You make a photo smaller, your computer says it is 98 KB, and the website still says it is over the 100 KB limit. You did nothing wrong. There are two ways of counting kilobytes, and your computer and the website are using different ones.
Two ways to count a kilobyte
Files are measured in bytes. The question is how many bytes make a kilobyte:
- 1 KB = 1,000 bytes. This is the metric way, the same as 1 kilometre = 1,000 metres. Macs, iPhones and most Android phones count like this.
- 1 KB = 1,024 bytes. This is the older computer way, based on powers of two. Windows counts like this, even though it writes "KB".
The same file therefore shows two different sizes:
| The file | Windows shows | A phone or Mac shows |
|---|---|---|
| 100,352 bytes | 98 KB | 100.4 KB |
| 51,200 bytes | 50 KB | 51.2 KB |
| 245,760 bytes | 240 KB | 245.8 KB |
The gap grows with bigger units. A megabyte is either 1,000,000 bytes or 1,048,576 bytes, a difference of almost 5%.
Why this gets files rejected
A website checks the exact number of bytes. If it treats 100 KB as 100,000 bytes, a file of 100,352 bytes is over the limit, even though Windows calls it 98 KB. If it treats 100 KB as 102,400 bytes, the same file passes. Most websites do not say which they use.
Minimum sizes can fail the other way. A file of 50,500 bytes shows as 50.5 KB on a phone, which looks fine for an "at least 50 KB" rule. But Windows shows it as 49.3 KB, and a website that counts 1,024 bytes per kilobyte wants at least 51,200 bytes, so the file fails.
The safe way: pass under both counts
To be sure a file is accepted, make it fit both ways of counting:
- For a maximum, use the smaller number. For "up to 100 KB", stay under 100,000 bytes. For "up to 2 MB", stay under 2,000,000 bytes.
- For a minimum, use the bigger number. For "at least 50 KB", go over 51,200 bytes.
SafeResize does this automatically. Every file it makes stays inside the limit whichever way the website counts, and the result screen shows the exact number of bytes along with both sizes, for example "82,262 bytes. Windows shows 80.3 KB; phones and Macs show 82.3 KB."
How to see the exact size yourself
- Windows: right-click the file, choose Properties, and look at "Size". It shows both, for example "98.0 KB (100,352 bytes)".
- Mac: select the file and press Command + I. "Size" shows the bytes.
- iPhone and Android: the Files app shows the size in 1,000-byte kilobytes.
Quick reference
| You see | It could mean |
|---|---|
| 1 KB | 1,000 or 1,024 bytes |
| 100 KB | 100,000 or 102,400 bytes |
| 1 MB | 1,000,000 or 1,048,576 bytes |
| 2 MB | 2,000,000 or 2,097,152 bytes |
If a website rejects a file that looks just under its limit, this is almost always the reason. Make it a little smaller with Make a photo smaller or Resize a PDF, or test it with Check my file.