Bicycle seatpost diameters: 25.4, 26.8, 27.2, 28.6, 30.9, 31.6 and 34.9 mm — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/bicycle-seatpost-diameters/  (CC BY 4.0)
Basis: Sizes from Sheldon Brown's seatpost list (27.2 the de facto size for 1 1/8 in seat tubes) and BikeRadar's dropper-post guide (30.9 and 31.6 most common, 34.9 and 27.2 also made)
Catalog version: 2026-10-20

Sizes: see table below
- A seatpost's diameter is set by the frame's seat tube; there are dozens of sizes in 0.2 mm steps, and the right one is usually stamped on the post near the minimum-insertion mark.
- Most common today: 27.2 (road, gravel, older mountain bikes), 30.9 and 31.6 (most dropper posts), and 34.9 (large droppers). 25.4, 26.8 and 28.6 cover many older frames.
- Files: each size as a Ø × 100 mm section, axis on X, plus a DXF of the circle: the outside of the post only (walls differ and are not drawn). A saddle-bag, light or bottle mount that clamps the exposed post uses this Ø.
- Seat-tube clamps (the collar on the frame) are sized by the frame's outside diameter, not the post: that is a different number.

post Ø (mm)  where it is used  from
Ø25.4  1 in posts on older and lower-cost frames  Sheldon Brown
Ø26.8  common on older frames  Sheldon Brown
Ø27.2  the de facto size for frames with a 1 1/8 in seat tube; still the usual road and gravel post  Sheldon Brown, BikeRadar
Ø28.6  1 1/8 in  Sheldon Brown
Ø30.9  one of the two most common dropper-post sizes  BikeRadar
Ø31.6  one of the two most common dropper-post sizes; common on mountain bikes  BikeRadar, Sheldon Brown
Ø34.9  large dropper posts  BikeRadar

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Sheldon Brown (Harris Cyclery): Seatpost sizes by make and model (27.2 the de facto size for 1 1/8 in seat tubes) — web — reference [verify license at link] — https://www.sheldonbrown.com/seatpost-sizes.html
- BikeRadar: The anatomy of a dropper post: 30.9 and 31.6 most common, 34.9 and 27.2 also made — web — reference [verify license at link] — https://www.bikeradar.com/us/mtb/gear/article/dropper-post-explained-49097/

Free files (CC BY 4.0, Xem Studio; STEP + STL, 7 parts): https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/index.json
API: https://xemstudio.com/api/geometry?object=bicycle-seatpost-diameters