The best tool I ever bought for working on bikes was a Digital Caliper. I found one for $35 at Auto Zone that reads both metric and inches.

It's still easy to mis-measure a seatpost though so you might as well take it to a bike shop and let them check it with their tools.

Edit: I meant it's still easy to mis-measure a seat tube when you don't have the original post to measure instead. The tubes usually aren't as round as a post.