Help which ruler is right

What is the best ruler to use for measuring I want to get the metal one to be more accurate when measuring. Now I have a rubber looking one you know the one kids use in school that bends and I have the paper one but I want to measure correctly cause sometime I measure and looks like it’s 7in then sometime 6 and a little bit, so I want to make sure that I have the right size. I say am 6 and a little bit like 3/4 or 1/3 some were in there any help please or it just don’t matter which one I use.

The hardest ruler you can find that doesn’t bend/wobble

Thin metal ruler is best but use the same ruler every time you measure. Otherwise you will get variations.

I use some string and an online ruler. Here is the one I use iRuler.net - Online ruler. Hope that helps