Ever bought a hard drive only to find you’ve got a smal­ler disk space than stated on the box? That’s not because your oper­at­ing sys­tem is tak­ing up that space, it’s because you’re not count­ing it as the hard drive man­u­fac­tur­ers did.

Com­puters count in base two – that is, bin­ary. In bin­ary, a kilo­byte is 1024 bytes (2 to the power of 10). Nor­mal people con­sidered a “kilo” to mean a thou­sand so a kilo­byte, to them, was 1000 bytes. If you sell hard drives, the obvi­ous choice is to use the meas­ure­ment that sounds like more value – the non-geek 1000 was there­fore used (1MB = 1,048,576 bytes in bin­ary or 1,000,000 bytes in met­ric – less is cheaper to make although less in usable capa­city). In 1998, the IEC defined 1GB as 1,000,000,000 bytes – using the non-geek/manufacturer method. To avoid con­fu­sion, they also decided to rename meas­ure­ments of bin­ary by repla­cing the last two let­ters with “bi” to rep­res­ent “bin­ary” (kib­i­b­ytes, mebi­b­ytes, gib­i­b­ytes, tebi­b­ytes). The thing is, though, these new names sound ter­rible and they will never be picked up. In real­ity, every­one uses the ori­ginal names and ignores the stated capa­cit­ies on hard drives.

If you’re math­em­at­ic­ally inclined, you’ll notice that the dif­fer­ence is 7%; to find out the real capa­city of any hard drive, just take off 7%.