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