KB vs KiB: Why a "20 KB" Photo Can Still Be Rejected

Two meanings of one unit

The kilo in kilobyte means 1000, just like in kilometre. Because computers work in powers of two, the industry also used kilobyte to mean 1024 bytes for decades. In 1998 the International Electrotechnical Commission introduced the kibibyte (KiB) for 1024 bytes so the two could be told apart, but many programs still write KB for both.

How the difference causes rejections

Imagine a form with a 20 KB limit, and a photo that is 20,300 bytes. Divided by 1024, that is 19.8 KB, so Windows shows it as under the limit. If the form's server checks against 20 × 1000 = 20,000 bytes, the same file is 300 bytes too large and is rejected.

The opposite also happens: a file shown as 20.4 KB on a Mac is accepted by a form that counts 20 × 1024 = 20,480 bytes. People see inconsistent results and assume the form is broken, when the two sides simply count differently.

The safe rule

Keep the file at or under the limit multiplied by 1000 bytes. A file of 20,000 bytes or less passes a 20 KB limit whichever way the form counts.

FitKB applies every limit this way and shows the exact byte count next to the rounded size, so you can see that a result passes before you upload it. Try it with the 20 KB preset.

How to check a file's exact size

• Windows: right-click the file, choose Properties, and read the number of bytes in brackets next to Size.
• macOS: select the file and press Command + I. Finder shows the size in decimal KB and the exact byte count.
• Android: most file manager apps show details or properties with the exact size; some round it.
• iPhone: in the Files app, long-press the file and choose Get Info.

Quick conversion table

The table shows how many bytes each common limit allows under both counting methods. The middle column is the safe target, because it passes either way.

Why the gap matters more for small limits

The difference between the two methods is 2.4 percent. On a 10 MB limit that is a quarter of a megabyte, which rarely matters because files are seldom that close to the edge. On a 20 KB limit it is only 480 bytes, and compressed photos very often land within a few hundred bytes of the target. That is why KB confusion causes far more rejections on strict exam and application portals than on general file-sharing sites.

If a form gives its limit in bytes, for example 20480 bytes, use that number directly: divide it by 1000 and enter the result as the KB limit in FitKB, here 20.48. The tool accepts decimals.