How To Determine Seatpost Size

While there are many reasons why you would need to find out the size of your seat post, the main reason is likely because you need to replace it.

Perhaps the seat post you currently have is broken or has aged. Or perhaps you need to purchase a larger seat post for your child that has outgrown theirs. 

Whatever the reason, you may be wondering how to determine the size of the seat post you will need to purchase. 

bike seat covered in leather

While it may seem like a daunting task, there are many ways you can determine this. Even if you do not have the measurements to hand, there are some simple ways you can find out this information manually.

In this article, we will be discussing a handful of ways to determine the size of the seat post you will need. We will also be going into some detail about what a seat post is and the importance of it.

What Is A Seat Post?

A seat post can be referred to in many different ways. You may have heard it be called a seat pin, a saddle post, or even a saddle pillar. However, all of these refer to the same thing.

A seat post is what the seat of a bike is attached to. It is a tube that connects the seat to the main frame of the bike.

It can be adjusted a certain amount to best suit the height needed for the person using the bike. They come in a variety of different sizes, depending on the bike you choose.

How To Determine Seatpost Size?

If you do not know how to determine a seat post size, you may be a little confused about where to begin.

However, we have discussed some simple ways you can do this below.

Size Stamp

Luckily, many brands and types of bikes clearly state the size of the seat post, using a size stamp. This stamp can usually be found on the seat post itself. While not all seat posts will have this, it is worth checking yours just in case. Sometimes the numbers will wear off due to age.

If your bike does have these numbers, then great, it will make your job of replacing the seat post easier! However, if it does not, it is always worth looking at the company’s website. They may have the measurements needed. 

Use A Seat Post Sizing Rod

If you cannot find the size on the seat post and you have had no luck finding the information on the manufacturer’s website, your next best option is to use a seat post sizing rod.

This is the best way you can accurately measure your seat post. The measurer is placed inside the seat post tube and will give you the accurate measurements you need.

Measure The Diameter Of Your Old Seat post

If your old seat post does not contain any size references, and you do not have access to a sizing rod, the next best option is to measure it manually. By measuring the diameter of your old seat post, you will have a better idea of the size you will need. 

You want to measure the outer diameter of your existing post to get a better idea of the size of the one you will need to purchase. Ensure that you are measuring as accurately as possible as the new seat post will not fit correctly if it is too thin or too wide. 

Measure The Seat Tube Of Your Old Seat post

After you have measured the diameter of your seat post, next you will want to measure the inside of the seat tube. You want to ensure that you are accurately measuring, as the seat tube is what your seat post will fit into. 

Ensure you are measuring the inside diameter of the seat tube and not the outside diameter to get the most accurate results.

Length Of The Seat Pole

The length of your existing seat pole is crucial. If you were happy with the length of the seat pole, you will want to purchase one as close to that length as possible. If you purchase one that is too short, you will not be at the correct height to ride your bike safely.

If you are unsure of the length, you should extend your old old seat post so that it is at its maximum extension.

When the seat post is placed into the tube, you are limited in how much you can adjust your seat up and down.

On average, you will be able to move it 100 to 150mm. If you need a higher adjustment than that, we would recommend purchasing a longer post. 

In contrast to this, you do not want a seat post that places the seat too high. Always ensure the seat can be placed at the correct height before purchasing.

The overall length of a seat pole can vary dramatically. This is to ensure that there are bikes suitable for everyone of all ages and heights.

Typically a seat post can range anywhere from 75mm up to 400mm. Given this, you can see why it is so important to determine the size you will need correctly.

The Type Of Bike

It is important to remember that the type of bike that you have will affect the type and size of the seat post you will need. For example, some bikes will need a shorter seat post. 

However, mountain bikes typically need a longer seat pole. This is because they will likely need adjusting according to the different types of terrain they are being ridden on.


We hope that this article has helped you to understand how to determine the size of a seat post. As you can see, there are many different ways you can find out this information. 

Even if the measurements are not printed on your seat post or the manufacturer’s website, there are many ways you can measure the size manually. If you do not have access to a seat pole measurer, it is simple enough to measure the seat post and tube manually.

Always remember to double-check the height and sizing before purchasing to ensure that the seat post will be at a correct and safe height for whoever is riding the bike. 

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