Why exact sizes matter
Most of the time a file's size is a side effect. When you are testing, it is the whole point. An upload form that claims a 10 MB limit should accept a file of exactly 10 MB and reject one a single byte over, and the only way to know which it does is to feed it files built to the byte. This tool assembles a file of precisely the size you ask for, in your browser, and verifies the finished size before offering the download. A KB here means 1,024 bytes and a MB means 1,048,576 bytes, the binary convention; the data size converter explains how that differs from the decimal units printed on hard drive boxes.
Worked examples
Random bytes, zeros or text?
The filling changes what the file is good for. Random bytes, drawn from the browser's cryptographic generator in 64 KB chunks, cannot be compressed, which makes them the honest choice for bandwidth and transfer-speed tests: a network or backup tool that silently compresses data will fly through a file of zeros and give you a wildly optimistic number. Zeros are generated fastest and are fine when only the size matters, such as probing an upload limit. Repeating text gives you a file you can open in any editor and recognise instantly, useful when a test involves a human checking the right file arrived.
Whatever the filling, the output is inert. There is no executable content and no metadata, just bytes. If you need to prove a file arrived intact after a transfer, generate it here, then run both copies through the file hash checker: matching hashes mean identical files, byte for byte.
Built and checked on your device
The file is constructed in memory from 64 KB chunks whose lengths are arranged to sum to exactly the requested size, then assembled into a single download. The verified row reads the size back from the finished file itself rather than trusting the arithmetic, so a discrepancy would be caught and shown. Because everything lives in memory until you save it, very large files are capped at 512 MB and you will see a warning above 100 MB; on a phone, staying under that warning line is kind to your browser.
Nothing about this process touches a server. The file is created on your device, the download is served from your own memory, and closing the tab discards everything. That also means it works offline once the page has loaded. The default filename is test.bin, but you can name the file anything that suits the system under test: a .dat for an import routine, a .txt for the repeating text fill, or a long awkward name to test how an upload form copes with one.
Frequently asked questions
Is the file really exactly the size I asked for?
Yes, to the byte. The file is assembled from chunks whose lengths add up to precisely the requested size, and the actual size row is read back from the finished file rather than from the request, so what you see is what was built. A KB here is 1,024 bytes and a MB is 1,048,576 bytes.
Which content type should I choose?
Random bytes for anything involving compression or transfer speed, because random data cannot be compressed and gives honest results. Zeros for the fastest possible generation when content is irrelevant. Repeating text when you want a file you can open and recognise in a text editor.
Is the file safe?
Completely. It contains only inert bytes: random values, zeros or plain text. It is not executable and carries no metadata. It is generated on your device, so nothing is downloaded from the internet and nothing you create is uploaded anywhere.
Why is there a 512 MB cap?
The file is built in your browser's memory before it can be saved, and both the chunks and the finished file exist together briefly. 512 MB is a size that modern desktop browsers handle comfortably; beyond that, tabs on lower-memory devices start to fail unpredictably. Above 100 MB the tool shows a memory warning for the same reason.
What are dummy files actually used for?
Testing upload size limits on forms and APIs, checking backup and sync software, measuring real transfer speeds across networks or USB drives, filling storage to test low-space behaviour, and creating placeholder attachments. Anywhere you need a file of a known, exact size rather than a particular content.