To calculate the semi-major axis or orbital period of a planet using Kepler's third law, input the known variable and the gravitational constant.
Kepler's third law relates the semi-major axis of a planet's orbit to its orbital period:
Orbital Period2 = Semi-Major Axis3 / G
This means that given the semi-major axis of a planet's orbit (in AU), you can calculate its orbital period (in Earth years) using this formula. Conversely, given the orbital period of a planet (in Earth years), you can calculate its semi-major axis (in AU).