What size scope rings do i need for a 40mm scope?

I'm waiting on my new tc dimension to come in i decided i wanted a nikon prostaff 4-12x40 what size scope rings should i need

You should get medium or low rings (in either 1" or 30mm diameter, whatever the scope body is). Head into a gun store and ask to try the different heights out so you can see what feels most comfortable for you. Better than having to send back rings you've bought on the internet for a different set.

You want the scope as low as possible, it largely depends on the height if your rail. I would say you'll need medium or high rings.

What is the scope body diameter?

there's many kinds of scopes, objective lens diameter doesn't determine what kind of rings.

All of the Nikon Prostaff lineup with 4-12x magnification have 1" tubes. You need 1" scope rings.

If you will be mounting this scope on a bolt rifle with a rail, you probably want the lowest scope rings possible. If you will be mounting this scope on an AR-15, you probably want the highest scope rings possible. You'll have to do some experimenting with your rifle to determine how high above the bore your eye will be when you are in your usual shooting position and then you can make a determination about the height your scope rings need to be.