Byte converter

Decimal or binary?

Drive makers and most systems use decimal (1 KB = 1000 bytes). Windows and many programs count in binary (1 KB = 1024 bytes): that's why a «1 TB» drive shows about 931 GB.

What it's for

Handy to understand how big a file is, how much data you're downloading or how much memory you need. Remember bits (lowercase «b») are used for network speed: 1 byte = 8 bits.

The right names exist, and nobody uses them

To remove the ambiguity between the two ways of counting, separate names have existed since 1998: kilobyte, megabyte and gigabyte (KB, MB, GB) mean a thousand, while kibibyte, mebibyte and gibibyte (KiB, MiB, GiB) mean 1024. Linux uses them, as do some technical programs and the specifications; almost nobody else does, and Windows still writes «GB» when it's counting in gibibytes. That's why the same thing has two numbers and it looks as though somebody is getting it wrong.

Where one is used and where the other

For a thousand: disk and memory-stick makers, phone operators, the Mac, Android phones. For 1024: RAM (which is built in powers of two, so there it's the only sensible way), Windows, and most programs that show a file's size.

The difference grows with size: it's 2.4% on kilobytes, 7% on gigabytes and 10% on terabytes. That's why a «1 TB» disk shows 931 in Windows: nobody stole anything from you, they're the same bytes counted another way.

Bits are yet another unit

The byte is written with a capital B, the bit with a lowercase b, and between the two there's a factor of eight. Network speeds are always measured in bits per second, files always in bytes: a «100 mega» line gives at most 12.5 MB a second, and that's the conversion that explains most of the disappointment about internet speed.

Nearby tools

To work out how long a file takes to download there is Download time, and for a video's size before export Video bitrate and size. If a file needs slimming down, the pages are Compress image, Compress PDF and Compress an Office file.