Often as a developer, for testing disk performance, or perhaps your internet or network connection speed, you need to generate a large file of a specific size. Windows includes a utility that allows you to quickly generate a file of any size instantly.

This is a command line (DOS) utility, so firstly, open a command prompt (as an administrator if you need to write to protected folders).

To launch a command prompt, hit start and just type command. You'll see "Command Prompt" appear as the closest match (Remember to right-click and choose "Run as Administrator" if you need admin priviledges). Alterntaively, use the key combination: [WIN]+R, and type cmd.exe to launch a command prompt.

Run the following command:

fsutil file createnew <file> <size in bytes>

For example, this command will create a 1GB file called 1gb.test in my temp folder:

fsutil file createnew c:\temp\1gb.test 1073741824

Note: the size of the file is specified in bytes. So, you'll need to be specific about the value you choose if you need a specific size.

Googling the converted value (e.g. 100gb in bytes) will give you a result, but to avoid answers with exponents, here are some pre-calculated equivalent values:

SizeSize in Bytes
1 MB1048576
100 MB104857600
1 GB1073741824
10 GB10737418240
100 GB107374182400
1 TB1099511627776
10 TB10995116277760

If you don't like using the command line there are various GUI utilities out there that can also do this for you:

Dummy File Creator: http://www.mynikko.com/dummy/

Disk Tools Toolkit: http://www.soft.tahionic.com/download-file-generator/