SeatPOST sizes?
 

How many sizes are there for seatpost? I askee this because I came to anbidea of switching one seat saddle to another bike but when I do, one seatpost couldnt fix on other bike seat hole. How do I know what size do I have?

Well, the Thomson Elite comes in:

25.0mm
25.4mm
26.0mm
26.6mm
26.8mm
27.0mm
27.2mm
27.4mm
28.6mm
29.4mm
29.8mm
30.0mm
30.6mm
30.8mm
30.9mm
31.4mm
31.6mm
32.4mm

As to yours, the diameter and length is often stamped near the bottom of the post, on the part that resides inside the frame.

About thirty trillion.

Most modern bikes probably use seatposts with a diameter of 27.2mm, 30.9mm, or 31.6mm.

Very carefully measure it with calipers.
Sometimes the diameter is written somewhere on the seatpost.

Always use a precisely-fitting seatpost.

Dozens, from about 24.0mm to more than 31.8mm, and that was just while I was in the business more than 15 years ago. Buy a good caliper and learn to use it.

Thank you guys! Very helpful!!

And if you can't measure or see the size stamped, Sheldon Brown has a pretty good database of many bikes and their sizes here:
https://www.sheldonbrown.com/seatpost-sizes.html

25.0mm
25.4mm
26.0mm
26.6mm
26.8mm
27.0mm
27.2mm
Kalloy , Much lower cost, stops, at 27.2..
in addition , a commonly available [Bike shop order] seat post sizing shim sleeve has ID of 25.4 (1") or 27.2mm, and made in all the previously listed OD.. * more likely in 1" ID being only 0.2mm larger than 27.2

27.4mm *
28.6mm
29.4mm
29.8mm
30.0mm
30.6mm
30.8mm
30.9mm
31.4mm
31.6mm
32.4mm





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If you plan on working on bikes buy at least a cheap calipers.

https://www.harborfreight.com/hand-t...per-63586.html

They start at 7/8"(22.2mm) and go up into the low 30's.
Then there is the whole proprietary aero designs. Even measuring your existing doesn't guarantee that someone didn't stuff the wrong size one in unless you know for sure it's original.

as indicated , Measure..

[math ; wrap a piece of paper around the seat post, mark it,
where it comes around to meet itself,
that is the circumference, of the post ,
Then, you can use a ruler to measure the length between marks.]

from there you can calculate the diameter..

.....

reaming at bike factories can have variation , as reamer wears,
over a production run,
and users bend the ears on the frame to compensate..

I have used a adjustable, hand , reamer , and gone up 1,
0.2mm diameter size post. to get a precise fit..

Was 27.0, to 27.2mm , had the post in hand and stopped cutting
as soon as it fit..




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