Start with the model’s chart
Measure your height and inseam, then open the size chart for the exact bike and model year. A medium in one brand can feel closer to a small or large in another. Road frames often use centimetres or letter sizes, while mountain and hybrid bikes commonly use letters. Treat the label as a way to locate the relevant geometry chart.
What geometry numbers tell you
Frame reach is the horizontal distance from the bottom bracket to the top centre of the head tube. Stack is the vertical distance between those points. They help compare frames, while stem length, bar shape and saddle position also affect how far you actually reach. Standover measures height above the ground at a point along the top tube. Check where each manufacturer measures it.
What to feel on a test ride
Ride in your usual shoes. Try a slow turn, a controlled stop and a short stretch of steady pedalling. You should be able to reach the brakes easily, hold the bars with relaxed shoulders and pedal without rocking side to side. On drop bars, try the hoods and drops. On a commuter, check how easily you can look behind you.
Between sizes
Ask the shop to set each saddle at a comparable height before you decide. A smaller frame can feel easier to reach and may give more standover room. A larger one may offer a higher front end or more room through the cockpit. Choose based on the position and handling you can comfortably sustain. If neither feels right, try a different model.
Further reading · 99 Bikes sizing charts
