Buyers guide to choosing the correct size road bike

If you’re browsing online for road bikes then this guide will help you narrow down what is going to fit you and what won’t. It’s all in the seat tube length and the reach length. Plus a few more tweaks that you could do with knowing about.

Choosing the correct size of bike is just as important as choosing the correct type of bike. An ill-fitting Racing bike will not be as fast as a correctly-fitting Sportive bike. A poorly-fitting Sportive bike will not be as comfortable as a suitable-sized Racing bike. Fit is everything.

Where your body ends up in relation to the handlebars, the front wheel and the bottom bracket governs how efficient and comfortable you will be on the bike. It also affects how well – or not – the bike handles.

Despite what some people may think, efficiency and comfort are not entirely mutually exclusive. It is perfectly possible to find a bike that is pleasingly fast and acceptably comfortable. Having said that, you can prioritise comfort over speed (or vice versa) if you want a bike to excel in that one particular area.

Bicycle fit does involves compromise. Compromises between comfort and performance, quick acceleration and handling stability, top speed and taking in the views.

sensa giulia ultegra 6800 di2 special

Frame sizes

Frame size is almost always related to the seat tube length. A 56cm size frame has a seat tube that is 56cm long. So that’s what the number relates to, in case you’ve always wondered.

The relatively recent move to compact frame designs (lower top tubes essentially) has thrown a lot of previous sizing recommendations out of the window. Someone who fitted at 58cm frame back in the 1990s will probably not fit a 58cm frame in 2014.

All bodies are different but assuming an averagely distributed body and limb length, there is a degree of agreement in the bike world about basic by-height sizings. See the chart below.

Recommended frame sizes by rider height

Rider height Frame size
5′ 3″ – 5′ 5″ 50cm – 52cm
5′ 5″ – 5′ 7″ 52cm – 54cm
5′ 7″ – 5′ 9″ 54cm – 56cm
5′ 9″ – 5′ 11″ 56cm – 58cm
5′ 11″ – 6′ 2″ 58cm – 60cm
6′ 2″ – 6′ 5″ 60cm – 62cm

If you have body dimensions that aren’t so average then you should factor these in and adjust the frame size that looking for.

If you have a short arms or torso go for a slightly smaller frame. If you have a long arms or torso go for a slightly longer frame (but check that the frame’s standover is going to be sufficient with your legs’ short-for-your-height inseam).

Measure your Ape Index

To discover if you have significantly short or long arms you can measure your Ape Index. The ‘ape index’ is a measure of the ratio of your arm span relative to your height. A typical ratio is 1:1, or where your arm span is equal to your height).

If your arm span is bigger than your height by 5cm or more, you may wish to consider moving up a frame size.

If your arm span is smaller than your height by 5cm or more, you may wish to consider trying a smaller frame size.


There’s a handy online Ape Index calculator here –

Changing components to improve bike fit

Bikes can also be tweaked to fit by changing various components – stem, bars and seat post usually – but the degrees of adjustability aren’t massive. Changing components should be about fine-tuning rather than trying to make an incorrectly sized frame fit you. Bikes with unsuitably proportioned components will handle extremely oddly and place undue stresses on your body.

But if a bike’s top tube is not the suitable length for you there’s very little that you can do to cure it. You need to choose a bike that has the correct length top tube for your body and your intended riding.

If we could turn back the clock – or overturn decades of cycling tradition – it would make far more sense to list frame sizes related to the length of their top tubes. It’s a shame that we are currently still stuck with the seat tube length based system but there we go.

We’re here to help

If in doubt, feel free to give us a call on 01772 432431, or email, and we’ll help you out.

Have a look at our current road bike range

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