Why Use This Calculator?
Bike sizing starts with the inseam, not the height — two riders the same height can need different frames. Road bikes size in centimeters using roughly inseam × 0.70; mountain bikes size in inches with a different factor. This calculator runs both from a single measurement.
Frame sizing gets you into the right ballpark; saddle height and stem length do the final fitting. When you land between sizes, reach (top-tube length) decides: longer torso, size up for a roomier cockpit.
How It Works
- Measure your inseam: barefoot, floor to crotch, standing straight.
- Read the road frame in cm and the mountain frame in inches.
- Use the S/M/L read against a brand's chart, since geometry varies.
Frequently Asked Questions
How does a bicycle frame size calculator work?
Road frames use inseam in centimeters times 0.70 for center-to-top frame size; mountain bikes use inseam in inches times roughly 0.226 for the frame in inches. Both give a strong starting point.
What if I'm between two frame sizes?
For aggressive, sporty positioning choose the smaller frame and longer stem; for comfort and endurance riding choose the larger. Smaller frames are easier to make bigger than the reverse.
How do kids' bike sizes work?
Kids' bikes size by wheel diameter — 12, 16, 20, 24 inches — matched to age and height rather than inseam formulas. Prioritize standover clearance: both feet flat when straddling.
Tips & Things to Watch
- Standover clearance matters most: both feet flat, top tube an inch below you.
- A professional fit beats any formula once you're riding serious miles.
- Different brands size differently — always check their geometry chart against your number.