Originally Posted by
LarDasse74
Since you don't own calipers, I suggest buying some calipers. A basic set is a lot less expensive than buying an extra seatpost in the wrong size.
+1 Buy a digital caliper. I picked one up at Harbor Freight for $10. That is a lot cheaper than buying seat posts that are the wrong size, and risking damage to your bike. (edit: mistyped micrometer, meant caliper).
+1 Seat posts usually come in 0.2mm size increments. And you have to get the right size (if a bike is sized for a 27.2mm seat post, then a 27.0 and a 27.4 will not work). So there are a LOT of different sizes.