What "file must be under 100 KB" actually means: KB vs MB, bytes, and why forms cap uploads

Your file says 100 KB in the folder. The form says the limit is 100 KB. The upload is rejected anyway, with a message that the file is too large. Nothing is broken: the folder and the form are counting kilobytes differently, and a file that sits right on the line will pass one count and fail the other. This guide explains the two definitions, shows where the real number is, and gives you a margin that works on every portal. If you just need the file fixed, Compress Image to 100 KB does it in the browser, and Compress PDF to 100 KB does the same for documents.

Two kilobytes

A byte is the only unit everyone agrees on. Above that, two systems have been in use for decades. The metric system says kilo means 1,000, so 1 kB is 1,000 bytes and 1 MB is 1,000,000 bytes. Early computing borrowed the prefix for 1,024 (2 to the power 10) because it was close enough, and that habit stuck in operating systems and programming languages. The IEC standardised separate names for the binary sizes in 1999 (kibibyte, KiB; mebibyte, MiB), and NIST recommends them, but most software still prints “KB” and means whichever it likes.

Unit as printed	Decimal reading	Binary reading	Who uses which
1 KB	1,000 bytes	1,024 bytes	Decimal: SI/NIST, macOS Finder, drive manufacturers. Binary: Windows Explorer, PHP, most upload tools
100 KB	100,000 bytes	102,400 bytes	Difference of 2,400 bytes — the whole reason this guide exists
1 MB	1,000,000 bytes	1,048,576 bytes	Difference of about 4.9 %
2 MB	2,000,000 bytes	2,097,152 bytes	The default PHP upload cap is the binary 2M

Apple switched Finder and iOS to the decimal reading with Snow Leopard and iOS 11; anything earlier used binary. Windows has never switched: File Explorer and the Properties dialog still divide by 1,024, which is why the same photo shows as “102 KB” on a Mac and “100 KB” on the PC next to it.

What the form is really checking

A browser upload control hands the page a File object whose size property is a plain integer of bytes. The person who built the form then wrote a comparison against a number they typed in. Sometimes that number is 100 * 1024 (102,400). Sometimes it is 100000. Sometimes the browser check is generous and the server behind it is not, because the server has its own limit in a different unit: PHP, common behind smaller portals, reads a limit written as 100K as 102,400 bytes, while 100000 is exactly that. From outside you cannot tell which reading a portal uses, and the error message says “100 KB” either way.

That is why files that show “100 KB” in a folder fail so often. A folder rounds. A file of 101,900 bytes is “99.5 KB” on Windows (101,900 ÷ 1,024) and “102 KB” on a Mac. Against a decimal check it is 1,900 bytes over. The form is not wrong; the rounding hid the overshoot.

Finding the number that matters

Ignore the rounded figure and read the byte count.

On Windows, right-click the file, choose Properties, and look at the Size line: it reads something like “99.5 KB (101,900 bytes)”. The number in parentheses is the truth. On a Mac, select the file and press Command-I; Get Info shows “102 KB on disk (101,900 bytes)”. On a phone, most gallery apps show only a rounded size, so use a file manager that lists bytes or simply compress with more margin. The compression tools on this site display the output size before you download, so you can re-run at a smaller target if the result is too close to the line.

Why a portal caps uploads at all

Limits look arbitrary from the applicant’s side, but three practical reasons produce them, and knowing them tells you how much room to leave.

The first is scale. A photograph printed on an admit card at roughly 3.5 × 4.5 cm needs only a few hundred pixels on each side, and a JPEG of that size is comfortably under 50 KB. Storing a 4 MB camera original for every applicant would cost eighty times as much for no benefit, so the portal caps the upload near what the printed output needs. That is where the 20–50 KB photo windows on IBPS and similar forms come from: they are pixel budgets expressed as byte budgets.

The second is the plumbing. Web servers and frameworks ship with default upload limits, and many portals simply inherit them: PHP’s default upload_max_filesize is 2M and its post_max_size is 8M, which is why “2 MB” is such a common ceiling on smaller sites. A limit that looks like a round number in one unit is often a default in another.

The third is legibility, and it works in the other direction. Some forms set a minimum size as well. IBPS asks for a signature of 10–20 KB and a handwritten declaration of 50–100 KB; a file under the floor has almost certainly been compressed into an unreadable smear, and the minimum rejects it before a human has to. If you hit a minimum, the fix is higher JPEG quality or slightly larger dimensions, not padding.

Landing under the limit with room to spare

Treat every stated limit as the decimal figure minus a safety margin, and you will not meet the problem again. For a 100 KB cap, aim for 90–95 KB; for 50 KB, aim for 45; for 20 KB, aim for 18. The margin costs nothing visible at these sizes and covers the decimal-versus-binary gap, metadata the encoder adds, and any difference between the browser check and the server check.

For an image, open Compress Image to 100 KB, drop the photo in, and read the size it reports. The tool searches JPEG quality settings for the highest one that fits, and reduces dimensions only if quality alone cannot get there. One thing to know: the size targets on this site use the binary reading, so “100 KB” means at most 102,400 bytes. If a portal rejects that output, it is counting in thousands; use the next target down, such as Compress Image to 50 KB, or the general Image Compressor where you can set the target yourself. For scanned documents, Compress PDF to 100 KB re-encodes the page images the same way, and the PDF Compressor handles larger targets. All of these run in your browser; the file is not uploaded to a server.

Two habits make the compression itself easier. Resize before you compress: a 4,000-pixel photo cannot reach 50 KB without visible artefacts, but the same photo at 600 pixels wide can. And crop to what the form wants; a signature scanned as a full A4 page spends most of its bytes on blank paper.

Common caps, translated

The form says	Binary bytes (what most tools produce)	Decimal bytes (what some servers check)	Aim for
20 KB	20,480	20,000	18 KB
50 KB	51,200	50,000	45 KB
100 KB	102,400	100,000	90–95 KB
200 KB	204,800	200,000	185 KB
500 KB	512,000	500,000	470 KB
1 MB	1,048,576	1,000,000	950 KB
2 MB	2,097,152	2,000,000	1.9 MB

The “aim for” column is a rule of thumb, not a specification. If the portal’s own instructions state a size in bytes, that number wins over anything here.

Checklist

• Read the byte count in Properties or Get Info, not the rounded KB in the folder.
• Assume the strictest reading: 1 KB = 1,000 bytes, and leave 5–10 % headroom.
• Resize and crop before compressing; bytes follow pixels.
• If a form has a minimum size, raise quality or dimensions rather than padding the file.
• Check the output size the tool reports before downloading, and re-run at a smaller target if it is within a few hundred bytes of the cap.
• The on-screen instructions of the form are the final authority; when they conflict with this guide, follow the form.