How do you measure a bicycle seat post?

I have just purchased a new seat for my bike and it doesn’t work with my current seat post so I have to buy a new one. Most of the bike posts are between 24 and 27.something mm. I measured the circumference of my current post and it’s 82 mm. Should be measuring diameter? and which part the top or the bottom? I looked online and cannot find the answer so I thought someone from here could answer.