With most boots, a 25 and a 25.5 will share the same shell, so they have the same boot sole length. Difference is really just the liner is a tad bit bigger or smaller. However, with certain boots, a 25, and 25.5 are different shells, the only ones I've really come across would be this years Atomics, I think the M:9? Not sure on that one though. But for most boots, 25 and 25.5 share the same shell. And like the guy above said, different brands have different sole lengths. They are usually within the same range however, a 25 in Salomon will be about 2-3 mm's off from a Tecnica for example.