Buying a bike in the correct size means it will be more comfortable and enjoyable to ride and you'll also reduce the risk of picking up an injury.
This guide will help you to understand the measurements you need to pick the right sized bike frame for you, no matter what your cycling discipline.
Why is bike size so important?
Even on a moderate ride, you repeat the same motions to drive the pedals thousands of times. Any excessive strain or muscle pulling quickly becomes problematic, resulting in discomfort and even injury.
We all know the health benefits of riding a bike. Getting the size right means you're more likely to enjoy it and less likely to find that it's so uncomfortable that you're compelled to give up. You'll also be able to ride more efficiently if the bike is the right one for you.
If the frame is too small, you could suffer an injury from holding a cramped position, clip the front wheel with your toe while turning, and you'll be unable to deliver your full range of movement, limiting the power you can push with.
If your bike is too big, it will be difficult to deliver your power efficiently or handle the bike properly.
Although there are many types of bikes with different riding positions to suit their purpose (for example road racing bikes which put the rider in a lower position or hybrids where the rider sits more upright), there are some general principles that apply no matter what kind of bike you're buying.
If the bike is the right size for you, you should be able to:
- Stand over the top tube with around two inches of clearance
- Pedal with a slight bend in your knee at the bottom of the stroke
- Reach the handlebars and brake levers comfortably
How do I choose the right size of bike?
Bikes are generally measured and defined by the length of the seat tube. This is the tube on the bike that the seat post (and saddle) slide into.
Traditionally, this measurement is given in metric (cm) for road bikes, and imperial (inches) for mountain bikes. Some manufactures use the seat-tube measurement to differentiate bike sizes while others use small, medium or large.
The way manufacturers measure their bikes can differ slightly from one brand to the next. At Wiggle, we measure all bikes in the same way and you'll see these universal measurement standards for each bike on our website.
What measurements do you need?
The main measurements you'll need are your height, your inside leg measurement, and your reach.
Your inside leg measurement will determine your standover height - this is the clearance between the top tube of the bike and your crotch when stood just in front of the saddle. We suggest a clearance of at least two inches to enable you to hop off quickly as and when required.
If your height and inside leg measurements put you in-between two sizes, then your upper body should be the deciding factor in selecting the best-sized bike for you. Your upper body dimensions determine your reach, and this affects how 'stretched-out' you'll be along the top tube of your bike. To work out if you have a long or short reach, you need to determine your 'Ape Index' (arm span minus height).
- If you have a positive Ape Index (i.e. your arm span is greater than your height), then we suggest you go for the larger of the two bike sizes.
- If you have a negative Ape Index (i.e. your height is greater than your arm span), then you will probably be more comfortable on the smaller of the two bike sizes.
Example: 152cm (arm span) - 163cm (height) = minus 11. In this example, you have a negative 'Ape Index', and should go for a smaller frame size.
If you're still not happy with the reach on your bike, it's possible to swap the stem (the part your handlebars attach to) for a longer or shorter one. Whilst this is a common modification which suits many riders, be aware that it can affect the bike's handling. The best solution is always to ensure your bike fits from day one.
If your bike is the right size, you can make adjustments to the fore and aft position of your saddle and its tilt, the angle of your handlebars, and the distance to the brake levers to get the perfect position for you.
Women-specific bikes and unisex bikes
The old assumptions that women have a shorter body and longer legs don't necessarily hold true and there are always exceptions and outliers. A women-specific bike might make sense for your size and shape, but you should always check the measurements and make sure it's right for you. Many women find that they are perfectly comfortable on a unisex frame.
We asked Ben Marvin, one of the Design Engineers over at Vitus, to explain the difference between the unisex and women's bikes in their range.
'At Vitus, we offer women's options in the Mythique, Sentier, Nucleus, Razor and Zenium ranges. Although the frame geometry is the same as the unisex models, the women's bikes are available in a smaller size curve.
'We know that women generally have narrower shoulders so we fit narrower bars and a shorter stem. The shorter stem-length isn't just to do with the reach - narrow bars with a longer stem would make the steering feel twitchy on a smaller bike. We also know that a women's pelvis has a different shape to men's so we fit a women's saddle which has a different cut.'
Whether you're a man or a woman, the most important thing is to make sure that the key measurements of inside leg and height are correct. If the bike fits, it's easy to fine-tune the details by swapping out things like saddles and bars.
How do you choose the correct size of kid's bike?
Children's bikes are measured and sized in a different way from adult bikes. They are usually categorised in age ranges and wheel sizes.
Below are some key points to keep in mind when sizing a bike for your child:
- Don't rely completely on an age range to size a bike for your child as this is only for guidance. If your child is tall for their age, then you may need the next size up.
- Height and inside leg measurement are just as important for kids as they are for adults.
- Whilst it's tempting to buy a larger size for your child to 'grow in to,' if the bike is too big it will be difficult for them to control and ride with confidence. Reduce the chance of them falling over and give them the opportunity to learn to ride with skill and passion by getting a bike that fits.
Bike sizing charts
For reference, here are the generic bike size charts we use at Wiggle.
Click on the bike size guide to take you to our ranges.
Buying your bike
We want you to be confident that the bike you've chosen is right for you so if you need any help, please don't hesitate to ask one of our advisers on our pop-up chat.
When you've chosen your bike, you'll be asked to confirm your height and inside leg measurement at the checkout. Our qualified mechanics will double-check that the bike is suitable before they build it.
There will be a small amount of assembly to do at home but the video below shows you what to expect. Enjoy the ride!