How many KB in a MB: 1,000 or 1,024?
Both, and that is the whole confusion. A megabyte is 1,000 kilobytes when sizes are counted in tens, the way storage makers, Macs and iPhones count. It is 1,024 kilobytes when they are counted in powers of two, the way Windows counts. Apple’s own support page says iOS 10 and earlier, and OS X Leopard and earlier, reported sizes the binary way; current Apple systems use the decimal one. Neither answer is wrong. They are two different kilobytes that happen to share a name.
| Size | In KB (1 KB = 1,000 bytes) | In KB (1 KB = 1,024 bytes) | Bytes |
|---|---|---|---|
| 1 MB | 1,000 KB | 1,024 KB | 1,000,000 or 1,048,576 |
| 2 MB | 2,000 KB | 2,048 KB | 2,000,000 or 2,097,152 |
| 5 MB | 5,000 KB | 5,120 KB | 5,000,000 or 5,242,880 |
| 10 MB | 10,000 KB | 10,240 KB | 10,000,000 or 10,485,760 |
| 0.5 MB | 500 KB | 512 KB | 500,000 or 524,288 |
For everyday sizes the gap is small, about 2.4% at the kilobyte level. It only matters when a number sits right at a limit, and that is exactly where upload forms put you.
What Windows shows for the same file
Windows divides by 1,024 and then cuts the number to three digits instead of rounding it. We created files of exact byte sizes on Windows 11 and read the size Windows reports for each one (the same figure you see under right-click → Properties, next to the exact byte count):
| File size in bytes | Windows shows | Counted in thousands |
|---|---|---|
| 100,000 | 97.6 KB | 100 KB |
| 102,400 | 100 KB | 102.4 KB |
| 500,000 | 488 KB | 500 KB |
| 512,000 | 500 KB | 512 KB |
| 1,000,000 | 976 KB | 1,000 KB (1 MB) |
| 1,024,000 | 0.97 MB | 1,024 KB |
| 2,000,000 | 1.90 MB | 2 MB |
| 5,000,000 | 4.76 MB | 5 MB |
Two things stand out. A file Windows calls “500 KB” is 512,000 bytes, which is over a 500 KB limit on any site that counts in thousands. And because Windows truncates, 2,000,000 bytes shows as 1.90 MB although the true figure is 1.907; it never rounds a file up to look bigger than it is. The converter above adds a “shown on Windows” line using the same rule, so you can see both numbers before you upload.
Converting a photo from MB to KB
Most people who search for MB to KB are not doing arithmetic. They have a 3 MB phone photo and a form that says “maximum 100 KB”. Converting the number does not help; the file itself has to get smaller. Drop it into the compressor on this page, pick the size the form asks for, and the tool finds the highest quality that fits. If quality alone cannot get there, it scales the picture down step by step, never below the minimum width you set.
The target is treated as a ceiling in thousands: 100 KB means at most 100,000 bytes. That figure is under 100 KB whichever way the form counts, so the file cannot fail on a technicality. If the form also has a minimum (“between 20 and 50 KB”), the minimum is counted the other way, 1,024 bytes per KB, for the same reason. A photo that is already small enough is left untouched rather than re-saved.
Need a file to be bigger instead, because the form says “at least 10 KB”? Increase image size in KB adds padding to the exact byte without changing a pixel. For PDFs, use Compress PDF; for videos, Compress Video.
Why a file that “fits” still gets rejected
Picture the usual chain: Windows shows your photo as 500 KB, the form says 500 KB maximum, and the upload fails. Windows meant 512,000 bytes; the server meant 500,000. Nothing is broken, and nobody can tell you in advance which kilobyte a particular site uses. The safe habit is simple: aim a little under the limit, or let the tool aim at the thousands figure for you. We measured where those rejections come from, and what each kind of server actually checks, in why a 500 KB file fails a 500 KB limit.
Quick reference
- MB to KB: multiply by 1,000 (decimal) or 1,024 (Windows).
- KB to MB: divide by 1,000 or 1,024.
- For a form limit of N KB: keep the file at or under N × 1,000 bytes.
- For a form minimum of N KB: make it at least N × 1,024 bytes.
- To check the exact bytes: right-click → Properties on Windows, Get Info on a Mac.
Everything on this page runs in your browser. The converter is plain arithmetic, and photos you compress are processed on your device and never uploaded. There is no account and no limit on the number of files.