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What are the standard measurement points for seatpost length?

I am buying a new seatpost, and need to compare my old post to the "marketed" length of possible replacement posts to make sure I don't buy one that's too short.

Options include:

  • The bottom of the seatpost to the rail bottom/center
  • The length of the post only (i.e. the bottom of the post to the bottom of the clamp)
  • something else?
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  • If in doubt, buy one that longer, can always cut it down (unless its a dropper post, that has a lot more to consider than length).
    – mattnz
    Sep 3, 2022 at 6:30

2 Answers 2

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Standard is bottom end to the saddle rails.

Minimum insertion length is included in this nominal length, but is not universal between posts, which can bite you in edge cases.

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I'm lazy - I simply read the nominal measurements off the stamping on the seatpost, and trust they're correct.

All seatposts will have their nominal diameter stamped in below the "minimum insertion" line, and many modern ones have their length there too.

Here's one that says 400mm and measuring from about where the rails are down to the very bottom, that's 400mm. Note its not the length of the part that could go in the frame.

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