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Orbital period of a planet

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Orbital period of a planet diagram
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.
Semimajor axis
Mass of sun
Mass of planet

Orbital period
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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.