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How to Choose the Right Bike Size

Whatever the type of bike, it is essential to choose the right size. The incorrect geometry or size can ruin the riding experience, reduce sport performance and lead to injury.

Image of a Specialized Bicycle Image of a Specialized Bicycle

Once you have decided on the type of bike you need, the first and most important decision is to choose the size and geometry of the frame. Never make the mistake of buying the wrong size to take advantage of a sale, a second-hand bargain or the last remaining unit of your dream model. It will be difficult or impossible to achieve a perfect postural fit using components such as the stem and seatpost, and this can alter the behaviour of the bike.

Ideally, you should seek professional advice from a specialised store, such as Mammoth or the advice of a biomechanic. But if this is not possible, or if you want to understand the process, here is a guide to the key points to consider when choosing the size.

Bicycle Measurements

Unfortunately, there is no standard measurement for bike sizes. Initially, size was determined by the length of the down tube from the centre of the bottom bracket axle to the centre of the top tube, which was almost identical for all brands. But the evolution of frame geometry, which began with the sloping top tube, has made sizes harder to decipher and they also vary from one brand to another. Because of this, other measurements are now used for choosing frame sizes, the most important of which are the Reach and the Stack. We explain both concepts in depth, in this video on Bike Geometry and they also appear in figure 1 below. In the last part of this article, we will see how to use these measurements to choose the right frame and size.

Image by Specialized Bicycle Figura 1 Bicyle geometry

Calculating the Size by Height and Leg Length

Let's start with the traditional measurements, which are the cyclist’s height and leg length, because, in spite of their limitations, they are still the most commonly used. Height is not always a valid reference, and leg length is much more accurate, as this varies considerably between people of the same height.

To measure your inner leg length, you need to stand barefoot and upright against a wall, preferably in cycling shorts. Hold a book between your legs, with its edge against the wall, to ensure it is completely horizontal and pull it up firmly against your crotch. Measure the distance from the top edge of the book to the floor and that is your inner leg length.

Measure your inner leg length Measure your inner leg length

There are a multitude of size charts on the internet based on height and leg length, with varying degrees of accuracy. We have made our own charts for calculating sizes of MTB and road bikes, using the leg length as a baseline and a rough estimate of the most common height ranges for each size. Hybrid bikes usually have the same sizing as mountain bikes and it is given in inches. Fitness and leisure bike sizes are very similar to road bikes and are in centimetres.

If your inner leg measurement is halfway between two sizes, you should usually choose the smaller of the two, because it is easier to compensate by adjusting the components. However, in our experience, we recommend you also take the following two aspects into account::

  • The build of the rider, cyclists with larger builds should choose the larger of the two sizes, who will find it more awkward to fold down to a smaller size.
  • The bike use, choose the smaller of the two sizes for more active use and the larger for more recreational use where comfort is more important.

Let's start with the size chart for choosing a mountain bike:

MTB Size Chart

For road bikes, the numerical size is expressed in centimetres and there is usually a wider range of sizes than for mountain bikes. For this reason, it will not always coincide with the size of a mountain bike, which usually uses alpha sizing. A rider who uses a size M mountain bike may use S for a road or cyclocross bike. It is therefore important to recalculate the size for each type of bike. Here is the size chart for road bikes.

Road Bike Size Chart

If the road bike is specifically for time trial or triathlon, the aero bars will reduce the numerical size by two to four centimetres on the size chart, because the frames are shorter and designed for an aerodynamic posture. However, the brands that use alpha sizing usually take this into account. This means that size M, instead of being a 54, for example, will be a 50-52. This can be seen in the following chart.

Triathlon Bike Size Chart

Children's bikes, are defined by the size of the wheel and these can be 10, 12, 14, 16, 16, 18, 20 ,24 inches and even 26 inches for older children and young adults. It is advisable to choose children’s bike sizes according to height, rather than age, as growth rates vary so much. In the following size chart, we indicate the wheel size according to the height of the child and the average age range, according to the average percentile of children in Spain.

children’s bike sizes

Almost all brands have their own size charts and the most advanced, such as Specialized, Scott or Orbea, for example, even offer size calculators that are based on a number of measurements: type of bike, height, leg length, upper limb measurement and even weight, to estimate the body build and body mass index. We recommend using this if you are considering purchasing a bike by any of these brands.

Specialized size calculator Specialized size calculator

Calculate Size According to Reach and Stack

For those who want greater precision, the following two measurements should be used: Stack, which is the vertical distance from the centre of the bottom bracket to the top mid point of the head tube, and Reach, which is the horizontal distance from the bottom bracket axle to the centre of the headset. You can view them in the diagram in figure 1 above. These measurements are not related to the other bike geometry and are the only standard measurements for comparing one frame to another, providing they are the same type.

The calculation for each rider and bike type can be complex. The correct Stack can be approximately calculated by multiplying the inner leg length by 0.67 and it is easy to compensate different angles of the seat tube, which is the most affected measurement. On the other hand, the calculation of the Reach involves your height and both arm and leg length. The correct Reach is important for achieving the correct posture angles. Ideally this should be calculated by a biomechanical assessment so that you can then look for the model that is closest to the recommended measurements, which are provided by all manufacturers in their technical data.

In the absence of a biomechanical study, there are some specialised websites that calculate these measurements for you, but be careful, because the results are not always very accurate.

In Mammoth stores and at our mammothbikes.com/en/ website, you can find professional advice to solve your queries on bike sizes and the largest selection of all types of bikes.

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