The horizontal distance from the pedals (bottom bracket) to the handlebars. A longer reach feels more "stretched out" and aggressive.
When standing over the bike's top tube with both feet flat, you should have at least 1 inch of clearance on a road bike and 2–3 inches on a mountain bike for safety. 3. Understanding Advanced Fit (Reach & Stack)
If you are between sizes, or comparing different brands, look at "Reach" and "Stack" on the bike's geometry chart. bike buying guide size
If you are on the cusp, sizing down is usually safer as it offers better control and maneuverability. Sizing up is better if you prefer a more stable, less "twitchy" feel.
Your height gives you a direction, but your inseam gives you precision. This is not your pant size; it is the distance from your crotch to the floor. The horizontal distance from the pedals (bottom bracket)
While pedaling, your knees should not hit the handlebars (too small), and your legs should not fully lock out at the bottom of the stroke (too large).
Height is the most common starting point for bike sizing. Use these general charts to find your baseline size. Sizing up is better if you prefer a
The vertical height from the pedals to the handlebars. A higher stack puts you in a more upright, comfortable position. 4. Sizing for Kids