Bicycle Mechanics - What seatpost size?

Ok, I have kind of a stupid question.

On my MTB, a 2000 Trek 8500 LT I have a crappy suspension seatpost I want to get rid of, and replace with a nice Thompson. The only problem is, I'm not sure what size (diameter, not length) to get because the old seatpost doesn't have any markings relating to size. I know I can measure it, but when you are working with units as small as millimeter, I don't want to take a chance.

If you know what size I should get, or know how I can figure out (preferably without having to leave home) please let me know.

Check Sheldon's website. He has been compiling a table of seat post diameters.

Dude..............take it out and measure it.........geezzzzzzzz ;)

27.2 is the correct diameter

http://www.sheldonbrown.com/seatpost-sizes.html

Take out old seatpost, read measurement stamped just below minimun insertion mark, make note of correct size, buy new seatpost of correct size, installed seapost of correct size, and install and adjust saddle.

The measurement is in tenths of a mm. You can't do that without precision measuring tools, like calipers, but with calipers, it's a snap. Take it to a bike shop or any good mechanic, they'll be able to tell you the size.

Thanks everyone, I appreciate the help.

I kinda figured I wouldn't be able to just take out a ruler and eyeball it, which is why I asked. I appreciate the input.

The measurement is in tenths of a mm. You can't do that without precision measuring tools, like calipers, but with calipers, it's a snap. Take it to a bike shop or any good mechanic, they'll be able to tell you the size.

It is standred practice in the industry to stamp the post size just below the minimum insertion line. I've never seen a post without the size stamped in that place.