When you buy a road bike, there are many things you need to consider. In addition to finding the right type of bike, e.g. road bike, mountain bike, etc., you also need to pick the correct bike frame size. Since bicycles aren’t getting any less expensive, you need to ensure that you’re asking the right questions. The guide below includes information about how to size bike frames for adults and children and as well as some information about whether to involve gender in the sizing process. Key content from this post is adapted with permission from Bicycle New England.
Bike Sizing Essentials
You can’t get the frame size wrong. If it is, things are bound to get dicey as you most likely won’t be comfortable and riding with an incorrectly-sized frame is downright dangerous. In some cases, you my be exacerbating health issues; especially in cases where risk factors are high. Lower back pain, elbow pain, neck pain, and knee pain can all be indicative of a poor fit (among other issues).
Step 1: Measure Your Frame Size
Bike frame sizes are based on the height of the bike. To determine the correct height of the hike, measure the length of the seat post tube (see image above). This measurement is expressed in metric (cm) for road bikes, and imperial (inches) for mountain bikes.
Step 2: Get Your Measurements
To figure out the right frame size, you need to take a few measurements:
- Your leg inseam
- Your torso length
- Your arm length
Leg inseam is the most common measurement you’ll see as you pick the right bike frame size. Many bike size charts also reference stand-over height. This measurement refers to your leg inseam plus another 1-2 inches for comfortable clearance of that top tube. Some sizing guides claim that road bikes need one to two inches of clearance while mountain or commuter bikes need two to four inches.
Another common formula is that of the top tube length. Finding a road bike that is on the compacter side with a shorter top tube makes a big difference; so don’t overlook this measurement. If you have a short torso, the distance between the saddle and the handlebars can really affect your ride. To determine your ideal top tube length, try the following:
(torso length + arm length) / 2 = x
x – 6 = top tube length
(Add your torso length to your arm length, divide that by two, and subtract six). This will tell you in inches what the ideal distance would be between your seat and handlebars.
If your bike has the right frame size, you will also be able to comfortably stand over the cross bar. You will be able to stop safely, rather than having to fall sideways until your foot hits the ground…ouch!
If your bike is too small, your knees and back may start to hurt, and you won’t be able to use your leg power efficiently.
Height and Inseam Length
The two most important measurements to know are your height, and your inseam length (the distance from your crotch to the end of your pant leg). Of these two lengths (height and inseam), inseam is the most important.
The reason the inseam length is the most important measurement is because it determines your stand-over height. For example, if a bike has a stand-over height of 27 inches, and you have an inseam length of 29 inches, you will be able to stand over the bike, and the cross bar will not hurt you if you have to stop suddenly. This is where you definitely have to get things right. Also, this bit of clearance will enable you to safely and easily hop off the bike when you need to; unless you have a bike with a tube that is purposely designed to dip down.
For road bikes you need to have about 1 to 3 inches of distance between the bar and your crotch; for mountain bikes it is safer to have an inch or two more.
Your arm length measurement becomes important if your height and inseam measurements put you in between sizes. In that case, use your arm measurement to decide.
You need to know if you have a long or short reach, which means you need to find out your “ape index” (which is your arm span compared to your height).
- If you have a positive ape index (your arm span is greater than your height) then you have a long reach and you should probably go for the larger of the sizes.
- If you have a negative ape index (your height is greater than your arm span) then you have a short reach and you should probably go for the smaller of the sizes.
For example, say you have an arm span of 160 cm and a height of 160 cm. This would mean your arm span is exactly the same as your height. So you have an ape index of 1, and therefore an average reach. However, if your arms are shorter, you have a short reach, so if you are between frame sizes, go for the smaller frame size.
Two Different Bike Frame Size Systems
Just to make it a little more challenging, there are two different bike-sizing systems. There’s one system for road bikes, and then a different one for mountain bikes and hybrid bikes. Sizes are given in metric (cm) for road bikes, and imperial (inches) for mountain bikes.
You can figure out your bike frame size with the Bike Frame Size Guides below
Even with the right frame size, if you are planning to do a lot of cycling, you should still get your bike fine-tuned with a professional bike fitting, if you can afford it. If you are buying a new bike, try to find a bike shop that offers some help with bike fitting. Some of them are especially good with this, and will give you a professional level of help. (Many of the smaller bike shops cannot afford this, however.) Getting a well fitting bike will protect you against back pain or cycling knee pain.
