What does 'KB' mean on an upload form: 1000 bytes or 1024?

A form says 'maximum 50 KB'. Windows says your file is 49 KB, the form says it is too large. Both are telling the truth, because 'KB' means two different things.

Two kilobytes

In the decimal system a kilobyte is 1000 bytes, as a kilometre is 1000 metres. In the older binary convention a kilobyte is 1024 bytes, because memory comes in powers of two. The binary unit has its own name, the kibibyte (KiB), but most software still writes it as KB.

Windows File Explorer divides by 1024 and writes KB. macOS Finder has divided by 1000 since 2009. Phone file managers vary. So a file of 50,500 bytes is '49 KB' on Windows, '51 KB' on a Mac, and over a 50,000-byte limit either way.

How portals count

An upload checker compares the file's size in bytes with a number someone typed into its code: 50 × 1024 = 51,200, or 50 × 1000 = 50,000. Most use 1024, some use 1000, and the notice rarely says which. The fit-to-size tool on this site counts 1 KB as 1024 bytes and stops a little under the limit.

The safe target

For a maximum, aim under the 1000-byte figure: it passes both kinds of checker. In practice, enter about 97% of the limit (for 50 KB, enter 48 KB). For a minimum, aim above the 1024-byte figure: for a 10 KB floor, enter 11 KB.

Reading the exact size

On Windows, right-click the file, choose Properties and read 'Size' in bytes. On a Mac, choose Get Info. The result row of every tool on this site shows the size of the file you are about to download.