Start with bytes, not the rounded label

An upload form says “maximum 200 KB.” Your computer shows “200 KB.” The upload still fails. One possible explanation is that the two programs count units differently; another is that the visible size has been rounded. The exact number of bytes is the useful comparison.

In decimal units, 1 KB is 1,000 bytes and 1 MB is 1,000,000 bytes. In binary units, 1 KiB is 1,024 bytes and 1 MiB is 1,048,576 bytes. Some interfaces display KB when their calculation actually uses 1,024. A label alone is therefore not enough to resolve an ambiguous upload rule.

A worked 200 KB example

A 204,000-byte file is about 199.22 KiB, so an interface using binary arithmetic could show a number below 200. But it is 204 KB in decimal arithmetic and exceeds a strict 200,000-byte ceiling by 4,000 bytes. Neither compression nor the uploader has to be broken for that mismatch to happen.

Our image-under-200kb landing preset uses 200,000 bytes. The general workspace KB control uses 1,024 bytes per displayed unit, so changing that control to 200 produces a 204,800-byte target. Read the exact byte ceiling and output report if the destination is strict. The size calculator lets you compare both conventions before processing.

Choose a target that leaves room for uncertainty

If official documentation gives an exact byte count, use it. If it only says KB or MB, the smaller decimal interpretation is a conservative starting point. A modest margin below that value can help when the destination requires strictly less than the limit or counts additional packaging. A margin is a practical choice, not a guarantee.

  • Check the actual downloaded output, rather than the original file still in your Downloads folder.
  • Keep the destination’s required dimensions and format alongside its size limit.
  • If a file is already comfortably below the ceiling, investigate other rejection causes before compressing again.

Size is only one acceptance condition

An uploader may reject a correctly sized file because its MIME type, extension, pixel dimensions, encryption or content differs from the expected format. Renaming a .png extension to .jpg does not convert the file. Use a real conversion and inspect the output.

For images, allow resizing only if the destination permits it. For PDFs, retain the required pages and inspect small text. A successful technical size check is useful evidence, but it does not replace the destination’s current instructions.