About this tool
The Orbital Period Calculator is a free, in-browser astronomy tool built on Kepler's third law in its Newtonian form, T = 2π√(a³/GM). Given the semi-major axis a of an orbit and the mass M of the central body it returns the orbital period T; switch modes and it inverts the relation to find the semi-major axis a = ∛(GM·T²/4π²) from a known period instead. G = 6.674×10⁻¹¹ N·m²/kg² is the gravitational constant.
Everything is computed locally in your browser, so nothing you enter leaves your device. The axis can be given in astronomical units, kilometres, or metres, and the central mass in solar masses (1 M☉ = 1.989×10³⁰ kg), Earth masses, or kilograms, letting you handle planets around the Sun, moons around a planet, or satellites around Earth with the same tool.
For a body orbiting the Sun the law simplifies to the familiar T(years) = √(a(AU)³): at a = 1 AU the period is 1 year, and at a = 4 AU it is √64 = 8 years. Period results are shown in seconds, days, and years, and the tool also reports the mean orbital speed v = 2πa/T for a circular orbit — about 29.8 km/s for Earth.