๐Orbital Period Calculator
Apply Kepler's third law to find the orbital period of any body from its semi-major axis and the central body's mass.
Free Orbital Period Calculator
Our free orbital period calculator applies Kepler's third law, T = 2ฯโ(aยณ/GM), to find how long a planet, moon, or satellite takes to complete one orbit. Choose a preset central body โ the Sun, Earth, Mars, or Jupiter โ or enter a custom mass, then type in the semi-major axis of the orbit in astronomical units, kilometers, or meters.
Switch modes to solve the reverse problem: given a known orbital period, find the semi-major axis the orbit must have. Every result shows the full substitution into Kepler's third law so you can follow exactly how the number was derived, not just see the final answer.
This makes the tool useful for astronomy homework, satellite mission planning estimates, and quick sanity checks against real orbital data like Earth's 365.25-day year or Mars's roughly 687-day year. No sign-up, no downloads โ everything runs instantly in your browser.