# Bicycle seatpost diameters: 25.4, 26.8, 27.2, 28.6, 30.9, 31.6 and 34.9 mm

> What diameter is a bike seatpost?

- Category: Everyday mounts › Bicycle
- Kind: Standard part
- 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)
- Page: https://xemstudio.com/objects/bicycle-seatpost-diameters/ · JSON: https://xemstudio.com/api/geometry?object=bicycle-seatpost-diameters · Plain text: https://xemstudio.com/objects/bicycle-seatpost-diameters/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part.

## Notes

- 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.

## Mating features (exact size; open press / slide / run / socket by your process clearance, leave designed and clamp as drawn)

| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| post outside | Ø25.4 / 26.8 / 27.2 / 28.6 / 30.9 / 31.6 / 34.9 | frame seat tube of the same Ø; mount clamps on the exposed post | slide | seatpost-* |

## Sizes

| 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 |

## Where the files are

- Sheldon Brown (Harris Cyclery): [Seatpost sizes by make and model (27.2 the de facto size for 1 1/8 in seat tubes)](https://www.sheldonbrown.com/seatpost-sizes.html) — web — reference (verify at link)
- BikeRadar: [The anatomy of a dropper post: 30.9 and 31.6 most common, 34.9 and 27.2 also made](https://www.bikeradar.com/us/mtb/gear/article/dropper-post-explained-49097/) — web — reference (verify at link)

## Free STEP/STL (7 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/index.json

- seatpost-25.4-L100: [STEP](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-25.4-L100.step) · [STL](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-25.4-L100.stl) · [DXF](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-25.4-L100.dxf) — Seatpost section Ø25.4 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø25.4 circle
- seatpost-26.8-L100: [STEP](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-26.8-L100.step) · [STL](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-26.8-L100.stl) · [DXF](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-26.8-L100.dxf) — Seatpost section Ø26.8 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø26.8 circle
- seatpost-27.2-L100: [STEP](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-27.2-L100.step) · [STL](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-27.2-L100.stl) · [DXF](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-27.2-L100.dxf) — Seatpost section Ø27.2 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø27.2 circle
- seatpost-28.6-L100: [STEP](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-28.6-L100.step) · [STL](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-28.6-L100.stl) · [DXF](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-28.6-L100.dxf) — Seatpost section Ø28.6 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø28.6 circle
- seatpost-30.9-L100: [STEP](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-30.9-L100.step) · [STL](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-30.9-L100.stl) · [DXF](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-30.9-L100.dxf) — Seatpost section Ø30.9 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø30.9 circle
- seatpost-31.6-L100: [STEP](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-31.6-L100.step) · [STL](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-31.6-L100.stl) · [DXF](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-31.6-L100.dxf) — Seatpost section Ø31.6 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø31.6 circle
- seatpost-34.9-L100: [STEP](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-34.9-L100.step) · [STL](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-34.9-L100.stl) · [DXF](https://xemstudio.com/files/hardware/bicycle-seatpost-diameters/seatpost-34.9-L100.dxf) — Seatpost section Ø34.9 × 100 long, axis on X: the outside of the post (the wall is not drawn); DXF = the exact Ø34.9 circle

## How it fits

### Parts that fit it
- [Specialized Power saddle](https://xemstudio.com/objects/specialized-power-saddle/): on the seatpost clamp

### Alternatives
- [Bicycle handlebar diameters: 22.2, 23.8, 25.4, 26.0, 31.8 and 35.0 mm](https://xemstudio.com/objects/bicycle-handlebar-diameters/): bar sections for mounts on the handlebar

### Also needed
- [Socket head cap screws, M2–M8 (ISO 4762 / DIN 912)](https://xemstudio.com/objects/socket-head-cap-screws-metric/): M4 / M5 screws for a clamp on the post

## Questions

**What diameter is a bike seatpost?**

Most often 27.2 mm on road and gravel bikes and 30.9 or 31.6 mm on mountain bikes with dropper posts; 34.9 mm on some large droppers. The size is marked on the post.

**What are the most common dropper post sizes?**

30.9 and 31.6 mm; 34.9 and 27.2 mm droppers are also made.

**Is a 31.6 post the same as 31.8?**

No. 31.6 is a seatpost size; 31.8 is a handlebar clamp size. Do not mix them.

## Fits with

- [Bicycle handlebar diameters: 22.2, 23.8, 25.4, 26.0, 31.8 and 35.0 mm](https://xemstudio.com/objects/bicycle-handlebar-diameters/)
- [Handlebar clamp rings: split rings for 22.2 to 35.0 mm bars, M4 / M5](https://xemstudio.com/objects/handlebar-clamp-rings/)
- [Bicycle fork steerer, 1 1/8 in (28.6 mm), threadless](https://xemstudio.com/objects/bicycle-steerer-1-1-8/)
- [Bicycle bottle-cage bosses: M5 × 0.8 on 64 mm](https://xemstudio.com/objects/bicycle-bottle-cage-bosses/)