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.

**Semi-Major Axis**: The average distance between a planet and the sun, measured in astronomical units (AU).**Orbital Period**: The time it takes for a planet to complete one orbit around the sun, measured in Earth years.**Gravitational Constant (G)**: A fundamental constant of nature that appears in the law of universal gravitation, with units of (AU^3)/(Earth years^2) and a value of 39.5.

Kepler's third law relates the semi-major axis of a planet's orbit to its orbital period:

**Orbital Period ^{2} = Semi-Major Axis^{3} / 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).