How to convert KB to MB — and work out how big a file really is

Published 12 October 2026 · SepoooTools blog

An upload form says "maximum 2 MB". Your photo says 1,950 KB. Will it fit? Most of the time the answer is easy. Sometimes it is not, because computers use two different ways to count. This guide explains both, with simple examples you can check on paper.

The short answer

In everyday use, one megabyte (MB) is 1,000 kilobytes (KB).

KB to MB: divide by 1,000.
MB to KB: multiply by 1,000.

You have	Calculation	Result
350 KB	350 ÷ 1,000	0.35 MB
1,950 KB	1,950 ÷ 1,000	1.95 MB
1.5 MB	1.5 × 1,000	1,500 KB
0.2 MB	0.2 × 1,000	200 KB
2 MB	2 × 1,000	2,000 KB

The same step works one level up: 1 GB is 1,000 MB, and one level down: 1 KB is 1,000 bytes.

Why you sometimes see 1,024 instead of 1,000

Computers work in powers of two. For a long time, people used "kilobyte" to mean 1,024 bytes, because 1,024 is 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2. Later, official standards fixed the names:

kB (kilobyte) = 1,000 bytes. MB = 1,000,000 bytes.
KiB (kibibyte) = 1,024 bytes. MiB (mebibyte) = 1,048,576 bytes.

The problem: not every program follows the new names. Windows still divides by 1,024 but writes "KB". A Mac divides by 1,000 (Apple switched with OS X Snow Leopard). So the same file shows two different numbers.

Example: a photo has exactly 5,821,027 bytes.
On a Mac: 5,821,027 ÷ 1,000,000 = 5.82 MB.
On Windows: 5,821,027 ÷ 1,048,576 = 5.55 MB.
Same file, same bytes. Only the maths is different.

Another small example: a file of 65,536 bytes shows as "64 KB" on Windows and as "66 KB" on a Mac.

Which one does an upload form use?

You usually cannot know. A limit of "200 KB" might mean 200,000 bytes or 204,800 bytes. A limit of "2 MB" might mean 2,000,000 or 2,097,152 bytes. The difference is small, but it is enough to make a file fail by a few kilobytes.

The safe rule is simple: aim for about 5% under the limit.

Limit 200 KB → aim for 190 KB or less.
Limit 2 MB → aim for 1.9 MB or less.
Limit 5 MB → aim for 4.75 MB or less.

Then your file fits, whichever way the website counts.

How to work out a photo's size before you save it

A photo is a grid of pixels. Each colour pixel needs 3 bytes when nothing is compressed: one each for red, green and blue. So:

Uncompressed size in bytes = width in pixels × height in pixels × 3

Example 1: a phone photo

A 12-megapixel photo is 4,000 × 3,000 pixels.
4,000 × 3,000 = 12,000,000 pixels.
12,000,000 × 3 = 36,000,000 bytes = 36 MB uncompressed.

Your phone does not save 36 MB, though. It saves a JPG (or a similar format), which compresses the picture a lot. How small it gets depends on the picture and the quality setting. A plain wall compresses better than a busy street.

Example 2: a passport photo

A 35 × 45 mm photo at 300 DPI is about 413 × 531 pixels.
413 × 531 = 219,303 pixels.
219,303 × 3 = 657,909 bytes ≈ 658 KB uncompressed.

So if a form wants under 100 KB, compression has to do most of the work. That is normal, and a good JPG at this size usually still looks sharp.

Example 3: a 10 × 15 cm print

If you know the print size in centimetres, convert to inches first (divide by 2.54), then multiply by the DPI.
10 cm ÷ 2.54 × 300 ≈ 1,181 pixels. 15 cm ÷ 2.54 × 300 ≈ 1,772 pixels.
1,181 × 1,772 = 2,092,732 pixels × 3 = 6,278,196 bytes ≈ 6.3 MB uncompressed.

Bits and bytes: why your internet speed looks bigger

Internet speed is measured in megabits per second (Mbit/s), not megabytes. One byte is 8 bits. So divide by 8.

Example: a 50 Mbit/s connection moves at most 50 ÷ 8 = 6.25 MB per second. A 10 MB file then needs at least 10 ÷ 6.25 = 1.6 seconds. In real life it is often slower, because upload speed is usually lower than download speed.

Your file is over the limit. Now what?

First check what the form is really limiting. If the message talks about KB or MB, the file is too heavy. If it talks about pixels, the picture is too big in width or height.

Too many KB or MB in a photo → Image Compressor.
Too many pixels → Image Resizer. Fewer pixels also means fewer KB.
A PDF that is too heavy → Compress PDF.
Many photos at once → Bulk Resize.
Not sure what is wrong? Read File too big to upload?

All these tools run in your browser. Your file is not uploaded to our server.

Tool idea for later: a tiny KB ↔ MB converter with a 1,000 / 1,024 switch and a "5% safety margin" button.

The short version

KB ÷ 1,000 = MB. MB × 1,000 = KB.
Windows divides by 1,024, a Mac by 1,000. The file is the same.
Stay about 5% under any upload limit.
Uncompressed photo size = width × height × 3 bytes.
Mbit/s ÷ 8 = MB per second.