KB vs MB: What's the Difference and Why It Matters for Uploads

A kilobyte (KB) is 1,024 bytes and a megabyte (MB) is 1,024 kilobytes, so 1MB equals 1,024KB. A 500KB file is therefore about half a megabyte, and a form that asks for "minimum 1MB" will reject a 900KB file. That single conversion causes most confused upload errors.

Why your computer and the upload form sometimes disagree

There are two conventions in circulation. The binary convention, used by Windows and by nearly every upload validator, treats 1KB as 1,024 bytes. The decimal convention, used by macOS since 2009 and by hard drive manufacturers, treats 1KB as 1,000 bytes. A file macOS reports as 105KB may be 102.5KB to the server checking it.

The gap is small at kilobyte scale — about 2.4% — but it widens as files grow. At megabyte scale the difference is roughly 4.9%, which is enough to fail a "must be at least 2MB" check on a file your Mac insists is 2MB.

How to read a size requirement safely

When a form states a minimum, aim a little above it rather than exactly at it. If it wants at least 100KB, produce 105KB. If it wants at least 1MB, produce 1.1MB. The margin costs nothing and absorbs any disagreement between conventions.

When a form states a maximum, aim below it by the same logic. A file that is exactly at the ceiling is the one most likely to be rejected by a validator using a different definition of the unit.

Checking the real size of a file

On Windows, right-click the file, choose Properties, and read the "Size" line rather than "Size on disk" — the second figure includes filesystem block padding and is usually larger. On macOS, select the file and press Command+I; the Info panel shows the byte count in parentheses, which is the number that actually matters.

If you need to move a file from one side of a threshold to the other, the file size tool on the homepage sets an exact figure in KB or MB rather than leaving you to guess with a quality slider.