The Larmor radius is the radius of gyration of a charged particle moving in a magnetic field. Balancing the Lorentz Force

$\vec F = q \vec v \times \vec B$

with the centripetal force, whose magnitude is

$F = \frac{m v^2}{r}$

one immediately obtains

$r_L = \frac{m}{|q|} \frac{v_\perp}{B}$

where only the perpendicular component of the velocity (with respect to the magnetic field) enters.