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What size bike do I need: The true beginner's guide

By Micah Ling

Published

In order to choose the right bike, you have to start by figuring out the right size for your body and your riding plans. And though certain measurements are important to know, a lot of it also comes down to how the bike feels. Never buy a bike because it’s technically the “right size” but doesn’t feel comfortable. For new cyclists, it’s always a good idea to have a professional fit done, so that you know all of the measurements that are important. Here are a few tips to help you get started

How are bikes sized?

Your height is a significant factor, but not everyone who is 5’9” has the same proportions or fits on the same bike. Several adjustments can be made to any bike, but the frame is the skeleton, so it’s important to start with the right bones. As frames get larger, everything gets farther apart, to accommodate longer limbs and a larger torso. The seat tube, top tube, head tube, and reach all get longer as you size up.

Road bikes

Road bikes are generally sized using centimeters. Traditionally, frame size equals the seat tube length. So a 54cm frame would have a seat tube that’s 54cm long. The top tube length will also be close to that same length. This has changed a bit with modern compact frame designs. Now, depending on the brand, some 54cm bikes might actually have a 52cm, 50cm, or even shorter seat tube. To eliminate the confusion this causes, a few brands have switched to lettered sizes: small (S), medium (M), large (L). At The Pro’s Closet, we equate 48cm-51cm with small, 52cm-55cm with medium, and 56cm-58cm with large.

Road bike geometry chart

Mountain bikes

Unlike a road bike, where your position tends to be stationary, on a mountain bike, you move around a lot more, and generally are not in the same position for very long. Therefore, the sizing is different, geared toward a good fit for riding in and out of the saddle, descending or climbing. Mountain bikes use lettered sizes: small (S), medium (M), large (L), and sometimes XS and XL.

Mountain bike geometry chart

Gravel bikes

Dedicated gravel bikes are usually sized like road bikes. When it comes to choosing a gravel bike, you’ll want to feel comfortable the same way you would on a road bike. The big decisions when it comes to gravel have more to do with wheel size, tire size, and tire pressure.

Are frame sizes standard across manufacturers?

Although numbers do mean something — 54 centimeters is always 54 centimeters — each manufacturer does things a little differently. So, buying a bike is a lot like buying a pair of pants. One version of the size might feel bigger, smaller, or better than another version of the same size. Check the manufacturer’s bike size chart, because key frame dimensions can vary based on model or brand.

What factors should I keep in mind when sizing?

Bike size charts usually recommend a range of rider heights for each size. For instance, it might say that a 54cm or medium frame will fit someone who is between 5’6” and 5’9”. If you’re in the middle of a range, that’s probably the best size for you. If you’re at either end of the range, you may have to decide if you want to size up or down. There is some overlap between sizes so a 5’9” rider could fit on a larger size depending on their proportions. Reach out to our Ride Guides if you’re between sizes and need advice!

Reach and stack dimensions are also critical. When you sit on a bike, reach is likely the first thing you notice. This measurement extends a vertical line straight up from the bottom bracket and measures from that point to the center of the head tube. As the name “reach” implies, it’s the distance from your body to the bars, no matter whether you’re standing or seated.

Similar to reach, stack measures the relationship between the bottom bracket and the top of the head tube, except this time it pertains to the bike’s height. Stack measures the vertical distance from bottom bracket center to a virtual horizontal line from the top of the head tube.

If you spend a lot of time pedaling in the saddle (road and gravel bikes) your top tube needs to be appropriate for your torso and arm length. It is typically measured on a horizontal axis from the center of the seatpost to the center of the fork's steerer tube. A bike’s top tube length and reach won’t always be perfect out of the box, but it’s easy to dial in the length of your bike with a shorter or longer stem.

Want to geek out on geometry? We've got a more in-depth article for you.

Standover height

You may remember some version of this measurement as a kid. Basically, stand over the top tube of the bike, if your feet comfortably touch the ground, and the top tube isn’t jammed up in your groin, that’s the bike for you! Obviously it’s not quite that simple, and standover height is just one part of the equation. But as long as you’re taking several other fit parameters into consideration, standover height isn’t a bad place to start. The Pro’s Closet lists the standover height for all bikes. You can compare this to your inseam measurement to make sure a bike you’re interested in isn’t too tall. To accurately measure your inseam, stand against a wall with a book or flat object between your legs and use that to measure from your groin to the ground.

Overall body size

No matter how many measurements you take, no two bodies are the same. Many brands are now creating women’s bikes that accommodate longer legs and shorter torsos, shorter reach, and lighter bodies.

It’s a good idea to not only know height and weight, but also inseam length. The more measurements you know, the easier it will be to find your perfect bike.

Riding a road bike on some sweet, sweet pavement

Conclusion

If you have a bike that fits well, but it’s time to upgrade, look up the geometry of your current bike. That’s a great starting point for a new bike. Look for something that has the same or similar stack, reach, and top tube length, for starters. If you’ve been riding a bike that doesn’t fit right, or if you’re a beginner, it’s not a bad idea to get a professional bike fit so that you have all the important measurements and a recommendation of what to look for.

Also know that frame size is just the beginning — adjusting things like stack height, handlebar width or shape, stem length, and seatpost setback will make your riding experience so much better. If possible, always test ride a number of different bikes, so that you can compare how different brands feel. There’s a lot that goes into finding the perfect bike, but when you do, you’ll never want to stop riding.

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