Buying a bike can be trying time, and one of the things that people stumble around on is bike sizing. How do I know if a bike is the correct size? Are there any hard and fast rules to bike sizing? That is why we have created this guide to bike sizing just for you.

Having a solid grasp of bike sizing will help you whether you pop into a bike shop or buy a bike online. It is obviously more important for buying a bike online as you can’t just sit on it and feel how it fits, you also can’t go for a test ride.

The Beginning of a Fit

What are the first things you need to know about choosing the correct bike size?

You need to know all your own measurements. If you don’t know your measurements how can you tell if a bike will or will not fit you? So, first of all, we will need you to grab a book, a friend (it is always easier when you have a friend) and a measuring tape. You will also need yourself.

First of all, we will want to find your inseam measurement. You will want to remove your shoes for doing this. 

① Find a flat wall.

② Stand with your back against the wall.

③ Move your feet shoulder width apart.

④ Take your book, preferably a hardback,  and run it up to the inside of your legs until it is snug in your crotch.

⑤ Get your friend to measure from the floor to the top edge of the book.

⑥ Congratulations you now have your inside leg length.

You will very likely notice that this measurement is different from the inside leg measurement of your clothing. Do not worry about this; tailors just use a different standard. 

You will also want to get your torso and arm length measured as well. These numbers will give you an excellent basis to know which bikes do and do not fit you. These measurements work better for road and gravel bikes. On a mountain bike, you will be moving around a lot, and you may find that you prefer a longer or shorter top tube nut making sure you can clear the top tube with your crotch is a great start to finding the correct mountain bike.

To get arm length

① Go back to your wall and stand flat against it again.

②Bring one arm level with your shoulder and straighten your hand.

③Measure the distance from the center of your collarbone to the center of your hand.

Your friend is doing a great job on measuring you here. Now get them to measure your torso length. You will want to go back to that wall and stand against it again.

①We want your feet about 6 inches apart.

②Get your friend to locate your sternum, it is the place just below your throat where your collarbone creates a “v” shape.

③Get your hardcover book back and again bring it up to your crotch.

④Measure from your sternum to the hardback book.

Congratulations, you now know all of your measurements.

Road Bike Sizing

Now on your road bike we will want to use a formula to get us a good guide to top tube length.

It is not a very hard form of mathematics, but when checking road bike sizes, I would look at their geo chart and start there. I would look at the top tube lengths. First, no one wants a bad back from cycling, which in the below chart is the measurement “0”.

If the answer was 540 mm and I was then in between two sizes. I would take my leg measurement and see which ones my crotch could clear. If my inside leg was 545 mm I would go for the 540 frame size, and if I were 530 mm I would go for the 510 mm frame size.

I would always prefer checking our geometry charts rather than a generic sizing guide. Our geometry charts are a much more valuable tool, and we are fitting you, not the average person.

It is not a very hard form of mathematics, but when checking road bike sizes, I would look at their geo chart and start there. I would look at the top tube lengths. First, no one wants a bad back from cycling, which in the below chart is the measurement “0”.

If the answer was 540 mm and I was then in between two sizes. I would take my leg measurement and see which ones my crotch could clear. If my inside leg was 545 mm I would go for the 540 frame size, and if I were 530 mm I would go for the 510 mm frame size.

I would always prefer checking our geometry charts rather than a generic sizing guide. Our geometry charts are a much more valuable tool, and we are fitting you, not the average person.

Stack and Reach

If you look at many bike geometry charts now, you will see a measurement called stack and reach. If you already own a bike you can use stack and reach measurements to get a much better idea of bike fit.

A bike’s stack is the distance from the center point of your bottom bracket to the center point at the top of your bike frame’s headtube. A bike’s reach is the horizontal version of this distance. These numbers are both measured from fixed points that will be identical on two bikes.

Sometimes sizing can be a bit fluid thanks to head angles, fork trail, or seat tube angles. By using stack and reach we get away from these discrepancies. You will know straight away whether or not a bike will fit.  

Mountain Bike Sizing

With our mountain bike sizing, I would start with my inside leg measurement. You will need to be able to clear your top tube if anything goes wrong when out riding and due to the amount of moving about you will do I would be less worried about top tube length.

So on the above diagram, you will see the frame’s seat tube measurement is “ST.” You may notice that this number is lower than a road bike, usually around 3- 4” smaller. This is again a reflection on the amount of moving you have to do on a mountain bike. You don’t want the frame continually getting in the way.

In order to get the correct frame size, we will again need to a little bit of mathematical work. This time we are going to use a formula created by a famous road cyclist, Greg Lemond.

Inside leg (cm) x 0.883

The formula gives you your seat height rather than a top tube height. The reason I have gone with this is that you will need to add your droppers drop to make sure you have enough room to drop your saddle. If you buy too big a frame you will not be able to do that.

If I have an inside leg of 73cm, I would expect a saddle height of 650mm. The size small with a 170mm dropper post would give a saddle height of 553 mm which leaves around a centimetre to play with. So that is where I would go.

If you need any further help with sizing please send us a message and we will help you.

TRIAERO Frame Geometry