What the calculator uses
The calculator uses height, inseam and bike category: road, MTB, trekking/city, gravel, e-bike and kids bikes. Inseam is especially important because it describes the relationship between legs, frame clearance and saddle height.
Why geometry matters
Two bikes with the same letter size can feel different. Stack, reach, standover and model intent change the real riding position, so the result should always be compared with the brand chart.
Correct use
Use BikeSize as a first filter: find a plausible range, read the explanation, compare the geometry and then test the real bike when possible.
The result is an estimate, not a medical assessment or a complete bike fitting.