Bike Size Guide: Adults & Kids
Getting the right bike size makes a bigger difference than most people expect. Ride a frame that is too large, and you are constantly stretching to reach the bars, which puts strain on your back and shoulders over time. Too small and your legs are cramped, your pedalling loses efficiency, and the whole ride feels like a battle. For children, it matters even more because a bike that does not fit properly is genuinely harder to control and stop, which can knock confidence before it even has a chance to build.
This guide walks you through everything you need to know before buying. It covers how to take your measurements properly, how to read sizing charts for every type of bike, how to match the right wheel size to a child, and what to do when you fall between sizes. The FAQ section at the end covers the most searched questions around bike sizing, written as direct answers you can act on right away.
Once you've read through our guide, check out the full range of bikes at GO Outdoors.
Step One: Take Your Measurements
Before you look at any sizing chart, you need two measurements: your standing height and your inside leg length. Most people only bother with height, which gives you a starting point, but the inside leg is the more reliable figure. Two people who are the same height can have quite different inside leg lengths depending on their build, and for mountain bikes, inside leg length is the main input in most manufacturer guides. Take both, and you will be in a much stronger position from the start.
How to measure your height
Take your shoes off and stand with your back against a flat wall, feet together. Put a hardback book flat on top of your head and mark where it touches the wall. Measure from the floor up to that mark in centimetres.
How to measure your inside leg
Shoes off again, back against a wall. Slide a hardback book between your legs and press it upward as firmly as feels comfortable, replicating the pressure you would feel from a saddle. Mark the top of the book on the wall and measure from that point down to the floor. It is worth having someone help you here, as it is tricky to get an accurate reading on your own.
One thing worth knowing before you start: mountain bike sizing works differently from road bikes. It uses inside leg as the primary input rather than height, so take both measurements before you look at any chart. If you end up between two sizes, measuring your arm span and subtracting your height can help point you in the right direction. A positive result usually means the larger frame will suit you better. And if you plan to ride on the road regularly, a professional bike fit at a specialist store will always give you more precision than any general chart can.
Children's Bikes: Sizing by Wheel Diameter
Kids' bikes do not work like adult bikes. Instead of frame size, they are sized by wheel diameter, starting at 12 inches for the very youngest riders and going up to 24 inches before most children are ready to move on to an adult frame. A child who is around 150 cm tall is generally at the point where it is worth switching to adult sizing.
Age brackets are included in the chart below; treat them as a rough guide only. Children grow at very different rates, so height is always the more reliable starting point. A taller eight-year-old could easily be ready for a 24-inch bike, while a smaller child the same age might still be perfectly comfortable on a 20-inch. Before you buy anything, make sure your child can stand over the bike and reach the brakes comfortably.
| Wheel Size | Approx. Age | Child Height | Min. Inseam | Notes |
|---|---|---|---|---|
| 12 in | 2 to 3 years | 85 to 100 cm | 35 cm | Balance bikes and very first pedal bikes |
| 14 in | 3 to 4 years | 95 to 108 cm | 40 cm | First pedal bikes for slightly older beginners |
| 16 in | 4 to 6 years | 105 to 120 cm | 48 cm | Entry point for most geared kids' bikes |
| 20 in | 6 to 10 years | 115 to 135 cm | 55 cm | Widest range; suits most primary school riders |
| 24 in | 8 to 12 years | 127 to 145 cm | 63 cm | Transition size before adult frames |
| 26 in | 11+ / small adults | 145 cm+ | 70 cm | Youth or small adult; some Calibre models |
Balance bikes versus pedal bikes
For children under three, a balance bike is almost always the better starting point. Kids who learn to balance and steer first, without pedals getting in the way, tend to make the switch to a pedal bike much faster than those who start with stabilisers. By the time they are around three (usually about 85 cm tall), most children who have spent a few months on a balance bike are ready to go straight onto a pedal bike without ever needing stabilisers.
How much clearance does a child need over the frame?
