{"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":"2025-10-27T02:53:29","modified_gmt":"2025-10-27T02:53:29","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

\n\n\n\n\n\n\n\n\n
Metric<\/th>\nComputer<\/th>\n<\/tr>\n
prefix<\/th>\nabbrev<\/th>\npower of 10<\/th>\nmultiplier value<\/th>\nprefix<\/th>\nabbrev<\/th>\npower of 2<\/th>\nmultiplier value<\/th>\n<\/tr>\n
Kilo<\/td>\nk<\/td>\n10^3<\/td>\n1,000<\/td>\nKibi<\/td>\nKi<\/td>\n2^10<\/td>\n1,024<\/td>\n<\/tr>\n
Mega<\/td>\nM<\/td>\n10^6<\/td>\n1,000,000<\/td>\nMebi<\/td>\nMi<\/td>\n2^20<\/td>\n1,048,576<\/td>\n<\/tr>\n
Giga<\/td>\nG<\/td>\n10^9<\/td>\n1,000,000,000<\/td>\nGibi<\/td>\nGi<\/td>\n2^30<\/td>\n1,073,741,824<\/td>\n<\/tr>\n
Tera<\/td>\nT<\/td>\n10^12<\/td>\n1,000,000,000,000<\/td>\nTebi<\/td>\nTi<\/td>\n2^40<\/td>\n1,099,511,627,776<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\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