If you purchase a new bike in a shop, the clerk will take your body measurements without explaining what is important for you to know. In case you opt for an online purchase instead, you need to be well prepared. Let’s see what measurements need to be kept in mind.

Not all brands use the same measurements to define the characteristics of a frame. Since selecting the correct size can be a little tricky, especially for beginners, we will try and simplify this operation. Please keep in mind that purchasing a frame in the correct size is essential, as this allows the rider to live the best bike experience.

The figures that define your bike:

• Horizontal tube length;
• Vertical tube length (middle-to-middle or middle-to-end);
• Reach and stack
• Wheelbase
• Chainstays
• Front Centre (the length between the middle of the bottom bracket and the front)
• The angle between the seat tube and the conjunction axle between the back wheel bolt and the seat tube; the angle between the head tube and the conjunction axle between the back wheel bolt and the head tube.
• Drop (or BBdrop);
• Bottom bracket height

Many models, be they road or mountain bikes, are nowadays measured in cm. However, you might find some brands that still use inches. Some even mix both measurement units. For us it is certainly easier to use the metric system but, should you find this piece of information in inches, just multiply the figure times 2,54 to obtain the measurement in centimeters.

Manufacturers usually attach a file with the technical information of all their bikes available at the moment, which is surely more helpful than taking the measurements manually. We, therefore, recommend you do not throw this document away, as it might still be of help in the future.

The top tube length

Every bike has a top tube. Nowadays, it might be slightly inclined, or “sloping”. In this case, you do not measure the actual tube size, but consider its projection or the distance between the middle of the saddle and the middle of the head tube. To sum it up, the top tube length can be “real”, when you actually have a horizontal top tube, or “virtual”, when the tube is sloping.

Many brands name their sizes according to their top tube length. This is not the case with mountain bikes, for which sizes are usually S, M, and L. This last system is getting used more and more often for road bikes as well. Sizes are basically determined by each manufacturer freely: a size S might be 54 cm long for one brand, which would be considered an M for another one. 

It is also important to point out that not all brands measure the top tube in the same way. For instance, some only consider the length that stretches from the middle of the head tube until a vertical line that is perpendicularly projected from the middle of the saddle. This method does not take into consideration the further projection of the Seatpost towards the back, therefore the sizes of this specific brand are a little smaller than those of other manufacturers. It is therefore important to pay attention to such details in order not to make mistakes in picking the correct size.

The seat tube length

The seat tube length is the line between the intersection of the vertical tube and the top tube axle and the center of the bottom bracket. Once again, this measurement appears to be easy but is actually pretty tricky. If you start measuring from the middle of the bottom bracket till the intersection with the axle of the seat tube (where the seat post is located), then you are applying the Center-to-Center measurement method. Some bikes have a particularly long seat tube, even longer than the top tube, and are assembled with an extension that creates a sort of integrated Seatpost. This is the End-to-Center method.

As you can imagine, all these variables make it hard to compare sizes.

Reach and stack

We saw how the top tube and the seat tube length are dangerous fields when comparing frames. In order to avoid chaos, many manufacturers nowadays have added the values of reach and stack for their bikes. The pro of this unit is that they are independent of the frame and its shape and they measure the perpendicular distance between two points: the bottom bracket and the top of the head tube. Basically, the “reach” unit represents the horizontal between two points, and the “stack” is their vertical distance. Their sum equals the reach&stack.

Wheelbase

The wheelbase is the distance between the two-wheel bolts. It is a determining distance for the riding experience and can interfere in the calculation of the whole size frame. It is pretty easy to measure.

The lower parts chainstays

This measurement also influences the wheelbase and significantly alters the manageability of your bike. Shorter chainstays make your bike a lot more agile than longer stays. This information is gathered by measuring the line between the middle of the bottom bracket and the back wheel bolt (or derailleur hanger).

Front center

It is another component of the wheelbase and is equal to the distance between the chainstays, measured from the bottom bracket and the front wheel bolt. This unit influences the agility of the bike, too. Watch out: the wheelbase is not the sum of chainstay and front center, because none of them are horizontal units.

The angle between the seat tube and head tube

In order to determine how maneuverable a bike is, these two measurements are the most relevant. The more vertical the tubes are assembled on a bike, the more agile the bike is.

Drop (BB Drop)

The drop is the difference between the wheel bolt distance from the ground and the distance from the bottom bracket to the ground. It is basically the difference between the two heights. This is one of the typical pieces of information one can find on the bike’s specs.

Bottom bracket height

Last, but not least, the bottom bracket height is the bracket’s distance from the ground. It is easy to measure, although you need to make sure that your bike stands in perpendicular to the ground position and is not inclined. As opposed to the bb drop, it might be influenced by the wheels and their pressure.

By now you surely have a general idea regarding all necessary measures to determine the bike size. Sometimes, reading these instructions to refresh your memory can be of great help before purchasing a new bike.