Data storage units and file sizes

Last time

Before we start: last time, in Representing sound, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

How big is a file?

A song, a photo, a whole film: everything on your phone takes up space. Cambridge wants you to measure that space in the right units, and to move between them without a mistake.

The smallest units

The smallest unit of data is the bit: one binary digit, a zero or a one. Four bits make a nibble. Eight bits make a byte, enough to store one ASCII character.

The binary prefixes

Above the byte, each unit is one thousand and twenty-four times the one before. One thousand and twenty-four bytes make a kibibyte. Then mebibyte, gibibyte, tebibyte, pebibyte and exbibyte, each one thousand and twenty-four times bigger. Why one thousand and twenty-four? It is two to the power ten, a round number in binary.

Remember the order

Here is a way to remember the order: bit, nibble, byte, then kibi, mebi, gibi, tebi, pebi, exbi. Cambridge often asks you to pick the largest from four of them.

Kibi or kilo?

A kilobyte is exactly one thousand bytes, but a kibibyte is one thousand and twenty-four. Cambridge uses the binary ones: kibi, mebi, gibi. This is why a five hundred gigabyte hard disk shows only about four hundred and sixty-six gibibytes on a computer.

Converting units

Converting is one rule. To go to a bigger unit, divide. From bits to bytes, divide by eight. From bytes to kibibytes, divide by one thousand and twenty-four. To go to a smaller unit, multiply.