KB or KiB: how big is “50 KB” on a form?
Some systems count a KB as 1,000 bytes, others as 1,024. Here is why it matters near a limit and how this tool makes sure your file passes either way.
Two meanings
A kilobyte is 1,000 bytes in the SI system, but many programs (and Windows) show sizes in units of 1,024 bytes, properly called KiB.
Why it matters
A 51,000-byte file is 51 KB but only 49.8 KiB. If the form counts in 1,000s, it fails a 50 KB limit; if in 1,024s, it passes.
What we do
We aim for N × 1,000 bytes, the smaller of the two readings, so the file passes whichever way the form counts.
Check it
The result shows the size in bytes, KB and KiB, so you can compare with what the form shows.
Questions
Is 1 KB 1000 or 1024 bytes?
Strictly, 1 KB is 1,000 bytes and 1 KiB is 1,024 bytes, but many programs use “KB” for 1,024. Forms rarely say which they use, so staying under N × 1,000 bytes is the safe choice.
Why does Windows show a different size than my phone?
Windows counts in units of 1,024 bytes but labels them KB, while phones and macOS usually count in 1,000s. The same 50,000-byte file shows as about 48.8 KB in Windows and 50 KB on a Mac.
Will a 50 KB file from PhotoSize pass a 50 KB limit?
Yes, a file made with a 50 KB maximum is at most 50,000 bytes, which is under the limit whether the form counts a KB as 1,000 or 1,024 bytes.
Updated 2026-11-06