The great thing about learning how to measure a bike frame is that you’ll be able to tell the exact size that fits you, or someone else. You could, for instance, be a particular kind of biker who needs the exact kind measure of the frame every time. Knowing what that measure is will, of course, be helpful when, say, purchasing a new bike, or when renting one.
It can also be helpful if you are, say, selling a bike. Knowing how to measure a bike’s frame will allow you to inform better potential buyers who need to determine whether the bike fits them before they make a decision.
1. The easiest way is to determine the length of your seat tube is to check out the frame’s size label, which is typically listed at the bottom of the seat tube. If you’re wondering which of the tubes in your bike is the seat tube, don’t fret, because it’ll be really easy to tell. The seat tube is simply the tube that the seat post slides into. At the bottom of the seat tube will be a label, typically a sticker, where the bike’s frame size will be stated.
Not all bikes will have a size label, but as far as we know most will. If yours does, then you can at least skip measuring this part.
2. If you don’t see the label—if you own one of those rare bikes that haven’t been labeled—then the first thing to do will be to measure the length of the seat tube itself. Don’t forget to measure from the center of the gear crank to the topmost part of the seat tube, where the tube literally ends.
3. Now measure the top tube, which is the tube that connects the seat to the handlebars. If your top tube is slanted, then measure the distance between your seat and handlebars. When measuring this distance, it might help to imagine a horizontal line that is parallel to the ground that connects both your seat and handlebars.
4. Next, measure the head tube, which is the frontmost tube of the bike that connects your handlebar stem to the front wheels. You want to measure the length starting from where the head tube ends at the top to where the tube begins to fork below.
5. Lastly, measure your inseam. Basically, you’ll need to be barefooted and standing up to do this with your legs about 6 to 8 inches apart. Measure the length from the floor to your crotch area, or where your leg meets your hip bone.
6. For road bikes in particular: to get the right seat tube length for your height, all you need to do is get your inseam measurement (in number 5) and multiply it by 0.67cm. As an example, if your inseam measurement is 80 cm, then your seat tube length should be around 53.6 cm in length.
7. The measurement of the horizontal distance between your handlebars and the seat (number 3) can be used to determine your reach. While there are a few calculations proffered on the internet to help you determine your reach, the fact is that this is one of the most subjective areas of frame measurement, and the absolute best way to determine your reach length is to simply try a bike out and see for yourself what distance you are most comfortable with.
If you’re buying a bike online, it would be good to know already the length of the top tube and head tube that you are most comfortable with, so you can easily determine if the bike you’re buying is the right size. One important thing to remember is that the proper reach will allow you to rest your hands comfortably on the handlebars while maintaining slightly bent elbows.
8. For Mountain Bikes, you can get the proper seat tube length by simply multiplying your inseam measurement with a smaller number, like 0.65. The reason for this is that a mountain bike will have bulkier tires which will effectively lift you a few cms higher from the ground. So one way you can compensate for this is by getting a seat tube that is slightly shorter.
9. One method for calculating the proper top tube length when choosing a mountain bike (or the proper distance between the handlebars and seat) is to get your inseam measurement (in number 5), multiply it by 0.67cm and subtract the result by 10 to 13 cm. For instance, if your inseam measurement is 80 cm, then the proper top tube length should be around 43.6cm, since: ((80 x 0.67) – 10 ). Again, just like the with road bikes above, your best bet in determining reach length is to simply try a bike out and see what length you’re most comfortable with.
10. Despite all that we advised above, it’s always better to try the bike if it’s right in front of you anyway. This is because while the calculations we’ve listed above are a good way to determine the proper frame size, the best way would still be to try the bike out and see if it’s one you’re comfortable with. Naturally, body geometry is not the same for everyone, so the question of whether a particular bike frame is right for you can only ultimately be answered after you try it out.