When choosing a new bike, some riders will find themselves caught between frame sizes. You have to choose between a larger frame and a smaller one, but which one do you go for?

Getting the right size bike is a combination of fit and feel. Ultimately, the decision should be on how comfortable and confident you will be while riding it.

If you’re struggling to decide on the right bike, the good news is that there’s a perfect bike out there for you – and Halfords is here to help you find it.

Why am I in between bike sizes?

Before we discuss how to decide on the right size bike, we need to explain why bicycle sizing is about much more than just one number. Getting the right bike fit is complicated. If you just use the standard size charts provided by manufacturers or use your old bike as the basis for buying a new one, then you could find that the bike you have just bought isn’t right for you – and here’s why.

  • Different brands measure frames differently
    There is no single universal sizing method, so two bikes labelled the same size can feel different depending on the brand’s geometry and measuring approach. That’s why comparing numbers alone doesn’t always guarantee the same fit.

  • Road bikes are usually sized in centimetres
    Road frames are typically listed in centimetres, which gives a more precise size range, but the measurement still represents only one part of the overall bike geometry.

  • Mountain bikes are often sized in inches
    Mountain bike frames are commonly measured in inches instead of centimetres, and some brands also use general size labels like Small, Medium, or Large, which can vary between manufacturers.

  • Frame size typically refers to seat tube length (bottom bracket to seat tube top)
    In most cases, the stated frame size reflects the length of the seat tube, measured from the centre of the bottom bracket up to the top of the seat tube, serving as a baseline indicator of overall bike size.

  • Some manufacturers measure to the centre of the top tube instead
    Rather than measuring to the top of the seat tube, some brands measure from the bottom bracket to the centre of the top tube, which can slightly change the listed size and create differences when comparing bikes across brands.

Other Important Fit Factors (Beyond Frame Size)

  • Top tube length affects reach
    A longer top tube means a more stretched position; a shorter one creates a more upright, compact feel.

  • Handlebar height affects comfort
    Higher bars support a relaxed posture; lower bars create a more aerodynamic position.

  • Stem length changes reach and handling
    A longer stem increases reach and stability; a shorter stem makes steering quicker.

  • Standover height affects safety and confidence
    You should have clear space between your body and the top tube when standing over the bike.

  • Riding purpose influences fit
    Racing favours a lower, more aggressive setup; comfort riding suits a more upright position.

Should I go bigger or smaller?

Some riders will find themselves facing a choice between sizes and facing the choice between a larger or smaller frame.

Here are three practical ways to help you choose whether to go bigger or smaller.

1. Touch your toes

Find somewhere you can stretch out and try and touch your toes. If you can reach them (or get close), you’re pretty flexible and might prefer a smaller size frame.

If you find the riding position is a little bunched up you can extend it by replacing the stem.

If you weren’t able to touch your toes (or even get close), then we recommend going for the larger size.

2. Check your Ape Index

Another way to decide which bike to go for is to look at your ape index.

Your ape index is a comparison between your arm span and your height. If your arm span is longer than your height, go for a bigger frame. If it’s shorter, get the smaller one.

On a bigger bike, the reach to the handlebars will be longer. If you have proportionally longer arms, you are likely to feel more comfortable on a bigger frame.

3. Take them both for a test ride

It’s not always possible to visit a shop and ride every bike you’re interested in, but if you can, you should.

Visit a Halfords store, and you’ll be able to see and ride a selection of bikes from manufacturers such as Boardman, Raleigh and our own exclusive Carrera range.

While you’re there, you can chat with our trained staff who can discuss the differences between makes and models and help you find the right bike for you.

If you’re in the market for an electric bike, you can take advantage of our Free Electric Bike Trials.

Remember: Bikes are adjustable

People come in all shapes and sizes, but there are typically only a few frame sizes of each model, so you will be forced into making a choice. The good news is that bikes are adjustable and some parts can even be replaced. With a little work, you can get the bike fit just right. The seat post can slide up and down, the saddle can move backwards and forwards, and the handlebars and stem can be moved around. You can make all of these adjustments at home with a few standard bike tools. You can even do them while out on a ride if you need to. If these minor adjustments don’t work, you can swap parts like the stem to increase or decrease reach. If you have space, you can use handlebar spacers to raise their height. Parts such as the stem, spacers and even replacement handlebars can be picked up cheaply and are simple to fit yourself.

To help riders find the best bike fit, the Carrera range of mountain bikes, road bikes, hybrids and e-bikes come in a wider variety of sizes which makes finding the right fit easier.

In summery, the right bike size is not just one measurement. It’s a mix of fit, comfort, riding style, and body proportions. If you’re between two frame sizes, the best choice is the one that feels most comfortable and confidence-inspiring while riding.

Still not sure what size you need?

If you still can’t decide which frame size might suit you best, don’t worry. Just head to your nearest Halfords store where one of our bike experts can help you choose.

Before you set off, be sure to read our guide to getting the right size bike which is packed full of useful information and advice to help you choose. Armed with that information, the advice in this guide and our expert in-store help, finding your perfect bike fit should be simple.

Head over to Halfords.com where you’ll also find a hybrid bike size guide and a guide to finding the right size mountain bike. Why not also explore our full range of bikes and cycling accessories?