How to Measure a Bicycle Frame Size

Measuring the Seat Tube

Look for a size label on the bottom of the seat tube first. The seat tube is the long tube that the bike seat post sticks into. If you look near the bottom of this tube, about 3 inches (7.6 cm) up from the chain sprocket, you’ll see a glued-on label that states the bike’s frame size. While not all bicycle frames have a size label, it’ll save you some work if yours does. If the size isn’t given, you’ll need to measure the seat tube manually. Keep in mind that the frame size may be listed in either inches or centimeters. Common bike sizes range from 48 cm–62 cm. Bikes at the lower end of that size range are meant for shorter individuals, while taller people will need bikes sized 56 cm and up.

Measure from the center of the gear crank to the top of the seat tube if there isn't a label. Place the end of a tape measure at the exact center point of the gear crank (the metal post that runs through the center of the bike’s chain sprocket). Then run the tape measure up to the top of the tube. Measure to the point where the tube ends. This will give you the length of the seat tube which is the frame size. If it helps you measure, you can remove the seat from the top of the tube and set it aside so the seat doesn’t get in the way. Most regular-sized bikes have a seat tube length that’s around 21–23 inches (53–58 cm).

Convert the measurement to centimeters if it’s a road bike. If you’re in the U.S., you’re probably used to dealing with imperial measurements. However, road bike frame sizes are always given in centimeters. Multiply the number of inches by 2.54 to find the length in centimeters. Say that the length of the seat tube is 22 inches long. Multiply this by 2.54, which 55.88 cm. If you’re working with a mountain bike frame, leave the measurement in inches.

Choosing the Right Size Bike

Measure from the ground to your crotch with your legs spread. Stand barefooted (or just wearing socks) on the ground with your legs spread apart. Move your feet apart so they’re separated by about 6–8 inches (15–20 cm) of space. Use your tape measure to find the distance from the ground to your crotch in centimeters. Make sure you’re measuring to your actual crotch and not just the crotch of your pants. If you find that it’s awkward to measure this distance on your body without falling over, ask a close friend or partner if they’d be willing to help.

Multiply your inseam length by 0.7 if you’re buying a road bike. If you bought a bike whose seat tube was the exact same length as your inseam, you’d have a very hard time pedaling the bike! So, multiply by 0.7 to shorten the inseam measurement if you’re planning to ride a road bike. Road bikes are a great option if you plan to do most of your cycling on paved paths. Say that you measure the length of your inseam at 65 centimeters. Multiply this by 0.7 and you’ll come up with 45.5. Rounding up, you’ll have an answer of 46.

Multiply your inseam by 0.66 if you’d like a mountain bike. Mountain bikes have thicker, bulkier tires than sleeker, more streamlined road bikes do, which means that you’ll be elevated a little higher off the ground. To compensate for this, multiply your inseam by a slightly smaller number to calculate your correct bike size. If you don’t like doing math in your head, find a handy calculator or use the one in your cell phone. Mountain bikes are your best bet if you’ll be doing the majority of your cycling off-road or over rocky terrain. For example, say that your inseam measured 76 centimeters. Multiply this by 0.66 and you’ll get roughly 50.

Purchase a bike with a frame size that matches the number you calculated. The number that you got as the result of the equation you just solved corresponds to the size of bike that will fit your body. It’s smart to try out a bike before you buy, though, to make sure it feels comfortable. Ride the bike around a parking lot and adjust the seat until each leg is bent at a very slight angle when they’re fully extended pushing down on the pedal. If the bike doesn’t feel right or if you struggle to reach the pedals, try a different size until you find a bike that suits you. If you purchase a used bike that doesn’t happen to display its size, you can pull out your tape measure and measure the seat tube to find the size.

Measuring Other Tubes

Find the length of the vertical head tube in millimeters. On a bicycle frame, the head tube connects the handlebars to the top of the fork. (The fork is the split piece of metal that connects to the center of the front wheel.) Set the end of your tape measure or ruler against the top of the head tube at the point where it intersects with the handlebars. Measure down to the point at which the head tube forks. Make sure not to include the base of the handlebar post in this measurement! The head tube is by far the shortest of the main bike tubes. Because of this, measurements for the head tube are typically given in millimeters, while most other measurements are typically given in centimeters.

Compensate for a slanted top tube by measuring the effective top tube. The top tube, as its name suggests, runs roughly parallel to the ground between the seat post and head tube (at the base of the handlebars). To measure the length of a slanted top tube, you’ll actually be measuring what’s called the “effective top tube.” Put 1 end of a level at the point where the top tube intersects the head tube. Hold the level flat (so the air bubble rests at the center of the liquid it’s suspended in) and note where it intersects the seat tube. This will likely be a few inches above where the physical top tube intersects with the seat tube. It wouldn’t hurt to place a tiny piece of masking tape at the point you measured, so you won’t forget where to measure the effective top tube length. The piece of tape should be straight across from the intersection of the head and top tubes at the exact same height. It’s important that you use the level to make sure these 2 points are as high as one another. If the top tube is flat, skip this step and move directly to measuring the length of the tube. Slanted top tubes are lower where they intersect the seat tube and higher where they intersect the head tube. This is fairly common on bike frames, especially for road bikes.

Measure the bike frame’s top tube length. Place the tip of a tape measure against the center point of the head tube, where it intersects with the top tube. Measure along the top tube to the center point of the seat tube and find the length in centimeters. If you’re dealing with a slanted top tube, measure instead to the effective top tube point that you measured and marked earlier. The total distance is the top tube’s length. The top tube is the tube that’s situated between your legs when you ride a bike.