Going Going Bike

Jul 072011
Bike Measurement

Buying a bike for the first time can fill many with trepidation. One of the most common questions asked to me by friends who want to buy a bike for the first time is what size bike do I need?

It can be quite complicated and it’s an important decision to make. Choosing the wrong bike size can affect your comfort, efficiency and enjoyment on the bike so it is critical that you know what bike size fits you.

Fortunately, we have a simple to use bicycle sizing guide to help you choose the right sized bike.

Stand over height

If you didn’t have the use of our handy bike sizing guide you would need to resort to more basic methods of getting the right sized bike frame for what size bike you need.

Traditional wisdom has been, and this was very much my experience when I bought my first bike from Cycle Surgery in 2006, that you stand over the frame of a bike and if there was an inch or two between the top of the top tube and your crotch then you had found the right size bike for you.

While “stand over height” provides a good indicator of what bike size you need for your height, this simplistic approach to bike sizing fails to take account of the vagaries of the human body and it should only be used as a rough guide to determine what bike size do I need.

For example if your legs are short and your torso is long then a stand over test will result in you getting a bike which is likely to be too small for you.

Alternative ways to get the correct bike size requires an appreciation of how manufacturers measure their bike frames. Knowing how to measure a bike frame size is an art…not least because there is a lack of uniformity amongst bike manufacturers.

How are bike frames measured?

Manufacturers base the size of their bikes on the size of the frames. Generally this bike frame measurement is taken from the length of the seat tube.

The measurement starts from the top of the seat tube down to the centre of the bottom bracket.

However, the situation is confused by certain manufacturers measuring frame sizes differently and this may confuse a buyer on the question of what size bike do I need. For example many measure from the intersection of the top tube and seat post to the bottom bracket.

Lack of conformity

In a similar way and in a further lack of conformity, frame sizes can be quoted in centimetres, or inches and sometimes just as Small, Medium or Large. Mountain bike frames are almost always measured in inches as opposed to centimetres.

Our own bike frame size guide is based on the height of a person and is a good starting point for anyone looking to find the right bike size online, where sitting on the bike is not always possible. We suggest you take an accurate measurement of your height to help you determine what size bike frame would be best for you.

Inside leg measurement

For a more accurate bike size fit, particularly, when buying in a shop, knowing your inside leg measurement is a good idea. To find out what size bike frame you require, take the measurement of your inside leg and multiply it by 0.67. My inside leg measurement is 84cm, so my road bike frame size would be 56cm as 84cm x 0.67 = 56cm (rounded down). The calculation is different for other types of bikes.

Ape Index

Another factor that may be important in what bike size to choose especially if your measurements are caught in between two frame sizes is the measurement of your Ape Index (arm length or reach), which is your arm span minus your height. If you have a positive ape index then go for the larger frame size, if you have a negative ape index go for the smaller frame size.

Other factors, such as top tube length, seat position and crank length, also play a part in finding what bike size best suits your body and riding style, Measurements of torso and arm lengths for instance can help determines the ideal top-tube length, which some see as far more important in determining the correct bike frame size for a person.

Our bike frame size guide covers Road Bikes (male and female), Mountain bikes (male and female), Hybrid, Triathlon/TT, BMX and Kids bikes. We hope this blog article will help you find out what size bike you need.

See also

Guide to buying a used bicycle

Endurance Coach: Eating for recovery

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