KB vs KiB: Why a '20 KB' File Gets Rejected by Upload Forms

You resize a photo to 20 KB, your computer says 19.8 KB, and the upload form still says it is over the limit. Nothing is broken. You and the form are counting kilobytes differently, and the difference is 24 bytes per kilobyte, which is 480 bytes on a 20 KB file. This guide explains the two ways of counting, how to read a file's real size, and the one rule that makes a file pass every form: aim for the stricter number.

Two kilobytes

In the metric system, kilo means a thousand: a kilogram is 1,000 grams, a kilometer 1,000 meters. By that rule a kilobyte is 1,000 bytes, and that is how the International System of Units defines it.

Computers, though, count memory in powers of two, and 2¹⁰ is 1,024, close enough to 1,000 that early software borrowed the word. For decades "1 KB" meant 1,024 bytes in most operating systems and 1,000 in others, and nobody could tell which from the label.

In 1998 the International Electrotechnical Commission (IEC) gave the binary units their own names: a kibibyte (KiB) is 1,024 bytes, a mebibyte (MiB) 1,048,576 bytes. The names exist, and Linux tools often use them, but most everyday software still writes "KB" for both.

The gap grows with every step up, which is why a new hard drive "loses" space when you plug it in: the box counts in decimal, the computer in binary.

Why your computer and the form disagree

Windows File Explorer shows file sizes counting 1,024 bytes a KB, and labels them KB. A file of 20,480 bytes shows as exactly 20 KB.

Macs have changed. Apple's support page on how storage is measured names "OS X Leopard and earlier" among the systems that use the binary count; current macOS shows sizes in decimal. So the same file can read 20 KB on a Windows PC and 20.5 KB on a Mac.

Upload forms are written by whoever built the site, and they use whichever their code uses. Some compare the byte count with 20 × 1,000, others with 20 × 1,024. The page rarely says which. Government rules are usually written in plain words, "240 kB" or "no more than 10MB", without saying which kilobyte they mean.

The rule that always works

You cannot know how a form counts, but you do not need to. Aim for the stricter reading.

For a maximum, use 1,000 bytes a KB. A 20 KB maximum becomes 20,000 bytes. A file of 20,000 bytes or less passes a site counting 1,000 and one counting 1,024.

For a minimum, use 1,024 bytes a KB. A 10 KB minimum becomes 10,240 bytes. A file of 10,240 bytes or more passes both ways.

That is all. The cost is tiny: you give up 480 bytes on a 20 KB file, about 2%, which on a passport-size JPEG is a barely visible step in quality.

Our resizer reads every limit this way, so the number it shows always passes. Try your own file's numbers here:

How to read a file's real size

The size a file app shows is rounded and in one of the two units. The byte count is exact.

Windows. Right-click the file, choose Properties. The General tab shows something like "Size: 19.8 KB (20,300 bytes)". The number in brackets is the one that matters.

Mac. Select the file and press Command-I (Get Info). The Size line shows something like "20 KB (20,300 bytes)".

Linux. In a terminal, ls -l file.jpg prints the size in bytes; ls -lh rounds it in binary units.

Phones. Gallery and file apps usually show only a rounded size. If you are close to a limit, check the file on a computer, or use a tool that reports bytes. Our file checker reads the file on your device and shows both the KB and the exact byte count.

Where the numbers show up

A few real rules, quoted from their official sources, and what they mean in bytes:

Notice the UK rule: 10 MB is 10,000,000 bytes one way and 10,485,760 the other, a gap of nearly half a megabyte. A large photo straight from a camera can land in that gap.

The DV Lottery photo guide and the photo size rules pages go through each of these in full.

Two worked examples

A DV Lottery photo at 245,000 bytes. On a Windows PC, Explorer divides by 1,024 and shows 239 KB, under the 240 kB limit. The State Department writes "240 kB", the decimal symbol, which is 240,000 bytes. If the entry system compares against 240,000, the photo is 5,000 bytes over. Re-save it a little smaller, at 235,000 bytes say, and it passes on any reading. A 600 × 600 photo has plenty of room: at good quality it is usually well under 150,000 bytes.

A signature that must be 10 KB to 20 KB. The resized signature comes out at 10,100 bytes. Divided by 1,000 that is 10.1 KB, over the minimum; divided by 1,024 it is 9.86 KB, under it. A form that counts in binary will refuse it as too small, while your computer may show "10 KB". Bring it to at least 10,240 bytes, by allowing a slightly larger image if the form permits or by padding, and the problem is gone.

In both cases the file was "fine" by one count and not by the other. The safe targets, 240,000 and 10,240 bytes, make the question irrelevant.

Why minimum sizes exist, and how to meet them

A maximum is easy to understand: a site does not want to store huge files. Minimums surprise people. Forms set them to stop people uploading tiny, blurry thumbnails, especially of signatures and photos, which the site would then have to reject by hand.

The trouble is that a small, clean image, such as a signature on white paper, compresses very well. A 140 × 60 pixel signature can be 4 or 5 KB even at the highest JPEG quality. You cannot make it bigger by raising the quality any further.

There are two honest ways to reach the minimum:

Make the image bigger in pixels, if the form allows it. More pixels means more bytes. Often the form fixes the size, though.

Add padding inside the file. A JPEG can carry comment segments that viewers ignore. Adding empty bytes there makes the file bigger without changing a single pixel of the picture. That is what our resizer does when a file is under your minimum, and it says so on the page. The form checks bytes, and the bytes are there.

What does not work is saving the file again and again at "100%" in an editor: each save re-compresses, the size barely moves, and the quality drops.

MB limits on PDFs and documents

The same gap applies to megabytes, and it is bigger: almost 5%. Many portals cap documents at 1 or 2 MB. A 2 MB limit is 2,000,000 bytes or 2,097,152; a scanned PDF of 2,050,000 bytes passes one and fails the other.

Treat MB like KB: for a maximum, multiply by 1,000,000. If you need to shrink a scan to fit, our PDF compressor measures the whole finished PDF against the limit read that way, and the guide to fitting a PDF under a limit explains what it trades away.

A checklist before you upload

1. Read the limit on the form, and whether it is a maximum, a minimum or both.
2. Turn it into bytes the strict way: maximum × 1,000, minimum × 1,024.
3. Check the file's exact size in bytes in Properties or Get Info.
4. If it is over, resize or compress until it is under the byte target. If it is under a minimum, add pixels if allowed, or padding.
5. Check the file type too: a PNG renamed to .jpg is still a PNG, and forms that check the real type will refuse it.