What Kepler’s Third Law says
Kepler’s Third Law relates a planet’s orbital period to the size of its orbit and the mass of the star it orbits. In convenient units where the semi-major axis (a) is measured in astronomical units (AU), the central mass (M) in solar masses, and the period (T) in years, the law simplifies to T² = a³ / M.
How this calculator works
Enter the planet’s orbital semi-major axis in AU and the mass of the star it orbits in solar masses (the Sun = 1). The calculator applies the simplified Kepler’s Third Law formula, T = √(a³ / M), to find the orbital period in years, and also converts it to days.
This simplified form assumes the orbiting body’s mass is negligible compared to the central star, which is an excellent approximation for planets, moons, and most artificial satellites. This calculator is for general astronomy education and planning purposes.