How do you measure a curling iron, to know what size it is?

I'm a fifteen year old girl and my mom used to curl my hair with a curling iron when I was little, but it's been years! I've been looking into curly styles, and I want to use a one inch curling iron. The one I have (which is like eight or nine years old, but still works ok) might be a one inch, but I'm not sure.... How do I know? What part do I measure? The part that clamps down? Or the whole thing? HELP!How do you measure a curling iron, to know what size it is?
post a picture, or look up the model online bc it will usually say on the package and if its sold online youll find it.How do you measure a curling iron, to know what size it is?
1inch curling iron:

http://www.stylebell.com/v/vspfiles/phot



1/ 1/2

http://ecx.images-amazon.com/images/I/31
Well it says on the packaging, so if you still have that you should look. Measure around the whole iron to see if it's one inch.
the box will say what size you should buy a new they are much safer than the old ones,they dont do much damage any more the old ones fry your hair

advice your mother gave you