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Seatpost diameter sizes (standards)

Updated: 09/09/2020.

This post explains what kinds of seatpost diameters are most commonly used and how they are measured – to avoid mistakes when purchasing, or changing seatposts. The post deals only with diameter (width). The length of the seatpost depends on frame geometry (design) and size – i.e. how much the saddle needs to be raised from the end of the seat tube. Separate post explains the maximum amount of seatpost extension from the frame (minimal insertion length). For seatpost height in terms of bicycle fitting, see: Setting up comfortable riding position.


0. Terms

It is important to understand two terms. “Seat tube” – which is a tube of the frame that holds the “seatpost” – a post the saddle is mounted on. Picture 1 shows what is what.


1. A bit of history

First bicycle frames were made mostly from steel, with steel tubes of a standard (outer) diameter. British and Italian standard for seat tube diameter was 1 1/8″ (28.6 mm). Older French bicycles used 28 mm tubing. Old US bicycles (mostly BMX and older bikes that used one piece cranks) was 1″ (25.4 mm).

Standard outer diameter dimensions were important so that derailleur clamps etc. could be made in exact matching size (inner clamp diameter). Frames of higher quality are usually made with thinned down tube walls, so that frame weight is reduced. This meant that a wider seatpost diameter usually meant a higher quality (and lighter) frame. So high quality frames with 1 1/8″ seat tubes usually had 27.2 mm wide seatposts.

More modern trend, mostly on mountain bikes, where stronger seatposts are required (without too much extra weight) lead to a new “oversized” standard of 27.4 mm (and wider).


2. Measuring required seatpost diameter

One (precise) measurement is often better than a thousand expert opinions” – author’s expert opinion.  🙂

How to measure the seatpost diameter? Easiest and the most accurate method is using Vernier calipers. Like it is shown in the picture 2.

Measuring seatpost diameter using Vernier calipers. Source: forums.mtbr.com Picture 2
Measuring seatpost diameter using Vernier calipers.
Source: forums.mtbr.com
Picture 2

It can be seen from picture 2 that the seatpost is marked as 31.6 mm wide (and 410 mm long), while it really measures only 31.42 mm. This means the seatpost will probably not fit firmly enough to stay in place inside a seat tube meant for 31.6 mm wide seatposts. Always measure! 🙂

Measuring seat tube diameter is often necessary before purchasing, or changing the seatpost. How to do that? Three methods will be explained here, but one can always be creative. 🙂

Method 1: if the frame already has a mounted matching seatpost – that attaches firmly enough so it doesn’t slide, while it is still narrow enough to allow easy mounting and dismounting, then it is sufficient to measure it’s outer diameter, as it was explained above – “How to measure the seatpost diameter?”.

Method 2: using Vernier calipers, as shown in picture 3.

Measuring inner diameter of the seat tube. Source: forums.mtbr.com Picture 3
Measuring inner diameter of the seat tube.
Source: forums.mtbr.com
Picture 3

Method 3: using special seat post sizing rods. These are rods with an increasing diameter from one end to the other, with a scale noting the diameter in standard sizes. The rod is simply placed in the seat tube and the matching diameter is the one just above the end of the seat tube (the first visible part of the rod still sticking out of the tube). Place a screwdriver through a hole at the top of the rod, just to make sure it doesn’t drop all the way in.  🙂

Seat tube sizing rods. Source: bikeforums.net Picture 4
Seat tube sizing rods.
Source: bikeforums.net
Picture 4

Since measuring often can’t be done to a 1/100 of a millimeter correct, the next chapter gives a list of most commonly used standard seatpost diameters. If a measured diameter differs, it can be assumed that the correct diameter is the standard one that most closely matches the measured diameter.


3. Standard seatpost diameters

Table 1 gives a list (in mm) of diameter standard sizes:

Standard seatpost diameters Table 1
Standard seatpost diameters
Table 1

Seatpost diameters are usually a multiple of 0.2 mm (but not always, as the tables 1 and 2 show). If a measured value differs, and most closely matches a value that is not a multiple of 0.2, there is a very high probability that the closes multiple of 0.2 mm size will fit. For example, a measured 25.5 will most probably fit a 25.4 mm wide (nominal and measured width) seatpost. A rule of thumb is that the widest post that slides in without forcing (other but hand pushing and/or twisting) is the right one. If it drops in, with play, before the pinch bolt is tightened, the seatpost is probably too thin.

For easier managing, table 2 gives an overview of seatpost diameters most commonly used on modern bicycles (practically from the end of the 20th century to nowadays).

Seatpost diameters most commonly used on "modern" bicycles. Table 2
Seatpost diameters most commonly used on “modern” bicycles.
Table 2


Related post – How to choose a comfortable saddle (the first in a series of 5 posts explains saddle materials):

Bicycle saddle materials
Bicycle saddle materials
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11 thoughts on “Seatpost diameter sizes (standards)”

  1. I want to use a 31.8 or 30,9 dropper post on my 34.9 seatpost. what shims would work for this?

    • I’d look for an adapter that notes those very dimensions: 34.9 to 31.8, or 34.9 to 30.9 (depending on the choice).

  2. What size diameter seatpost do I need for my 1960’s 3 speed Tote Cycle and where can I get original old stock chrome one. Thank you.

  3. Another way to measure the circumference, albeit not super exact, is to use a measure tape and tie it around the seatpost.

    • If memory serves me well, dividing that with the number Pi should give the diameter.
      Haven’t tried the method, but I suspect it to be too imprecise for this use.

    • I’m upgrading my bike and my post I’m cangi g from a 26.8 to a 31.6. I’m buying a vernier because I cannot get a accurate measurements for a setpost adapter. The spacer I’m hoping that will work is a 26.8-31.8. My question is will I have to purchase to seat post clamps for this to work and be safe?

    • Not sure I understand the setup, but, if a narrower seatpost is to be inserted, using an adapter to fit the difference between the seat tube, and the seat post – then no need to change the clamp, if it’s in good condition.

      However, if seatpost is to be extended by more than some 20 centimetres, I would look for a well matching seatpost, not use an adapter – because adapters are usually not very deep (long), so won’t offer much of a support for the seatpost. Hope I’ve explained well what I mean.

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