One of the most frequent doubts when buying a second-hand bicycle is finding the right size. Am I better off with a size M or a size L? Is 53 better suited to my height or should I buy 55?

One thing you should know is that the size of a bicycle is indicated differently depending on whether it is for the road or for the mountains.

In mountain biking, inches are used as a measure to indicate size. A size 17 mountain bike,5″ means that the size of the vertical tube that connects the bottom bracket to the seat post is 17.5 inches. This is beginning to change and some manufacturers already use sizing in centimeters.

The size of a bicycle comes from the length of the vertical tube. Or at least it was until a few years ago, when the geometries of the frames changed radically. Thus, on a size 54 road bike, the length of this tube is 54 centimetres.

Some brands use a system of sizes expressed in centimeters, others in inches and others using the standard denomination of S, M, L and XL.

Until the 1990s, bicycle manufacturers used very similar designs to make their sizes. A size 54 of Orbea hardly varied from a size 54 of Colnago. They all indicated the same thing: that the length of the vertical tube was 54 centimetres.

However, everything changed when Giant introduced the sloping frames to the market. The sloping is the angle of fall of the horizontal tube that joins the steering pipe with the seat tube.

Until the 1990’s this tube was practically horizontal, but with the arrival of sloping the geometries of the picture design changed. Although the vertical tube had the same length, depending on the angle of the horizontal tube the distance between axes was longer or shorter. In other words, two bicycles with a vertical tube of 54 centimeters could actually be two different sizes.

Since then each manufacturer began to use (and continues to use) their own frame angles, so the sizes began to vary from one brand to another. This is especially noticeable on mountain bikes. A 1.70m person can do better with a size 17 in one brand and a size 19 in another.

And, to complicate matters, we have some manufacturers that offer their sizes following the alphabetical system: XS, S, M, L, XL and XXL. And others who use the numerical system: 50, 52, 54, 56, etc. What a mess!

We created the manual after analyzing the frame geometry provided by more than 100 manufacturers.

As if this were not enough, for some years there have been brands that maintain their system of numerical sizes, but definitely forgetting the classic pattern that this number corresponds to the height of the vertical tube. For example, in a 2017 Orbea Orca the vertical tube measure in a size 53 is not 53 centimeters, but 50.

### Equivalence of sizes

When we buy shoes we have a pattern that serves as a reference. If we use a size 44 of shoes, when we buy new ones we will ask for the size 44. It is probable that there are small differences from one brand to another, but as a general rule the lasts are the same.

Now let’s imagine we have an Orbea Avant size 54 and want to buy a Giant Defy. Giant offers its sizes in alphabetical patterns: XS, S, M, ML, L and XL. Which one do we have?

We have seen that each brand uses its own patterns to configure the angles of the frames it manufactures. So sizes don’t have to match. But in Tuvalum we have compiled the geometries of more than 30 manufacturers to produce the following table, which collects the approximate equivalences between numerical and alphabetical sizes.

Don’t forget that these equivalences are indicative. Although they fit most of the bicycle models analysed, there may be variations.

### Calculate your bike size from your height

Okay, you know that a size 54 road or 18″ mountain equals an M in most manufacturers. Now comes the next question: how do we know if the bike size we need is the M, the L, the 56 or the 19″?

If you look at the websites of the bike brands you will see that most of them offer size guides based on height. In other words, as much as you measure, such is the size that corresponds to you.

A first problem is that each brand will recommend a different size, even one brand can recommend two different sizes! For example, the Trek size guide indicates that someone who measures 1.75m. can wear both a size 54 and a size 56. How is this possible?

These differences are due to the fact that a person’s height is a guideline for calculating their correct bicycle size, but it is never a definitive value. Not all people who measure 1.75m. have the same long legs. That’s why some will do better a size 56 and others a size 54.

The following table contains a recommendation for bicycle sizes based on height. To make it we have taken as a reference the guides provided by the manufacturers.

This table adjusts to the average proportions of a person of a certain height. What happens if your legs are longer or shorter than the average?

In order to know exactly what your most suitable bicycle size is, it is best to have a fitting or biomechanical study.

### Calculate your size according to your leg length

The important thing when choosing the right bike size is not the height of your body, but the height of your hip. That’s why one of the most reliable factors in determining a bike size is the length of your legs. This is the data that is usually used as a reference in bicycle shops to recommend one size or another.

To do this, it is essential that you measure the internal length of your legs. Take off your shoes, put your feet on the ground with your legs straight and measure the distance from the ground to your crotch. You can help by holding a book between your legs at English height and measuring from the floor to the top corner of the book. This will make it easier for you.

That measurement will give you an X value, for example 90 centimeters. If you want to know your mountain bike size, multiply that number by 0.21. So, following the example, we have 90 x 0.21 = 18.9. In this case our mountain bike size would be 19, which corresponds to a vertical tube length of 19 inches.

If we want to buy a mountain bike whose manufacturer has expressed the size in centimeters, we will multiply that value by 0.54.

What if we are going to buy a road bike? What we will do is multiply the height of our crotch in centimetres by 0.65. That’s 90 x 0.65 = 58.5. A size 58 or 59 is ours!

This formula is more reliable for calculating our bike size than based solely on height. However, the result is not entirely accurate – although it does work for the majority of the population. Because manufacturers and brands change the geometries of their frames every so often, other measures have an influence, such as forearm length or elasticity.

### How to calculate the size of a triathlon bike

For time trial bikes, such as tiathlon or long distance goats, there are differences in the size calculation.

In this type of bicycle, both the frame geometry and the position of the cyclists and triathletes change with respect to road bicycles. They are usually shorter frames, with less wheelbase, that look for aerodynamics.

For this reason, it may be advisable to choose a frame size between 2 and 4 centimetres smaller than that which corresponds to your height and leg length. For example, if the calculation of your size gives you a value of 54, choose a size 52.

### What if the size you give me is between two measurements?

Sometimes it can happen that, when calculating our bicycle size, the result is between two sizes. For example, give us 54.3 centimetres. What do we do?

We can think that the solution is to round off the result. If you give us 54.3, we will keep the size 54. However, for these cases it is advisable to follow these two criteria:

- Biomechanical criterion: Measure your wingspan (the length from the tip of one hand to the other with your arms extended across). If the result is greater than your height, choose a larger size (following the example above, we would keep the size 55). If your wingspan is less than your height, choose the smallest bicycle size (size 54).
- Use criteria: The use you make of your bike is also a criterion to take into account when choosing your size. If the calculated value is between two sizes, choose the smallest size when you are going to use your bike for more sporting and competitive purposes. Choose the largest size for a smoother walking pace or routes where comfort over speed is paramount.