When your child stands with a leg on either side of the bike, there should be at least 2 cm of space between them and the top tube. They also need to be able to reach and squeeze the brakes without straining. If either of those things is not quite right, the bike is too big. It is always better to have a bike that fits well now than one that offers growing room but is harder to control and less safe to ride.
Need more guidance on kids' bikes? Check out our full kids' bike guide.
Adult Bikes: Sizing by Frame
Adult bikes are sized by frame size, which is the length of the seat tube measured from the centre of the bottom bracket up to the top. Road bikes use centimetres. Mountain bikes use inches, although plenty of brands now label things as S, M or L instead. Worth knowing: you may well need a different size depending on the type of bike you are buying, because the geometry and riding position vary quite a bit between categories.
Mountain bikes: size by inside leg
Mountain bikes are built with lower, sloping top tubes specifically to give you more freedom of movement when things get technical. Because of this, inside leg is a better starting point than height when sizing a mountain bike. The table below covers the typical frame size ranges you will see across most brands.
| Inside Leg | Frame Size | Label |
|---|---|---|
| 63 to 70 cm | 14 to 15 in | XS / S |
| 68 to 75 cm | 16 to 17 in | S / M |
| 73 to 80 cm | 18 to 19 in | M / L |
| 78 to 85 cm | 20 to 21 in | L / XL |
| 83 cm+ | 21 in+ | XL / XXL |
Road bikes: size by height
Road bike sizing is where getting it right matters most. You are in a fairly fixed position for long stretches of time, which means small fit errors that you might not notice in the first ten minutes will start to make themselves felt over longer rides. Road bikes are sized in centimetres, and you want roughly 2.5 cm of standover clearance as a baseline.
| Rider Height | Frame Size (cm) | Label |
|---|---|---|
| 155 to 162 cm (5'1" to 5'4") | 48 to 50 cm | XS |
| 162 to 170 cm (5'4" to 5'7") | 50 to 54 cm | S |
| 170 to 178 cm (5'7" to 5'10") | 54 to 56 cm | M |
| 178 to 185 cm (5'10" to 6'1") | 56 to 58 cm | L |
| 185 to 193 cm (6'1" to 6'4") | 58 to 61 cm | XL |
| 193 cm+ (6'4"+) | 61 cm+ | XXL |
Hybrid and commuter bikes
Hybrids are the most forgiving type of bike to size. The upright riding position suits a wide range of builds, and a small sizing error is unlikely to cause the same problems it would on a road bike. Most are labelled in inches or S / M / L, and they work well for everything from a daily commute to a relaxed park ride.
| Rider Height | Frame Size (inches) | Label |
|---|---|---|
| 155 to 165 cm (5'1" to 5'5") | 14 to 16 in | S |
| 165 to 175 cm (5'5" to 5'9") | 16 to 18 in | M |
| 175 to 185 cm (5'9" to 6'1") | 18 to 20 in | L |
| 185 cm+ (6'1"+) | 20 to 22 in | XL |
Browse the full adult bike range at GO Outdoors. Each product page includes the manufacturer's own size chart, which is worth checking alongside the general tables above.
Standover Height: The Safety Check
Standover height is the distance between the top tube and the ground, measured at the midpoint. When you stand flat-footed with a leg on either side of the bike, the gap between you and the frame is your standover clearance. How much you need varies depending on what type of bike you are riding.
| Bike Type | Recommended Standover Clearance |
|---|---|
| Road bike | At least 2.5 cm (1 inch) |
| Mountain bike | At least 5 cm (2 inches) minimum |
| Hybrid / Commuter | 2.5 to 5 cm |
| Kids bike | At least 2 cm; the child should stand over comfortably with feet flat |
Mountain bikes tend to have naturally lower standover heights than road bikes because of the sloping top tube design, so getting enough clearance is usually straightforward. If you are torn between two mountain bike frame sizes, checking the standover measurement on the product page is one of the most reliable ways to make the call. Pay attention to reach too. If the larger frame feels stretched when you sit on it, and the smaller one feels right, trust that instinct over the chart.
