Rider height is typically the primary factor in determining frame size, but it’s not 100% reliable. People’s geometries vary, from longer torsos to shorter legs and vice versa, and bicycles are no different. And no standardised system exists in the cycling industry for measuring bicycle frames. A size 54cm in one brand may not fit you in another. This is where rider inseam comes into play, and we are talking more than how long your jeans are.
Once measured correctly, your inseam length will be your first step in choosing the frame size best suited for your build. Name brands can guide you, but they aren’t enough. A frame’s geometry needs to correspond to your standover height. You’ve gotta do your homework. Measuring is easy. You can do it at home, alone or with a friend, with just a few household items. Let’s see how it’s done.
Collect the following bits before you start.
- Something to mark the wall with, a pen, pencil, bluetack, or a bit of masking tape, for example.
- A hardcover book with a rigid spine, or a spirit level that is long enough so you can see the bubble.
- A tape measure, or a piece of string (you can measure the string length after)
- A scrap of paper to record your results
Measuring the inseam
1. Find a closed door or some free wall space that is large enough for you to stand with your back up against it. You can take your measurement anywhere you are standing, but a fixed wall or closed door helps to make sure you are standing straight.
2. Remove your shoes, barefooted or in your stocking feet is fine. Have your marking instrument, a measuring tape or a piece of string handy. If a friend is helping you, ask them to hold them for you.
3. With your back against the wall, and your feet flat shoulder width apart on the floor, take the book or spirit level and place it between your legs. Slide it up to your groin. It should be in close contact with your person, more than just touching, as if sitting on a saddle. Make sure the entire length of the book is in direct contact with the wall. The spirit level, my preferred method for accuracy, has that handy bubble you can watch to guarantee it is parallel to the ground.
4. Firmly hold the book or spirit level in place (a friend can help with this) against the wall as you advance just enough to mark its point of contact. Step away from the wall and measure the height from the floor to your mark.
Some may prefer to measure directly with the book or level in place, but moving yourself out of the way before you do makes it easier. Repeat the process three times. Now break out your elementary math and calculate the average. Voila, you have the length of your inseam.
Now the fun part! Take your inseam measurement and begin looking for your future soulmate. If you have a brand or model name in mind, start there. You can also do an advanced search by numbers only by supporting the website for a small fee. The bike sizing will vary based on the type of riding you wish to do, as well as your experience. For example, mountain bikes won’t have the same geometry as a competition road model. And advanced riders will have different needs than beginners. Compare your calculation to any frame’s standover measurement to find your ideal size starting point. You should find this metric about halfway down the figures listed. The number defines what it measures, how much of an inseam a rider needs to standover the bike.
A rider should be able to straddle a bike comfortably between the legs when not riding, such as when setting out, stopping for red lights, and in case of a crash. Not sure? Check if you can place at least two fingers between yourself and the frame. If you can, you should be good to go. If not, try the next size down.
Other factors such as your overall height, reach and stack further determine which frame size is best for you, but having your inseam measurement is the perfect starting point. Your local bike shop or fit professional should be able to help you from there.
Ride before you buy
Numbers are just that, numbers. The best method, if possible, remains trying a bike out before you buy it. Once you’ve nailed the geometry, other components on the bicycle, such as the saddle position and height, stem length and handlebar width, will dial the bike to your anatomy.
If you are new to riding or have signed up for a special event such an endurance ride or a grand fondo in the coming months, consider getting a professional bike fit. It’s money well spent, an investment in yourself, that will make your entire experience more enjoyable and protect you from injury.
Depending on where you purchase your bicycle, the cost of a professional bike fit may be subtracted from the final price tag, or the shop may offer a healthy discount on accessories you’ll need such as a helmet, riding clothing, tools, and maybe even a car bike rack! Discounted totals can add up, covering the cost of the fit session.
If purchasing your bike online only, read the fine print regarding the trial and return policy to avoid any surprises. If a brand you are interested in doesn’t have a physical showroom, check rental locations near you. You may get lucky and find the exact model or brand you are looking for.