,000,000,000 bytes) and 1 tebibyte<\/strong> (1,099,511,627,776 bytes) is almost 100 gigabytes!<\/p>\n

To avoid confusion, and no doubt quite a few lawsuits, manufacturers will need to be much clearer in the future which prefix they are using!<\/p>\n