KB vs MB: what image file sizes actually mean
“Maximum 2 MB”, “under 200 KB”, “at least 1000 px”: upload rules mix three different units. Once you know how they relate, you can tell in seconds whether a photo will pass.
Short answer: 1 MB is 1000 KB (or 1024 KB, depending on who is counting), and 1 KB is 1000 or 1024 bytes. So a 2 MB limit is roughly 2000 KB, and a 350 KB photo fits comfortably. To be safe with strict limits, stay a few percent under the number.
The units, from small to large
| Unit | Decimal meaning | Binary meaning | Typical example |
|---|---|---|---|
| Byte (B) | One character of plain text | The letter “a” | |
| Kilobyte (KB) | 1,000 bytes | 1,024 bytes (KiB) | A small signature image: 10 – 20 KB |
| Megabyte (MB) | 1,000 KB | 1,024 KiB (MiB) | A phone photo: 2 – 6 MB |
| Gigabyte (GB) | 1,000 MB | 1,024 MiB (GiB) | Roughly 300 – 500 phone photos |
Why the same file shows different sizes
There are two ways of counting, and different software uses different ones. The international standard says kilo means 1000, and uses KiB (“kibibyte”) for 1024. Windows File Explorer, however, divides by 1024 but still writes “KB”. macOS divides by 1000. Websites can do either.
The result: a file of 51,000 bytes is about 51 KB on a Mac and 49.8 KB in Windows. If a form's limit is 50 KB and it counts in thousands, the file is rejected even though Windows said it was under. The fix is simple: aim a little below the limit. PicShrinker always lands under the target you pick, so choosing 48 KB for a 50 KB limit covers both counting methods.
KB is not the same as pixels
File size (KB, MB) measures storage. Pixel dimensions (for example 1200 × 900) measure how big the picture is. Upload rules often state both, and they are checked separately. A photo can be the right number of KB and still be rejected for having too few pixels, or the other way round. The guide to form size limits covers how to satisfy both.
How big is a photo at different resolutions?
File size depends on how much detail is in the picture and how strongly it is compressed, so these are rough ranges for an ordinary photo saved as a JPG at good quality:
| Dimensions | Megapixels | Typical JPG size |
|---|---|---|
| 400 × 400 | 0.16 MP | 20 – 60 KB |
| 800 × 600 | 0.5 MP | 60 – 200 KB |
| 1920 × 1080 (Full HD) | 2 MP | 250 – 800 KB |
| 3024 × 4032 (12 MP phone) | 12 MP | 2 – 5 MB |
| 6000 × 4000 (24 MP camera) | 24 MP | 6 – 12 MB |
The same photo saved as a PNG is typically several times larger, because PNG does not throw away any detail. That is why converting photos to JPG is usually the quickest way to get under a limit.
Quick conversions
- 0.5 MB ≈ 500 KB
- 1 MB ≈ 1000 KB
- 2 MB ≈ 2000 KB
- 100 KB = 0.1 MB
- 20 KB = 0.02 MB
If you want to be exact for a form that counts in 1024s, multiply by 1024 instead: 2 MB = 2048 KB. The difference only matters when you are right on the edge.
How to check a file's size
- Windows: right-click the file and choose Properties. The Size line shows the exact number of bytes in brackets.
- Mac: select the file and press Command + I (Get Info).
- iPhone: in the Files app, long-press the file and tap Get Info. In Photos, swipe up on a photo to see its details on recent iOS versions.
- Android: in most gallery apps, open the photo and tap the details or info button.
- PicShrinker: shows the original and compressed size, plus pixel dimensions, for every file you add.
Frequently asked questions
How many KB is 1 MB?
1000 KB in the decimal system used by most websites and macOS, or 1024 KB in the binary system Windows uses. For everyday purposes, 1 MB ≈ 1000 KB.
Is 500 KB bigger than 1 MB?
No. 500 KB is half a megabyte, so it is smaller than 1 MB.
How can I make a photo under 1 MB?
Open PicShrinker, type 950 into the custom size box and press Compress. Most phone photos reach that size with no visible loss.
Why is my PNG so much bigger than the JPG?
PNG is lossless: it keeps every pixel exactly. That suits logos and screenshots, but photos have so much fine detail that a PNG is often five to ten times larger than a good-quality JPG.