Confused about curtain pole sizing

[limetrees32]

I want to order new curtain rings but seems to have a block about working out correct size for my poles

If a curtain pole size is given as 28mm how is this calculated?
I've measured the circumference of mine and divided by pi and got 32mm
But this image has confused me .
if

The 28mm is the diameter of the pole that the rings should go on. Try measuring your pole diameter instead

Thank you ,i think what i needed to know was that suppliers quote diameter .
Which is what you get when you divide the circumference by pi.
( Or that's what Google says!)

Normally a pole and rings come together. Poles are always measured in diameter.

Totally overthinking this one OP.
Just measure the diameter of the pole.

I'm an overthinker for sure.
In my defence the poles are up with curtains on them ( hung with nasty cheap rings which i want to change ) and they have finials on both ends which makes measuring the diameter of the rod tricky.