How big is 100 KB, really?

The quick answer

100 KB means one hundred kilobytes. A kilobyte is a thousand bytes (or, in some places, 1,024 bytes), and a byte is the space of one small character. So 100 KB is about 100,000 bytes, a very small file by today's standards. For a sense of scale, a 4-page colour scan from an office scanner, like our sample, is 9.96 MB. A form that allows 100 KB is asking for something about one hundredth of that.

If your PDF is under 100 KB by the stricter way of counting, it is under 100 KB on every website, however that website counts. The rest of this post explains why there are two ways of counting, why one file can show three different sizes, and how to check yours.

Bytes, KB, MB and GB in one table

A byte is the basic unit of digital storage. Everything bigger is a multiple of it. The prefix says how many: kilo for a thousand, mega for a million, giga for a billion. Here is the ladder, written the strict, decimal way:

The gap between the two columns grows with each step: about 2.4% at the kilobyte, about 4.9% at the megabyte. For a 100 KB limit, that gap is 2,400 bytes, which is why it shows up so often: a file can be a little over 100 KB by one count and a little under by the other.

Why there are two ways to count a kilobyte

Computers store things in powers of two, and 1,024 is two multiplied by itself ten times. Early software called that a kilobyte, since it was close to a thousand. Storage makers and many modern systems prefer exactly 1,000, because it matches how kilo means a thousand in every other unit, such as a kilometre. Both habits stayed, and both are still written KB.

As a general pattern, Windows counts 1,024 bytes to a KB, while Macs count 1,000. Phone file apps and websites differ from one to the next. A form's own upload checker can use either. You usually can't tell which, and the form doesn't say.

That is why we take the safe side. On our tools, "under 100 KB" means at most 100,000 bytes. Anything that small is under 100 KB by both counts, so there is no need to aim for 95 KB "to be safe". The sizes our pages print after a file, like "98 KB", are counted in 1,024-byte kilobytes, the way many file apps count.

Why one file shows different sizes

Here is a real example from our tests. One result was a file of 99,933 bytes. It is the same file in all of these places, and all of them are right:

The file is under 100,000 bytes, so it passes either way. Four things cause most of the confusion:

• Different counting, as above.
• Rounding. One app writes 98 KB, another 97.6 KB. The extra decimal is just more detail.
• Size and size on disk. Some computers show two numbers for a file. "Size" is the real length of the file. "Size on disk" is the room it takes in storage, which is always a little more because storage is handed out in blocks. A form checks the real length.
• A different file. The original and the compressed copy have different sizes, and it is easy to look at the wrong one. Our tools add the limit to the new file's name, like marksheet-100KB.pdf, to help.

KB, Kb, MB, Mb and "Mbps"

Capital and small letters matter. A small b means bit, and a capital B means byte. A byte is eight bits. So:

• KB and MB measure the size of a file, in bytes.
• Kb and Mb are kilobits and megabits. They show up in internet speeds, like Mbps for megabits per second. A speed of 8 Mbps moves about one megabyte of file every second, if nothing slows it down.

A form that says 100 KB means bytes. If you see "100 Kb", it is almost certainly a typing slip for KB, but check the notice around it. File sizes are always given in bytes.

Your data pack is counted in bytes too: in MB or GB. That is why a tool that works on your phone matters. Once a page like ours has loaded, converting a file doesn't use your mobile data, because nothing is uploaded. Google's ad code still loads with the page, so the visit itself isn't free of data.

What fits in 100 KB? Some real files

File size depends on what the file holds. Typed words are tiny. Pictures are heavy. These files are all from our tests:

Two lessons. First, a typed PDF can be small without any help, but it can't be squeezed much below its text and fonts. Second, a scan can always be made small, because it is a picture, and the price is detail. The detail you keep depends on the number of pages that share the 100 KB: how many pages fit in 100 KB has the full table.

The post Why is my PDF so large? explains the causes of a big file, and Why won't my PDF go under 100 KB? covers what to do when a file refuses to fit.

Common limits, side by side

Forms mix KB and MB, and some write decimals like 0.1 MB. This table turns the usual limits into bytes the strict way, so you can compare them:

If a form gives a range, such as "50 KB to 100 KB", the same rule applies: stay inside it by the strict count. For a range, aim for the top. A bigger file is clearer, and nobody gains from a smaller one. Compress PDF to 100 KB keeps trying until the file is within 15% under your limit, so it usually lands between 85 KB and 100 KB.

For limits other than 50, 100 and 200 KB, Resize PDF lets you type any size, and Compress PDF to 200 KB is the page for 200 KB.

How to check a file's size

Menus differ from phone to phone and system to system, so we don't give exact paths. In general:

• On a phone, open the Files or file manager app, find the file and look for Details, Info or Properties. Many apps show the size there.
• On a computer, find the file, right-click it (or Control-click on a Mac) and choose Properties or Get Info. Some computers also show the exact number of bytes in brackets, which is the safest number to compare with a limit.
• On our tools, the result line shows the size before and after, with "Under 100 KB" in green when it fits.

Check the file you will upload, not another one. After a compress step, that is the new file with -100KB in its name, not the original.

Mistakes that make a file look the wrong size

• Renaming a file doesn't change its size. Putting "100KB" in a name only helps you remember. The number of bytes decides.
• Changing the ending doesn't convert the file. Renaming photo.jpg to photo.pdf gives you a broken PDF, not a PDF made from a photo. Make a real PDF with JPG to PDF.
• A ZIP is a different file. A ZIP holding your PDF has its own size, and a form that wants a PDF usually refuses a ZIP.
• A screenshot of the document makes a picture, not a PDF, with its own size. Convert it into a PDF properly if the form asks for one.
• Squeezing a file that was already squeezed loses detail for good. Always go back to your first scan.
• Waiting for the last hour. If a file needs a second try, you want time for it.

Your files stay on your device

A file that needs to be under 100 KB is often a private paper: an ID card, a marksheet, a bank page. Our tool has no upload step and never sends your file anywhere. The page contacts only this site and Google's ad service, which shows the ads that keep the tools free. Our code never gives your files to the ad code. The privacy policy says more.