 Orbital period of a planet

 
The fully defined version of Kepler's third law is used to calculate the orbital period of a planet. Note that this calculation does not include the effect of relativity.
Of course, this calc is not limited to planets and suns  satellites, moons, comets, asteroids etc. may also be input.  
For the special case of circular orbits, the semimajor axis is equal to the radius. You can check this calculation by setting the masses to 1 Sun and 1 Earth, and the distance to 1 astronomical unit (AU), which is the distance between the Earth and the Sun. You will see an orbital period close to the familiar 1 year. Kepler's laws of planetary motion were formulated at the beginning of the 17th century and are still important today, although they do not take relativity into account.  