Saddle Height: Getting the Position Right
Picking the right frame is the starting point. Getting the saddle height right is what actually makes the bike feel like it fits. Most bikes have a quick-release seat post clamp so you can adjust it without any tools.
Adults
When one pedal is at its lowest point, your leg should have a slight bend at the knee. Not fully straight, not deeply bent. A useful starting formula is to multiply your inside leg in centimetres by 0.883 to get an approximate saddle height from the centre of the bottom bracket. Treat it as a starting position rather than a fixed answer and adjust from there as you ride.
Children
For children, set the saddle low enough that they can put both feet flat on the ground while sitting. That ability to stop themselves easily builds confidence faster than anything else. As they become more capable, raise it so just the balls of their feet are touching the ground. Once they are riding confidently and in control, move to the same slightly-bent-knee position used for adults.
A quick way to check: sit on the bike on a flat surface and put one pedal at its lowest point. Your heel should just reach the pedal with your leg almost straight. When you move to the normal riding position with the ball of your foot on the pedal, your knee should have a slight bend. If your hips are rocking from side to side while you pedal, the saddle is too high. If your knees are coming up toward your chest, it is too low.
What to Do When You Are Between Sizes
It is genuinely common to sit between two frame sizes, especially with road bikes, where the gaps between sizes are fairly small. When that happens, reach is usually what decides it. Measure your arm span and subtract your height. If the result is positive, meaning your arms are longer than you are tall, you will generally feel more comfortable on the larger frame. A negative result usually means the smaller frame suits you better.
| Choose the smaller size if... | Choose the larger size if... |
|---|---|
| You want a more agile, responsive feel | You want a relaxed, stretched-out riding position |
| Your arm span is shorter than your height | Your arm span is longer than your height |
| You are buying a mountain bike for technical riding | You are buying a road or hybrid for long distances |
| The larger frame feels stretched when you sit on it | The smaller frame feels cramped through the shoulders |
Whenever you can, try both sizes in-store before committing. It removes all the guesswork, and for road bikes and mountain bikes especially, sitting on the bike tells you more in thirty seconds than any chart will. GO Outdoors carries a range of sizes in-store so you can get a proper feel before you buy.
Choosing by Situation
A child's first bike
Start with your child's height and match it to the wheel size chart. Check they can stand over the frame comfortably and reach the brakes without stretching. If they are under three or have never ridden before, a balance bike is almost always the right place to start. Try to go for a lighter frame wherever you can. A heavy bike is genuinely harder for a young child to handle, and that friction takes the fun out of it quickly. Our Wild Bikes collection usually contains a reliable choice for your child's first bike.
Leisure riding and family cycling
For most adults who want a bike they can just get on and ride without thinking too hard about it, a hybrid or commuter with an upright position is the right choice. Use the hybrid sizing chart, check that the standover feels comfortable, and pick a frame where you sit naturally rather than leaning forward. Hybrids are the most forgiving category, so even if your size is not exact, you will likely still feel comfortable.
Trail riding and mountain biking
Inside leg is your primary input here, not height. Aim for at least 5 cm of standover clearance and check the reach figure on the product page alongside the frame size, as the two together tell you much more than the size label alone. If you are not sure where to start, the Calibre range at GO Outdoors covers a wide range of budgets and experience levels, from entry-level hardtails through to full-suspension trail bikes.
Road cycling and commuting on roads
Start with the height chart and then check the standover data for the specific model you are looking at. If you are between sizes, go in-store and try both before deciding. For anyone planning to ride regularly, a professional bike fit is worth considering. It gives you a level of precision that no sizing chart, however detailed, can fully match.
Frequently Asked Questions
What size bike do I need?
