KB, MB and GB Explained in Plain Words
File sizes confuse almost everyone at some point. A form asks for "under 200 KB", your photo says "3.4 MB", and it is not obvious how far apart those numbers are. Here is the simple version.
What the letters mean
All files are made of bytes, tiny units of stored information. Because files hold huge numbers of bytes, we use bigger units, in the same way that we use kilometres rather than metres for long distances.
| Unit | Short for | Roughly equals | Typical example |
|---|---|---|---|
| KB | Kilobyte | 1,024 bytes | A scanned signature: 20 KB |
| MB | Megabyte | 1,024 KB | A phone photo: 3 MB |
| GB | Gigabyte | 1,024 MB | A two-hour film: 2 GB |
Quick ways to convert
For everyday use, think of it as "about a thousand" at every step. So 1 MB is about 1,000 KB, and 1 GB is about 1,000 MB. That means a 3 MB photo has to shrink about 15 times to fit under a 200 KB limit.
Why 1,024 and not 1,000?
Computers count in twos, and 1,024 is a power of two. Some companies, however, use a neat 1,000. Windows shows sizes using 1,024, while macOS shows them using 1,000. The same file can therefore appear as 1.00 MB on one computer and 1.05 MB on another. Neither is wrong.
Why a website rejected your file
Different counting is the usual reason that a file "just under the limit" gets refused. If a form says 2 MB and your file shows 1.98 MB, the website may count differently from your computer. The easy fix is to leave a margin of about 5%. Aim for 1.9 MB rather than 2 MB.
File size is not the same as pixel size
File size is how much storage a picture takes. Pixel size, such as 1200 × 800, is how many dots it has. A form may ask for either, or both. If it gives KB or MB, use a compressor. If it gives pixels, use resize. If it asks for both, resize first and then compress.
Once you know these few ideas, size limits stop being a puzzle. You can tell quickly how far a file has to shrink and pick the right tool for it.