How to size a bike

Bicycles not only come in different types and models, but also in different sizes. In this blog we’re going to look at how to size a bike including what the sizes mean and how to choose the correct bike. A bike that is sized correctly will not only perform better, it will be more comfortable and easier to handle too.

Style of bike

If you’re in need of a new bike but not sure how to find the right size then the first thing to decide is what style of bike you need. Choosing the right type of bike is crucial. Frame geometries vary between the styles of bikes, offering the best riding position and performance on designated terrain. 

how to size a bike

Frame sizes are not transferable between the styles of bikes so don’t stick to one size. A large mountain bike frame which is 20″/51cm is equivalent to a small road bike frame. This is because the rider position is different when riding on a mountain bike than when riding on a road bike and the frame geometry is different.

how to size a bike

Once you have established what style of bike you want, it’s time to determine what size suits you best.

How to size a bike

How to size a bike

The range of bike sizes creates a scale to give you a rough indication of which bike fits you, based on your height.

How to size a bike

This is expressed either in inches, used on mountain and hybrid bikes, or centimetres, used on road bikes.

To make it easier for consumers, some manufacturers also introduced a simplified scale, using letter sizing. 

How to size a bike

This measurement is the starting point. Along with the sizing chart on the manufacturer’s website will be a recommended rider height.  

There are only 5 or 6 different bike sizes so a range of heights will fit on each size of bike. Don’t feel confused if your height means that you overlap between two sizes.

How to size a bike

If you can’t get to try both sizes, a good rule of thumb is to go for the smaller size. It will be easier to handle and the smaller the bike, generally the more rigid and lighter the frame.

How to size a bike

For example; if you are 5ft 7″ or 170cm then, according to the sizing charts, you can fit on a small or a medium sized bike. 

The size of the bike is simply the measurement from the middle of the crank and either to the top of the seat tube

How to size a bike

or vertically to the top tube.

bike size

Because these different ways of measuring give different sizes, one manufacturer may have a sizing 1 or 2 inches different. The reason for using the crank and the seat tube is because it joins the seatpost and the pedals and is where your bum and feet rest on the bike and is therefore a crucial part of whether the bike fits you.

which bike size

Now we are able to go online and check how manufacturers measure their bikes. It’s important to note that if a shop assistant has told you that you “need to buy an 18″ bike” this may only be for one brand.  If you choose a different brand be prepared to buy a different size and have a bike that feels the same. This is because, just like in the clothing industry, there is no standardisation when it comes to bikes, sizing can vary between bike companies.

How to size a bike in person

If you are with the bike that you are sizing, the best thing to do is to sit on it.  Stand over the top tube (if there is one) and make sure that you have about 2″ between you and the top tube.  

How to size a bike

Next sit on your bike against the wall or ask someone to hold the front of the handle bars. Put both feet on the pedals and pedal backwards until one of your feet is at the bottom of the stroke (6 o’clock).  Now move that foot forward so your heel sits on the pedal and your foot is flat and in line with the ground.  You should have a straight leg. This is so that when you are pedalling on the ball of your foot you have a slight bend in the knee.

If your heel can’t touch the pedal then you will need to lower the seat and then sit on the bike again. Undo the nut or allen key where the seat post is inserted into the seat tube. Lower the seat and then sit on the bike again. Keep repeating the process until the saddle height feels right.

If, while having your heel on the pedal, you still had a bend in the knee then you should raise the saddle. Undo the nut or allen key where the seat post is inserted into the seat tube. Raise the seat post and then sit on the bike again.  Repeat the process until the saddle height feels right. If the saddle is at the correct height and you are still able to lower and raise the seat post (different shoes or a different saddle may require some adjustment so allow for that) then the frame of that bike is correct for you. 

Step through and stagger frames

This method of choosing the size of a bike is not applicable for the step through frames as they don’t have a top tube so it’s not possible to stand over the frame in the same way. As long as you can reach the saddle and handlebars comfortably then the bike will be fine for you. 

How to size a bike- upper body placement

Once you have got the saddle at the correct height, you can now have a look at how your upper body is placed.  If you feel too upright or too stretched out, a larger or smaller frame probably won’t make any difference. You will need to look at a different style of bike.  Don’t be disheartened. 

Example: if you really like to ride a bike with dropped bars, but the one you are looking at makes you feel uncomfortable, have a look at a different style of dropped bar bike.

Adjustments to handlebars can be carried out, but only make minor differences.  It’s also possible to buy different bars and stems to change the riding position. Find out the correct height for bike seat and handlebars in one of our previous posts.

How to size a bike online

If you are buying the bike without seeing it, for example when ordering online, find out the stand over height.

This measurement is taken from the floor, to the top of the top tube just in front of the saddle. It is normally on the manufacturer’s website.

Once you have this measurement it’s time to measure your inseam. The inseam length is measured from the bottom of the foot to the crotch.

The inseam measurement needs to be greater than the stand over height. About 2 inches is recommended.

This is so that you can safely stand over your bike when you dismount from the saddle. For example, if the stand over height of the bike is 28″ and your inseam is 30″ then the frame should fit you.

How to size a bike: Do’s and Don’ts

Putting a bit of effort into choosing the right size initially can pay off in the long run and give you a bike that gives you consistent comfort and performance. We have put together a list of how to size a bike do’s and don’ts to help you avoid making any mistakes. 

Do’s:

  • Do buy a bike that’s comfortable (unless you are competing at a high level).
  • Do check that the size sounds feasible if you are buying the bike without seeing it in “the flesh”. Some people think that the wheel size indicates the frame size. Road bike frames will be between 44 – 63 cm and mountain bikes and hybrid frames will be between 13 – 23″.

Don’ts

  • Don’t choose a bike because of the size. It’s important to note that if a shop assistant has told you that you “need to buy an 18″ bike” this may only be for one brand.  If you choose a different brand be prepared to buy a different size to have a bike that feels the same. 
  • Don’t buy a bike that is too big or too small because of the price tag. Sometimes retailers will sell end of season bikes cheap.  The sizes at the extremes are often the ones that get left.
  • Don’t feel confused if your height means that you overlap between two sizes. If you can’t get to try both sizes, a good rule of thumb is to go to the smaller size. It will be easier to handle and the smaller the bike, generally the more rigid and lighter the frame.  

Don’t use your old frame size to automatically choose a new frame size.  Even if you are buying the same brand, manufacturers change frame design every year, so treat a bike purchase afresh every time.

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