The right bike size depends on the type of bike and your measurements. For a road bike, use your standing height: 170 to 178 cm suits a 54 to 56 cm frame. For a mountain bike, use your inside leg: 73 to 80 cm suits an 18-to-19-inch frame. For a hybrid, height is the main input, and the fit is more forgiving. Measure both before consulting any chart.
What size bike do I need if I am 5 ft 8 in?
At 5 ft 8 in (173 cm), a 54 to 56 cm road bike, a medium to large mountain bike frame (17 to 19 inches), or a large hybrid frame is typically the right starting range. These are general figures; always check the specific manufacturer's size guide for the model you are buying, as geometry varies between brands.
What size bike do I need if I am 5 ft 5 in?
At 5 ft 5 in (165 cm), a 50 to 54 cm road bike or a small to medium mountain bike frame (15 to 17 inches) is usually the right starting point. For a hybrid, a medium frame (16 to 18 inches) typically fits well. Check the specific model guide before buying.
What size bike do I need if I am 6 ft?
At 6 ft (183 cm), a 56 to 58 cm road bike, a large mountain bike frame (19 to 21 inches), or a large hybrid frame (18 to 20 inches) is typically the right range. Riders at this height often fall cleanly into the large bracket across most brands. Always confirm by checking the manufacturer's size guide for the specific model.
What size bike does a child need?
Children's bikes are sized by wheel diameter, not frame size. Match the wheel size to your child's height: 12 inch for 85 to 100 cm, 16 inch for 105 to 120 cm, 20 inch for 115 to 135 cm, and 24 inch for 127 to 145 cm. Always use height rather than age as the guide, since children develop at very different rates. The child must also be able to stand over the bike with clearance and reach the brakes comfortably.
What size bike does a 7-year-old need?
Most 7-year-olds suit a 20-inch wheel bike, which fits children roughly between 115 and 135 cm tall. Taller children of the same age may already be ready for a 24-inch. Always measure your child's height and check they can stand over the bike and reach the brakes before buying.
What size bike does a 5-year-old need?
Most 5-year-olds suit a 16-inch wheel bike, which typically fits children between 105 and 120 cm tall. Some taller or more experienced 5-year-olds are ready for a 20-inch. Measure your child's height first and use the sizing chart rather than relying on age alone.
What size bike does a 10-year-old need?
Most 10-year-olds suit a 24-inch wheel bike, which fits children roughly between 127 and 145 cm tall. Taller children at the upper end of this range may be ready to move to an adult small frame. Confirm the child can stand over the frame comfortably and reach the brakes before buying.
How do I measure a child for a bike?
Measure the child's standing height with their back against a wall, shoes off. Then measure their inside leg by placing a hardback book between their legs, pressing it upward firmly, and measuring from the top of the book to the floor. Match the height to the wheel size chart above. Also, confirm the child can stand over the frame with at least 2 cm of clearance and can reach and squeeze the brakes without difficulty.
At what height does a child move on to an adult bike?
Most children are ready for an adult-sized frame once they reach approximately 150 cm in height. At that point, use adult frame sizing charts rather than the kids' wheel size chart. Most children at this height suit an extra-small or small adult frame, depending on the type of bike and their inside leg measurement.
How do I know if my bike is too big?
A bike is too big if you must stretch uncomfortably to reach the handlebars, if you struggle to control the front wheel in corners, or if your hips rock from side to side while pedalling, even with the saddle at the right height. When stationary, you should be able to stand flat-footed with a leg on either side of the frame with at least a small amount of clearance. If you are on tiptoes or feel unstable at a standstill, the frame is likely too large.
How do I know if my bike is too small?
A bike is too small if your knees rise toward your chest at the top of the pedal stroke, even with the saddle raised to its maximum height, if your back feels hunched and cramped, or if you feel generally restricted in your movement on the bike. At the correct saddle height, you should have a slight bend in your knee at the bottom of the pedal stroke. If the saddle is already at maximum and your leg is still too bent, the frame is too small.
Should I size up or size down when between bike sizes?
