#2 Invisible Zippers are 2mm across the closed zipper.
This is the size you would probably find at your local department store, unless they stock my invisible zippers.
It is the lightest weight invisible that I stock. If you need a little more "beef" try the #4. And if you are doing a slipcover or need a strong, heavy duty invisible use the #6.
Some manufacturers use other numbers for their zippers. On my site, a standard coil zipper or a dress zipper is either a #3 or #5. That is 3mm or 5mm across the exposed teeth. Invisible zippers are #2, #4, & #6 but other manufacturers call them #2, #3, & #5.
Too confusing for me!
If you are confused..... just give a call.