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Use your total height and a road bike sizing chart for a basic guideline. Measure your inseam Multiply your inseam measurement (in centimeters) by 0.67 for a suggested size. Measure your road bike’s seat tube. Compare your inseam measurement to the road bike’s C-T measurement.
Standing against a wall, measure your total height and record it in centimeters, as this is how road bike frames are measured. Through an online search or by borrowing a chart at a road bike store, find the bike height that is associated with your total height. Use the chart to find whether you need an extra small, small, medium, large, or extra large road bike frame.  You can convert your height from inches to centimeters by multiplying it by 2.54. Road bike height charts should not be used as the only resource for sizing a road bike, but are a good starting point for the basic frame size you should be looking for. Some road bike charts are more specific, with up to 6 or 7 categories of bike frame sizes. with measuring tape. Spread your feet 8 to 10 inches (20 to 25 cm) apart, like you're on a bike. Then, place a book vertically between your legs, touching your crotch to simulate a bike seat. While holding the book in place with your hands, have a friend measure from the top of the book to the floor and record these measurements for later.  The book should be held between your upper thighs, simulating a road bike seat. If you recorded your measurement in inches, convert the measurement to centimeters by multiplying it by 2.54. You can take this measurement several times and record the average for a more accurate number. This equation will produce your ideal vertical frame size, or the seat tube, in centimeters. You will use this number to find a seat tube that fits your body. Record this number in a notebook or on your cell phone, as it is what you will be using when visiting the bike shop or measuring your current road bike.  This formula will provide you with a suggested size, but it's not a guaranteed fit. Double check your math and if you feel that you have made a mistake start over, as this number is essential to sizing a road bike. For example, if your leg inseam is 80 centimeters, your frame size will be approximately 54 centimeters. Identify the top of the seat tube, where the seat clamp holds the seat post. Find the center of the bottom bracket, which is where the axle binds to the road bike’s the crank arms. With a tape measure, record the distance between these 2 points to find the length of your seat tube. This distance should match your inseam measurement for a proper road bike frame size. This measurement is known as a C-T measurement and is used to identify the frame size. Identify the correlation between the inseam measurement and the seat tube measurement and record their correlation. The difference between these measurements should be within 2.54 centimetres (1.00 in) for a properly-sized road bike frame.  If you are buying a new road bike, choose a frame that has a seat tube measurement matching your inseam. If you are measuring your current road bike, consider purchasing a new frame if the bike’s seat tube measurement differs from your inseam.