Use your reach to guide the decision. Measure your arm span and subtract your height. If the result is positive (arm span longer than height), the larger frame tends to suit you better. If it is negative, the smaller frame generally fits more naturally. For mountain bikes, the larger size usually handles better on the trail. For road bikes, the smaller size can feel more controlled. When in doubt, try both sizes in the store before deciding.
Can I adjust a bike to make it fit better?
Yes, to a point. Saddle height is the most impactful adjustment and can be changed easily without tools on most bikes. Handlebar height can sometimes be raised or lowered by moving spacers on the stem. Stem length can be swapped out to increase or reduce the reach to the bars. However, all these adjustments have limits. A frame that is significantly too large or too small cannot be corrected through component changes alone. If you are pushing any adjustment to the extreme just to make the bike feel rideable, then frame size is probably wrong.
What size mountain bike do I need?
Mountain bikes are sized primarily by inside leg rather than height. Measure your inside leg and use the chart in this guide: 63 to 70 cm suits a small frame (14 to 15 inches), 73 to 80 cm suits medium to large (18 to 19 inches), and 78 to 85 cm suits large to extra-large (20 to 21 inches). Aim for at least 5 cm of standover clearance when standing flat-footed over the frame. Always check the specific manufacturer's guide for the model you are considering.
Do men and women need different size bikes?
The sizing method is the same for both: height and inside leg determine the right frame size, regardless of gender. Women-specific bikes often have a shorter top tube to reduce the reach, along with narrower bars and a different saddle. These are genuine fit benefits for many women, but there is no reason a woman cannot ride a unisex frame if the dimensions suit her proportions. Choose the frame that fits best, whether it is labelled women-specific or unisex.
What size bike does a woman who is 5 ft 4 in need?
At 5 ft 4 in (163 cm), the typical starting range is a 48 to 52 cm road bike, a small to medium mountain bike frame (14 to 16 inches), or a small to medium hybrid (14 to 16 inches). Women-specific models in these sizes often offer a shorter reach and a more upright position. Check the manufacturer's guide for the specific model, as sizing varies between brands.
What size electric bike do I need?
Electric bikes use the same frame sizing as standard bikes. Use the same height and inside leg charts in this guide for road, mountain and hybrid e-bikes. The key practical difference is weight: e-bikes are considerably heavier than standard bikes, which can make a slightly smaller frame easier to handle at low speed and when manoeuvring. If possible, test ride an e-bike in store before buying, as the weight distribution feels noticeably different to a standard bike of the same frame size.
What size frame do I need for a gravel bike?
Gravel bikes are sized in centimetres like road bikes and use the same height-based charts. However, gravel bikes have a more relaxed geometry than race road bikes, and some riders prefer to size up by one increment for a more comfortable, upright position on long off-road rides. Check the specific manufacturer's guide for the model you are considering, as gravel bike geometry varies considerably between brands.
What is the difference between frame size and wheel size on a bike?
Frame size is the length of the seat tube and is what determines whether an adult bike fits you. Wheel size (26, 27.5 or 29 inches on mountain bikes; 700c on road bikes) affects how the bike rolls and handles, but does not determine which frame size you need. Children's bikes are the exception: they are categorised by wheel diameter rather than frame size because young riders' needs change more rapidly than frame geometry can reflect.
What is the difference between 27.5 and 29-inch wheels on a mountain bike?
29-inch wheels roll over obstacles more easily, carry momentum better and suit taller riders and cross-country riding. 27.5-inch wheels are lighter, more agile and easier to flick through tight corners, which suits shorter riders and technical trail riding. Neither wheel size changes the frame size you need: the inside leg sizing charts for mountain bikes apply regardless of which wheel diameter a model uses.
Can I check standover height before buying a bike online?
Yes. Most manufacturers publish full geometry data for each model on the product page, including standover height. Compare that figure to your inside leg measurement to confirm you have adequate clearance before ordering. For mountain bikes, also check the reach figure: standover and reach together give a much more complete picture of fit than the frame size label alone.