Step 3. Consider Buying a Gender-Specific Frame
On average, women tend to have longer legs and shorter torsos than men of equivalent height. Women specific bikes offer smaller frame sizes, shorter top tubes, narrower handlebars, female specific saddles and smaller brake handles.
However, you are unique, and a women specific bike may not be right for you, even if you are a woman. The bike you choose should be the perfect fit for you individually, regardless of gender.
If you are shorter than about 5 ft 5 in (165 cm) or have felt too stretched out on unisex bikes before, you may be more suited to a women specific bike. Try them out and see what works for you. I have known women who have been absolutely ecstatic on riding their first women specific bike, because for the first time they felt like the bike really fitted them. On the other hand, other women find no benefit in them.
Also, bear in mind that if you are a short man, a woman specific bike may be right for you. No one needs to know! The reality is that the average male height in some countries is shorter than the average female height in others. For example, average male height in Indonesia is 5 ft 2 in, while average female height in The Netherlands is 5 ft 7 in (must be all that cycling that makes them grow so tall and strong)! So obviously, we cannot really generalize too much about this matter. Everyone is unique.
Step 4: Select a Frame for Your Child
Kid’s bikes are measured and sized differently. Often they are categorized in age ranges and wheel sizes, as you can see from the table. But keep these points in mind:
Don’t rely on n age range to pick a bike size for your child. There are tremendous variations based on individual differences. Just as with gender, age does not necessarily predict height.
- Height and inside leg measurement remain the key factors. Keep this in mind and make sure your child can comfortably stand over his or her bike. This is especially important for safety for kid’s bikes – they need to be able to stop safely, and get off safely. Don’t put them off bikes for life by making their early experiences scary. Which brings us to the next point:
- Do not buy a bike that is a bit bigger, thinking your child can grow into it! The bike needs to fit from the get-go, so that your child does not get hurt or frightened. Unlike a jacket, a child can actually fall off a bike, so buying big is not a good idea. You may be able to extend the useful life of your child’s bike by raising the saddle when he or she grows. But you must have the right frame size to begin with.
Note: The tables showing how to pick the right frame size are extracted from my Bike Buyer’s Guide (aka How to Buy Used Bikes Online). Subscribe to this blog to get a free download of the entire book!
Questions and Answers
What if I purchased the wrong frame ?
Then ask your merchant, if you can send your bike back to get the right one. If you don’t want to do that, then you can do the following things:
- Move your saddle to the front or rear
- Buy a longer or shorter stem. It changes your body position
- Buy a longer seat post
Give it time to get the feeling right. I remember when I got my first 29er mountain bike. It felt like a ship and seemed difficult to ride in narrow trails of the forest. Now I start laughing when I sit on a 26″ mountain bike because it looks so tiny.
Should I buy a bike online?
It’s the most obvious and overused advice out there, but it’s true: Seeing a bike in person, trying it out, and getting a feel for it is often the best way to know if it’s right for you. Many bicycle brands and bike shops offer demo days or demo events to allow customers to test ride bikes. It also helps to ask questions and speak to knowledgeable staff members. As an added bonus, you avoid the potential damage from shipping, and you won’t have to assemble anything yourself.
How do I size my road bike tires?
Looking for the right tire size can feel a little like navigating a minefield at times, but understanding the particulars can help you get things spot on for a much easier ride.
On a road bike, tire sizes are described using dimensions.
Example: If a road bike measures 700 x 25c:
- 700 is the standard rim size diameter
- 25c is the width of the tire as you look down from a riding position.
Generally speaking, wider tires provide a smoother ride as the volume of air in each tyre is larger. Tyre manufacturers now make lighter, faster tires that are wider to meet the needs of today’s riders.
Several wheel manufacturers make wider 23mm or 25mm wide rims. These offer a high level of strength, performance and comfort when matched with wider 23mm/25mm tyres.
Most road bikes will have enough clearance to fit 25mm-wide tires, which can offer a good amount of comfort for the rider. Newer disc-brake road, gravel and cyclocross bikes often have enough clearance for 35-38mm off-road tires to cope with more challenging surfaces, such as mud and gravel.
If you’d like additional information about sizing your bike, check out REI’s bike sizing guide. And check out our best bike rack guide if you’re wondering how you’re going to transport your newly purchased road bike; or, bike lights for that dark ride.