KB vs MB: how to check and meet file upload size limits
Updated · 4 min read
An application form says 'maximum 1 MB', your file shows as 1,010 KB, and you're not sure whether it fits. Part of the confusion is real: kilobytes and megabytes are counted in two different ways, and your computer, your phone and the website may not agree. Here's what the units mean, how to see a file's exact size, and how to get under a limit with room to spare.
Bytes, kilobytes and megabytes
File sizes are counted in bytes. Because files are large, sizes are shown in bigger units: kilobytes (KB), megabytes (MB) and gigabytes (GB). A typical form photo is tens or hundreds of kilobytes; a scanned document is often a few megabytes.
The trouble is what 'kilo' means. There are two conventions in everyday use, and both are written the same way:
| Decimal (powers of 1,000) | Binary (powers of 1,024) | |
|---|---|---|
| 1 KB | 1,000 bytes | 1,024 bytes (strictly 'KiB') |
| 1 MB | 1,000 KB = 1,000,000 bytes | 1,024 KB = 1,048,576 bytes (strictly 'MiB') |
| 1 GB | 1,000 MB = 1,000,000,000 bytes | 1,024 MB = 1,073,741,824 bytes |
The decimal meaning matches the metric system and is what storage makers use. The binary meaning comes from how computer memory is built, and it's still widely used. Strictly, the binary units have their own names — kibibyte (KiB) and mebibyte (MiB) — but most software writes KB and MB for both. Neither is wrong; they just differ by about 2.4% at the kilobyte level and almost 5% at the megabyte level.
Different devices pick different conventions. Windows File Explorer counts in 1,024s while labelling the result KB and MB; macOS Finder and iPhones count in 1,000s; Android file managers vary by phone. So the same 1,000,000-byte file shows as '1 MB' on a Mac and about '977 KB' on Windows.
Tip: Don't confuse megabytes (MB) with megabits (Mb). Internet speeds are given in megabits per second; a byte is 8 bits, and upload limits are always given in bytes.
How to see a file's exact size
The surest way to compare a file with a limit is its size in plain bytes, which both conventions agree on.
- Windows: right-click the file and choose Properties. 'Size' shows the rounded value and the exact number of bytes in brackets. Use Size, not 'Size on disk'.
- Mac: select the file and press Command-I (or choose File → Get Info). The size is shown with the exact byte count.
- iPhone and iPad: in the Files app, touch and hold the file, then choose Get Info.
- Android: in your Files app, open the file's menu and choose Info or Details. The exact wording depends on the phone.
Which one does a website mean?
Upload forms check the file's size in bytes, but rarely say which 'MB' they mean. A site using the binary meaning accepts up to 1,048,576 bytes for '1 MB'; a site using the decimal meaning stops at 1,000,000. A file between the two is accepted by one and rejected by the other.
Our compression tools use the binary meaning: 1 KB is 1,024 bytes, and the 1 MB presets aim for 1,024 KB. That suits most portals, but if a form is strict and you don't know its convention, aim a little lower so the file fits either way:
| Form says | Safe target to type (KB) | Exact size |
|---|---|---|
| 50 KB | 48 | 49,152 bytes |
| 100 KB | 97 | 99,328 bytes |
| 200 KB | 195 | 199,680 bytes |
| 500 KB | 488 | 499,712 bytes |
| 1 MB | 975 | 998,400 bytes |
| 2 MB | 1,950 | 1,996,800 bytes |
Getting a PDF under the limit
Compress PDF has a Target size option in KB. It tries up to six progressively stronger settings, from high-quality images down to strong JPEG compression, and keeps the first result that fits — so images lose no more detail than they must. Text and vector graphics are never re-encoded, so words stay sharp. If the file can't reach the target without becoming unreadable, you get the smallest safe version with a note saying so.
There are presets for common limits: 2 MB, 1 MB, 500 KB, 200 KB and 100 KB. Text documents and short scans reach these easily; long, photo-heavy PDFs may need splitting or removing pages as well.
Set a target size and shrink your PDF in the browser — nothing is uploaded:
Getting a photo under the limit
Compress Image has a Target file size mode for JPG, PNG, WebP and AVIF. It lowers the quality step by step first and only reduces the pixel dimensions if that isn't enough. PNGs are lossless, so they're only scaled down — switching the output to JPG under Advanced usually reaches small sizes with a better-looking result.
Presets cover the sizes forms ask for most: 20 KB, 50 KB, 100 KB and 200 KB. For more on photos for applications, see how to reduce a photo for online forms.
Compress a photo to an exact size in your browser:
Questions
Is 1 MB 1,000 KB or 1,024 KB?
Both conventions are in use. In the decimal system 1 MB is 1,000 KB (1,000,000 bytes); in the binary system it's 1,024 KB (1,048,576 bytes). Windows uses the binary values, while Macs and iPhones use the decimal ones.
Why does my file show a different size on Windows and on a Mac?
The file is the same; the counting differs. Windows divides by 1,024 and macOS by 1,000, so Windows shows a slightly smaller number. Compare the exact byte counts to be sure.
My file is just under the limit but the upload still fails. Why?
The site may count 1 MB as 1,000,000 bytes while your device counts it as 1,048,576. Compress the file to a slightly lower target, such as 975 KB for a 1 MB limit, and try again.