When measuring a seat post, where are the measurements taken to determine if the post is a 300mm, 350mm, or 400mm post? From the insertion point, is it measured to the point where the post will no longer slip further into the seat tube, or is it measured to the center of the rail holes?

The reason I ask is because I want to replace the seat post in my son's bike with a longer one, but I have to figure out how long it is first. The post tells me that it is a 30.8 width post, but nothing on the length.

Thanks in advance.

Seatpost length is normally specified from the center of the saddle rail clamp holes to the very end of the post. The minimum insertion mark is typically 60 to 100 mm above the end.

Here is Thompson's dimensioned drawing showing how they are specified:

https://www.lhthomson.com/docs/update...0Dims_revA.pdf

Great! Thank you very much!