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Seatpost diameter sizes (standards)

Updated: 20/01/2021.

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 a seatpost (1), and a seat tube (2):

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 calipers (Vernier, or digital), like it is shown in the picture 2.

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

With a few notes, just in case:

  • If a seatpost wobbles/rocks left-right (before tightening the clamp), it is most probably too narrow.
  • Don’t punch it in. If it can’t go in using hand force, it’s probably too thick. Punching it in makes it very, very difficult to move, or take out later. It might even damage some frames.

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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15 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.

    • Measuring the circumference of the post and dividing by Pi is a good method. There are even circumference tape measures to do it.
      If you don’t have calipers, you’re hard-pressed to get the diameter. An adjustable wrench can be used as calipers, then measure the gap on the wrench. However, it’s almost impossible to distinguish 0.2 mm increments with a tape measure.
      But measuring the circumference means you only have to distinguish a 0.6 mm difference (because everything is multiplied by 3.14…), which is doable.

      Measuring the size of the hole in the seat post—well, that requires ID calipers, gauges, or sticking something into it then marking it so you can measure it like the seat post. That will introduce more error. For example, take some poster board (stiff paper), roll it up, insert it, and let it expand to fill the hole. Making sure it’s fully expanded & secure, mark the edge of the paper with a marker. Pull out the paper, unroll it, and measure between the edge and the mark (and divide by Pi).
      I’ve done it before, and it does work well enough.

  4. Your article is confusing, because of some obvious errors. First, “Picture 1” is not labeled, and nowhere do your explain which blue arrow is pointing at which part.

    Also, at several parts you refer to Vernier calipers, but none of your images have Vernier calipers. You show just plain old digital calipers that don’t have a Vernier scale. Look up what a Vernier caliper actually is.

    Finally, the suggestion that a less than 0.2mm difference in seat post diameter would make the post too small to fit snugly in the seat tube is laughable. Any halfway decent clamp should easily account for 1mm error or more.

    • Although it seems a bit snarky, your comment has been very useful.
      One of the WordPress updates seems to have removed labels on picture galleries (where more than one picture is placed side by side). Hadn’t noticed it. It’s fixed now, to the best way I could fix it with this “version” (doesn’t allow labeling each picture separately, without the label being put over a picture, instead of beneath it).

      “Vernier calipers” – English is not my native. The name of those calipers, whether analogue (with a Vernier scale), or digital, is the same in my native (“Šubler” – an “imported” German term). I will correct the term once I double check the proper English technical terminology.

      As for the 0.2 mm difference, it is probably counter intuitive for many people, but in practice it can cause one, or several of the following:
      – Seatpost slipping under rider weight and road bumps (either down, or turning sideways – or both).
      – Seat tube cracking near the seatpost clamp area (steel frames are the least susceptible to this, while carbon fiber ones are the most “sensitive” to such problems).

      I would normally say: “don’t take my word for it, try it, with a smaller diameter seatpost (0.2 mm is enough, but you can go 0.5, or 1 mm, as you said, to get “faster” results)”. However, this does pose a risk of damaging a frame, so do it only on a frame you are willing/prepared to damage.

      Thank you for what is in effect a very helpful, constructive feedback.
      If you find any other mistakes, feel free to note them. I try to keep this as correct, and up-to-date as possible.

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