If I have straight piece of PE or styrene and I measure the length do I or divide this length (circumference) by pi?

I can't quite follow now...
Do you mean how to calculate the diameter of a (circular) object if you know the circumference, or what?

If you mean the first, you can calculate it using the circumference formula c = π d, where c is circumference and d is diameter; Therefore, d = c / π.

But I can't quite see the reason for this, isn't it easier to just measure the diameter of an object using a caliper or something, that fiddle with the circumference to calculate the diameter.
But perhaps I didn't get the idea in the first place...

to clarify: I intend to roll the straight piece into a circle (hinge) so would like an idea of what size wire to use. hopefully more clear

Are you going to make a round rod of a square one? If you only need a small piece for a hinge perhaps stretching some sprue would do the trick?

Cheers

What I have is a hinge set form Aber if you are familiar with these you know that there are, in this case two straight pieces that are meant to wrapped around a round piece of wire or sprue to make the hinge. What I would like to know is by first measuring the straight pieces BEFORE I roll it can I calculate the diameter of the wire I need to insert. E.G. if the length of the two straight pieces are 2 mm when I roll them what would the diameter be for the wire I need to roll them on. Hopefully this makes sense if not I might need to introduce pictures

Oh, those things, now I know exactly what we're dealing with.
If the instructions don't give any clues about wire diameter, then I would experiment with different kinds of wire to see which one would look the best. It's not that exact science.
I don't if the sets are designed that the hinges are measured exactly to the lenght to make for the circumference of the needed wire.

If you do wish to do calculations, the circumference formula will do the trick.
d = c / π
The diameter of the wire is equal to the circumference (the lenght of the hinge part to be rolled up) divided by pi.

Thanks!!

In a similar situation (barrel rifling) I use a piece of paper cut to the same size as the PE, and experiment with that first.