How to create a test file of an exact size, and find where an upload limit really is

The short answer. 5,242,880 bytes, exactly, when you ask the Dummy File Generator for 5 MB. Choose TXT, BIN, CSV, PDF, JPG, PNG or ZIP, tap a size or type any number of bytes, KB, MB or GB up to 1 GB, and press Make file. The file is made on your device and is a valid file of its type.

An upload form that says "under 2 MB", an email server that bounces attachments over 25 MB, an API that refuses large bodies: each needs a file at its limit and one just past it. A real document is the wrong test file. Its size is whatever it happens to be, and it may hold something private. A padded dummy is the size you asked for and holds nothing.

How to make one

1. Open the Dummy File Generator and choose the type the form expects.
2. Tap 100 KB, 1 MB, 5 MB, 10 MB, 100 MB or 1 GB, or type any size and pick B, KB, MB or GB.
3. Read the line under the choices: it says what the file really is and the smallest size that type can be.
4. Press Make file. A large file is written to disk as it is made, with progress and a Cancel button.

KB, MB and GB count in 1,024s, as Windows and most upload limits do. Under More options you can switch to 1,000s, as macOS and disk makers count: then 2 MB is 2,000,000 bytes instead of 2,097,152.

A worked example: where is a "2 MB" limit?

A form says "PDF, maximum 2 MB". That could mean 2,000,000 bytes or 2,097,152, and 97,152 bytes is a lot to be wrong by. Make four PDFs, typing each size in bytes:

Size typed	File you get	Accepted means
2,000,000 B	dummy-2000000B.pdf	the limit is at least 2,000,000 bytes
2,000,001 B	dummy-2000001B.pdf	the limit is not decimal 2 MB
2,097,152 B (2 MB)	dummy-2MB.pdf	the limit is at least binary 2 MB
2,097,153 B	dummy-2097153B.pdf	the limit is above 2 MB, or not checked at all

Upload them in order. Where the first refusal comes is the real limit: refused at 2,000,001 means 2,000,000 bytes; refused at 2,097,153 means 2,097,152. If a form refuses 2,000,000 bytes, it is probably checking "less than" rather than "up to", and a file of exactly 2 MB will never pass.

Each one is a real PDF: one A4 page saying it is a dummy and giving its size. In dummy-2MB.pdf that page and the PDF's structure take 866 bytes, and the other 2,096,286 bytes are padding in a stream no page draws, so it opens instantly in any reader.

What each type really is

Type	What is inside	Smallest size
TXT	One readable line, repeated	1 byte
BIN	Zeros, or random bytes	1 byte
CSV	A header and numbered rows of Test Users at example.com	65 bytes
ZIP	One uncompressed entry, dummy.txt	116 bytes
JPG	A 16 × 16 grey image; padding in comment segments	163 bytes
PNG	A 16 × 16 grey image; padding in a text chunk	360 bytes
PDF	One A4 page; padding in a stream no page draws	850 bytes

A size below the minimum is refused with the minimum stated: a 500-byte PDF cannot be made, because its fixed structure alone is larger. A 100 KB CSV holds 1,604 data rows below its header, every email at example.com so none can reach a real person. For chosen columns and seeded data, use the Mock Data Generator.

There is no MP4. A video padded to size would not play, so a test that passed with it would pass for the wrong reason.

Big files and speed tests

A 1 GB file is written to disk in 128 pieces of 8 MB as it is made. On Chrome and Edge on a computer you choose where to save it and the pieces go straight there, so memory use stays small; elsewhere they go to the browser's private storage on disk first. The Create Large Test File page opens on BIN at 100 MB.

For a transfer speed test, choose Random bytes rather than Zeros. Zeros compress to almost nothing, so a zip or a compressed connection can make them look far faster than real data. Random bytes come from your browser's cryptographic source and do not compress.

Questions people ask

How do I create a file of a specific size?

Open the Dummy File Generator, choose a type such as PDF or TXT, tap a common size like 1 MB or type any size in bytes, KB, MB or GB, and press Make file. The file is written on your device and is exactly that size, down to the byte: 1 MB is 1,048,576 bytes, up to 1 GB.

Is 1 MB 1,000,000 or 1,048,576 bytes?

Both are in use. Windows and most upload limits count 1 MB as 1,048,576 bytes; macOS and disk makers count 1,000,000. The Dummy File Generator uses 1,024s by default, with 1,000s under More options. To learn which a form uses, upload files of 2,000,001 and 2,097,153 bytes and see which one it refuses.

How do I test an upload size limit?

Make one file exactly at the limit and one a single byte over it, of the type the form accepts. For a 2 MB PDF limit, that is 2,097,152 and 2,097,153 bytes, plus 2,000,000 and 2,000,001 in case the limit is decimal. The first file refused shows where the limit really sits.

Can I open the dummy file?

Yes. Each is a valid file of its type: the PDF opens as one A4 page saying it is a dummy and giving its size, the JPG and PNG show a 16 by 16 grey square, and the ZIP holds dummy.txt. The extra size is padding stored where readers skip it, so the files open quickly.

Can a phone make a 1 GB file?

Usually, if it has 1 GB of free space. The file is written to disk in 8 MB pieces as it is made, 128 of them for 1 GB, either to the place you choose or to the browser's private storage, so memory use stays small. Progress and a Cancel button stay on screen.

Sources

Checked 15 Oct 2026. Every size, minimum and file name on this page is the Dummy File Generator's own output, from its functions:

• The unit conversion (1,024 by default, 1,000 under More options), the 1 GB maximum (1,073,741,824 bytes) and the 8 MB pieces.
• The smallest size of each type, the file names, the PDF's 866 bytes of structure at 2 MB, and the 1,604 rows in a 100 KB CSV, counted from the generated files.