KB and MB: why a file that looks small can still be too big

What KB and MB mean

File sizes are counted in bytes. KB and MB are just shorter ways of writing big numbers of bytes.

The “k” in KB is the metric prefix kilo, and the “M” in MB is mega. The International Bureau of Weights and Measures (BIPM) defines kilo as 10³ (a thousand) and mega as 10⁶ (a million). So:

• 1 KB = 1,000 bytes
• 1 MB = 1,000,000 bytes = 1,000 KB
• 500 KB = half of 1 MB, and 200 KB is a fifth of 1 MB

The US National Institute of Standards and Technology (NIST) is clear that this is the only correct meaning. Its guide to the SI says it is unacceptable to use kilo to mean 1,024.

The 1,024 version

Computer professionals started using “kilo” to mean 1,024, because 2¹⁰ (1,024) is very nearly 1,000. NIST says the confusion that followed is real. In December 1998, the International Electrotechnical Commission (IEC) approved separate names for binary multiples, as NIST explains:

• 1 KiB (kibibyte) = 1,024 bytes
• 1 MiB (mebibyte) = 1,048,576 bytes

Not all software uses the new names. Some shows a 1,024-based size and still labels it “KB” or “MB”. Apple, for example, notes that some apps and operating systems, including iOS 10 and earlier and OS X Leopard and earlier, report storage in the binary system.

So the same file can show two different numbers. Here is how that catches people out:

• A file of 510,000 bytes is 510 KB counted in thousands. Counted in 1,024s it is about 498. If your computer shows “498 KB”, it looks under a 500 KB limit, but a form that counts in thousands sees 510 KB.
• A file of 2,050,000 bytes is 2.05 MB counted in millions. Counted in 1,048,576s it is about 1.96. It can look under 2 MB and still be over a 2,000,000-byte limit.

Other reasons a small-looking file is too big

Units aren’t the only trap. Check these too:

• Rounding. A size shown with one decimal hides the rest. 2,040,000 bytes rounds to “2.0 MB”, but it is over a 2,000,000-byte limit.
• KB and MB mixed up. A limit of 500 KB is not 5 MB, or even 1 MB. A 1.2 MB photo is more than twice as big as a 500 KB limit allows.
• The wrong file. If you made a smaller copy, make sure it is the copy you pick in the form, not the original.

You can’t always know how a form counts. The safe answer is to make a file that is under the limit whichever way it is counted.

How Make My File Fit counts

Make My File Fit uses the decimal meaning everywhere: 1 KB is 1,000 bytes and 1 MB is 1,000,000 bytes. You can set any limit from 1 KB to 20 MB.

• The tool measures the real file it made, and aims a little under your limit. A copy is only called a fit when it is under the limit.
• Sizes on screen are rounded up, never down, so a size is never shown smaller than it is.
• On the Done screen after a fit, the result also shows the copy’s exact size in bytes, so you can compare it with the form yourself.

Counting in thousands is also the safe choice. A form that counts in 1,024s allows slightly more: 2 MiB is 2,097,152 bytes, which is more than 2,000,000. So a copy under the decimal limit is under the binary one too.

A quick check before you upload

1. Read the limit on the form, and note whether it says KB or MB.
2. Turn it into bytes: multiply KB by 1,000, or MB by 1,000,000.
3. Compare it with your file’s exact size in bytes, not a rounded one.
4. If the file is over, or too close to call, make a copy under the limit.

If the form still says the file is too large, see “File too large” when uploading for the other things to check.