The sizes come from the Wheels Manufacturing seatpost gauge, which covers the most common diameters, and from the Fox and RockShox dropper post catalogs and Hope’s seat clamp range, checked in October 2026. The seatpost is the tube that connects the saddle to the frame; its outer diameter has to match the inner diameter of the seat tube, with no play. Seatposts are sold in millimeters in the US too.
Most common seatpost diameters
| Diameter | Common size (Wheels) | Fox Transfer | RockShox Reverb AXS |
|---|---|---|---|
| 25.4 mm | Yes | No | No |
| 27.2 mm | Yes | Yes, gravel only | No |
| 30.9 mm | Yes | Yes | Yes |
| 31.6 mm | Yes | Yes | Yes |
| 34.9 mm | Yes | Yes | Yes |
Dropper posts (the ones that lower the saddle with a lever on the bar) come in 30.9, 31.6 and 34.9 mm, with travel from 50 to 240 mm from Fox and from 100 to 250 mm from RockShox. 27.2 mm is the only rigid post size Fox makes a gravel dropper for.
There are plenty of in-between sizes: the Wheels Manufacturing gauge covers 25 diameters between 25.0 and 34.9 mm (25.0, 25.4, 25.6, 25.8, 26.0, 26.2, 26.4, 26.8, 27.0, 27.2, 27.4, 28.2, 28.6, 28.8, 29.2, 29.4, 30.0, 30.2, 30.4, 30.8, 30.9, 31.4, 31.6, 31.8 and 34.9 mm).
Seat tube clamp
The clamp isn’t sized by the seatpost but by the outer diameter of the frame’s seat tube, which is larger.
| Clamp diameter | Made by Hope |
|---|---|
| 28.6 mm | Yes |
| 30.0 mm | Yes |
| 31.8 mm | Yes |
| 34.9 mm | Yes |
| 36.4 mm | Yes |
| 38.5 mm | Yes |
| 39.7 mm | Yes |
There’s no fixed match between seatpost and clamp size: it depends on the seat tube’s wall thickness. Measure the outside of the tube with calipers or check the size etched on your current clamp.
How to tell which seatpost your bike takes
- Look for the etched number on the post, usually next to the minimum insertion mark (“27.2”, “30.9”…).
- Measure with calipers across the post in two directions and go with the larger number. If the post is badly scratched, measure the part that sits inside the frame.
- Measure the seat tube from the inside if you don’t have the post: a gauge like the Wheels Manufacturing one slides in until it fits snugly and gives you the size.
- Dropper posts: besides diameter, check the maximum insertion your frame allows. RockShox and Fox list the total length and minimum insertion for each travel, and the minimum insertion can’t end up outside the tube.
Between two close sizes, like 30.9 and 31.6 mm, there’s only 0.7 mm: get it wrong and the post either slips or won’t go in. If your frame takes an unusual size, the fix is a reducer shim from a standard post to your frame’s size.
Once the post is in, work out your saddle height with the saddle height calculator or look it up in the saddle height chart by inseam; the saddle height guide explains how to fine-tune it. Clamp and saddle rail bolt torque is in the bike torque specs chart, and the names of every part in six languages are in the components glossary.
Frequently asked questions
Can I put a 27.2 seatpost in a 30.9 frame?
Only with a 27.2 to 30.9 mm reducer shim. Without one, the post will move around and can damage the frame. The other way around is impossible: a post thicker than the seat tube won't go in.
How do I know my seatpost diameter?
Nearly every seatpost has its diameter etched near the minimum insertion mark. If yours doesn't, measure the post with calipers or the inside of the seat tube with a seatpost gauge.
What diameters do dropper posts come in?
Mountain bike droppers are made in 30.9, 31.6 and 34.9 mm (Fox Transfer and RockShox Reverb AXS). Fox also makes a 27.2 mm Transfer just for gravel, with less travel.