Frame. Bicycle. Sizes. Three seemingly different words, but together they are crucial in determining the quality and longevity of your ride. Yes, because buying a bicycle online is just the first step. First, you need to choose the right model (road bike, gravel bike, etc.) and the correct size. And this is where a cyclist, if unprepared, might face the first challenges. That's why, especially when buying a bike online, it's essential to know how to measure the frame. Only then can you be sure you're making the best purchase based on your body’s unique measurements.
Measuring a Bike Frame: Everything You Need to Know (and More)
Measuring a bike frame isn’t as straightforward as it seems. Since not all brands use the same measurements to define their frame characteristics, choosing the right bike size can be tricky — especially for beginners. So, here are three key tips for picking the correct bike size:
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be curious — get used to checking all the technical information on the official website of the bike brand you're considering. This will give you a solid starting point for choosing the right size;
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pay attention to measurements — many bikes are measured in centimeters, though some brands still use inches. In that case, simply multiply the value by 2.54 to convert it to centimeters;
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don't overlook biomechanics — buying a bike online or from a bike shop is just the first step. The second is visiting a skilled bike fitter who will help you find the perfect riding position and provide a file with all the measurements taken for your specific frame. Keep this document safe — it’s useful for future adjustments or when purchasing a new bike.
Bikeroom Calculator: Find the Right Size for You
Measuring a bike frame and choosing the right size directly affects your comfort and performance, so precision is key. That's why we created the Bikeroom Calculator — a quick and easy tool to get an accurate estimate of the perfect bike size for you. Simply enter a few details (gender, height, and inseam length) to discover the best frame size for your body.
There are several ways to determine your bike size, but the most reliable method is based on your inseam height. Our bike size calculator uses this measurement — the distance from the floor to the top of your inner thigh — to suggest the ideal frame size.
The Bikeroom Calculator Home Screen
How to Measure Your Inseam Correctly
In simple terms, your inseam height is the distance from the floor to your crotch. Here’s how to measure it in 4 easy steps:
- grab a measuring tape and a hardcover book;
- stand barefoot with your back and heels against a wall, keeping about 15-20 cm (6-8 inches) between your feet;
- place the book between your legs to simulate a bike saddle;
- ask someone to measure from the top edge of the book to the floor — that’s your inseam height.
Bike Frame Analysis: Digging Deeper
Now that you know how to find your bike size, let's dive a bit deeper into the technical side of frame design, breaking down the key measurements used to define a bike's geometry. Generally, a bike frame's geometry consists of:
- Top Tube Length (Horizontal) - from the center of the seat tube (where the saddle is mounted) to the center of the head tube;
- Seat Tube Length (Vertical) - from the bottom bracket to the top of the top tube;
- Reach / Stack - key measurements defining the bike's fit, with reach indicating horizontal distance and stack the vertical one from the bottom bracket to the top of the head tube;
- Wheelbase - the distance between the front and rear wheel axles;
- Chainstays - the length of the rear stays from the bottom bracket to the rear axle;
- Front Center - the distance from the center of the bottom bracket to the center of the front wheel axle;
- Seat Tube Angle - measured between the seat tube and the rear wheel
- Head Tube Angle - measured by the angle between the head tube and a horizontal line where the fork sits;
- BB Drop (or Bottom Bracket Drop) - the vertical distance between the bottom bracket and a line drawn between the front and rear wheel axles;
- BB Height to Hub - the height of the bottom bracket relative to the wheel hubs.
Credit: bikexchange.com
Top Tube Length
Many modern bikes have a slightly sloping or "compact" top tube. In this case, to measure the effective top tube length, you need to consider its reach — the distance between the center of the saddle tube and the center of the head tube. The top tube length can be either:
- actual: when the top tube is completely horizontal;
- virtual: when the tube is sloped, representing the horizontal distance between the head tube and the seat post.
Many brands size their bikes based on top tube length. This doesn't apply as much to mountain bikes, which are usually available in Small (S), Medium (M), and Large (L) sizes — although this sizing method is becoming more common for road bikes too. Each manufacturer assigns their own sizes according to their range: for example, a "54 cm" might correspond to a size S for one brand or an M for another.
It's also crucial to highlight that not all brands measure the top tube in the same way. Some only account for the length from the head tube center to a vertical line projected perpendicularly from the saddle post's center. This method ignores the seat post’s rearward extension, meaning measurements might appear slightly shorter compared to other brands.
Pro tip: Pay attention to these details to avoid choosing the wrong size!
Specialized S Works Tarmac SL7 Disc, Peter Sagan collection | Credit: specialized.com
Seat Tube Length
The seat tube length is the line from the intersection of the vertical tube, the axis of the top tube, and the center of the bottom bracket. It sounds simple, but it's actually quite tricky.
If you measure from the center of the bottom bracket to the intersection with the axis of the seat tube (where the seat post is inserted), this is the "center-to-center" method. However, some bikes have an especially long seat tube—sometimes even longer than the top tube—and are built with an extension that creates an integrated seat post.
In this case, the measurement follows the "center-to-end" method, starting from the center of the bottom bracket and ending at the top of the seat tube. These variations often make it challenging to compare sizes across different models of the same type of bike.
The Top Tube of the Bianchi GreenEDGE
Reach and Stack
Since, as we've just seen, the top tube and seat tube length can be tricky fields when comparing frames, many manufacturers have introduced reach and stack measurements for their bikes—two additional metrics that help avoid confusion and make comparisons more straightforward.
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Reach is the horizontal distance from the center of the bottom bracket to the top-center of the head tube. In mountain bikes, for example, reach has been increased because stems have become shorter and handlebars wider. This change makes pedaling more stable at higher speeds.
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Stack is the vertical distance from the center of the bottom bracket to the top-center of the head tube, along the virtual steering axis. A low stack can enhance aerodynamics, but if it's too low, your back and neck might suffer during long rides.
Reach and stack go hand in hand and are crucial for determining the bike's optimal front-end position before adding a stem, as they take into account the length, angle, and any spacers used.
Credit: wilier.com
Wheelbase
The wheelbase is the distance between the bolts of the bike's wheels, and it plays a crucial role in the riding experience as it directly impacts the overall frame size. Fortunately, this is a relatively simple measurement to take.
Chainstays
This measurement also affects the wheelbase and significantly influences handling. Shorter chainstays make the bike more agile, while longer ones provide greater stability. The chainstay length is measured from the center of the bottom bracket to the rear wheel axle (or the derailleur hanger).
The chainstay of the Wilier Filante from Team Astana | Credit: wilier.com
Front Center
The so-called front center is another component of the wheelbase, representing the distance from the bottom bracket to the front wheel axle. This measurement also affects the bike's agility. However, it's important to note that the wheelbase is not simply the sum of the chainstay length and the front center, as neither of these is a purely horizontal measurement.
Seat Tube and Head Tube Angles
These two angles are key in determining how a bike handles. The more vertical the seat tube and head tube are, the more responsive and agile the bike will be.
Drop (or BB Drop)
The drop is the difference between the height of the wheel axle from the ground and the height of the bottom bracket from the ground. Essentially, it's the difference between these two measurements. This is a common spec you'll find listed in a bike's geometry chart.
Bottom Bracket Height
Last but not least, bottom bracket height is the distance from the ground to the center of the bottom bracket. It's fairly easy to measure, but you need to ensure the bike is upright and not tilted. Unlike the bottom bracket drop, this measurement can be influenced by wheel size and tire pressure.
