Bicycle Mechanics - Seatpost height

Hello to everyone,

Need some advice as far as the correct way to measure/determine the height of a seat post. I want to purchase a new seat post but I am confused as far as where to measure to determine what length I need. I would like to get a carbon one and don't want to cut it if it's too long. Thanks......

Measure the one you've got and get the same length. Basically, though, Road posts are 250mm and Mountain posts are 350. You should never have to cut a seat post; they are not like fork steerer tubes that you cut to accomdate the HS. The most important size in a seat post is the Diameter. Your old seat post should have the size embossed on it near the bottom. If not, you'll need to measure it in MilliMeters.

Thanks ljbike,
What I want to know is do I measure from the end of the post to where the seat rail attaches to the post? My current post is a little too short and I want to get a longer one but not too long. I need to get a real accurate measurement. Thanks for the reply....

If you have to raise your seat by changing the seatpost, then the handlebar stem is probably too short too. That would be a typical shortcoming.

That is, you can use a longer seat post, raise the seat, and find that even with the stem maxed out, the handlebars are still too low.

In most cases, you want the seat to be about level with the handlebars.

If you are replacing the seat-post, check the stem too just to be sure.

Contact the frame manufacturer because they will have a maximum on seatpost length allowed - then just buy that. Anything longer can cause frame damage. Trek's max on my bike was 360mm of post length.

An overly long seatpost makes a great lever to bend your frame.

Many manufacturers make seatposts well in excess of 400mm.

I'm sorry everyone, what I meant to ask is how do you measure a seat post LENGTH? Like, when a manufacturer says the post is 350mm long what points are the measurements taken from? Thanks for all of the info sent to this post; I learned a few more things about seat post installations.

The measurements are from the center of the seat clamp mounting bracket to the end of the post.

Thanks for the reply. Armed with this information I can go looking for just the right length seat post. The post I have now is about 35mm too short and I don't want too much excess post sticking out of the frame tube (full suspension bike) or else it might hit the rear shock on full compression. Thanks again for all the help......