Forms and uploads
Why Your “50 KB” File Gets Rejected: KB vs KiB Explained
By the AllToolsDesk team · Updated
You compress a photo, your computer says 49 KB, and the portal still says the file is larger than 50 KB. Or the reverse: a form says the file is too small when it looks fine. Often nothing is wrong with the photo. The two sides are counting kilobytes differently.
Two kinds of kilobyte
A kilobyte can mean 1,000 bytes (the decimal kilobyte, KB) or 1,024 bytes (the binary kibibyte, KiB). Both are in everyday use, and most software just writes “KB” for either.
| File size in bytes | In KB (1,000) | In KiB (1,024) |
|---|---|---|
| 50,000 | 50.0 | 48.8 |
| 51,000 | 51.0 | 49.8 |
| 51,200 | 51.2 | 50.0 |
| 20,000 | 20.0 | 19.5 |
A 51,000-byte file passes a portal that divides by 1,024 (49.8) and fails one that divides by 1,000 (51.0). Windows File Explorer divides by 1,024 and labels the result KB; macOS Finder divides by 1,000. So the same file can show different numbers on two computers.
Rounding makes it worse
File managers also round. Windows Explorer's Size column rounds up to the next whole kilobyte, so a file of 49.1 KiB shows as 50 KB. Some portals round down, some compare exact bytes. A file within a few hundred bytes of the limit can pass on one day's portal and fail after an update.
A simple rule that always passes
- For a maximum, count 1 KB as 1,000 bytes. A file under 50,000 bytes is under 50 KB by either definition.
- For a minimum, count 1 KB as 1,024 bytes. A file over 20,480 bytes is over 20 KB by either definition.
- Check the exact size in bytes: Properties on Windows, Get Info on a Mac, or the size shown under each result in our tools.
Our KB tools apply exactly this rule. Compress Image to 50 KB keeps files at or under 50,000 bytes, and the exam photo presets pad files that would otherwise fall below 1,024 bytes per kilobyte of the stated minimum.
The same confusion exists for megabytes: 1 MB is 1,000,000 bytes, while 1 MiB is 1,048,576 bytes. For a 1 MB limit, stay under 1,000,000 bytes.
How to see the exact size in bytes
- Windows: right-click the file, choose Properties, and read the first number on the Size line, in bytes. Ignore “Size on disk”, which depends on the drive, not the file.
- Mac: select the file and press Command + I. Get Info shows the size in bytes in brackets.
- Android: most file manager apps show details or properties for a file; some round to one decimal place, so check bytes where the app offers them.
- iPhone: in the Files app, long-press the file and choose Get Info.
If the size is within about 2% of the limit, assume it might fail and make the file a little smaller. The difference in quality between a 49 KB and a 47 KB photo is invisible, while a rejection costs you another attempt and sometimes a session timeout on the portal.