Are you are making bias TAPE (not bias BINDING)? Then it will depend upon what method you are using to make it.
I use Clover's bias tape makers and cut my strips two (2) times the width of the finished bias tape. So, for 1/4", cut it 1/2" wide and for 3/8", cut it 3/4" wide.
If you are using some other tool, then check the directions that came with it.