Choosing the correct size bike is essential if you are to be comfortable when riding, it will also make the bike more efficient and easier to handle. This guide offers basic guidance on choosing the most suitable frame size for you.

How do you choose the correct size bike?

A good starting point  is to measure your inseam which will determine your stand over height. The stand over height is the clearance between your crotch and the top tube of the bike when you stand just in front of the saddle and becomes critical in a sudden dismount; this is more likely on a Mountain Bike,  so we  recommend 2 inches with a minimum of 1 inch of clearance . As well as your inseam length you should also measure your height, this gives you two points of reference to help choose the size of bike from the bike size charts, which are located near the bottom of this page. Bike frame sizes usually increase in 2cm increments for road bikes and 2” increments for mountain bikes, so get someone else to measure you carefully as accurate measurements are essential.

How do you determine your reach?

 As frame size increases so does the top tube and head tube length to keep the bike in proportion.  If you find yourself in between sizes based on your height and inseam length the upper body should be the deciding factor, because your reach to the bars will be affected. To help work out if you have a long or short reach take  your arm span (finger tip to finger tip) minus your height. If you have a positive result' (your arm span is greater than your height) then go for the larger of the two sizes. If you have a negative results (your height is greater than your arm span) then go for the smaller of the two sizes. 

Correct frame size

 If you already have a bike that you are comfortable on you can measure the frame size. The two most commonly stated frame measurements are center to top (a to c on the diagram to the right) or center to center (a to b). These measurements refer to the center of the bottom bracket axle to either the top of the seat tube or middle of top tube in line with the seat tube. Other dimensions to take into account are the top tube and head tube, these determine your reach and your handlebar height. All dimensions have a certain amount of adjustment, saddle height and set back is adjusted via the seat post, handlebar reach is adjusted with stem lengths and saddle fore/aft adjustment, while handlebar height can be adjusted via spacers or different angle stems. But, it is essential you get the most appropriate frame size so that all these dimensions can be achieved. Manufacturers can state either measurement on your frame, so always check which they have used when viewing their range. When comparing frame sizes you should also take into account the style of frame, especially road bikes which come in traditional, semi compact or compact geometries 

How do you choose the correct size child's bike?

Choosing the correct size of bike is classed by age as opposed to height. Kids are still learning to ride, so it's important the bike you buy them fits correctly and is not too big! This is a common mistake in parents wanting to stretch out the life of a bike. The fact remains that when a child is learning, they often need to step forward off the saddle, so they straddle the top tube. It's important the frame isn't so big they risk hurting themselves when this happens.

Our guide table below suggests which wheel sizes are most suitable for which age range.  You know your child best.  If he/she is tallest in the class and is at the top of a size range, then go for the next wheel size up. If your child's age is in the middle of the range, try and resist the urge to 'upsize' them to get extra life out of the bike. This will be counter-productive to the child's enjoyment of learning to handle a bicycle

Time Trial (TT)/Triathlon bike positioning

 Time Trial and Triathlon bikes pose different sizing challenges from other bikes.  The aim of a TT/Tri frame is to get a flat back and reduce your aerodynamic drag. A TT or Tri bike will have a slightly steeper seat tube angle; this moves your saddle position forward so the angle between your torso and thighs is not reduced excessively.  This is because, if your thighs come up too high you will lose power.  By moving the saddle position forward and your bars lower you do not lose any power while gaining an aerodynamic benefit. The diagram (right) shows the two positions, notice how the TT/Tri position results in lower front end whilst retaining the angle between thigh and torso, you will also see how the point of contact with the saddle is further forward while the reach to the handlebars roughly unchanged.

Bike Sizing Charts