Orbital Period Calculator
Kepler's Third Law solver: given a central body and a semi-major axis, compute the orbital period. Or solve for any of period / semi-major axis / central mass. NASA / IAU body data populated automatically.
Limitations: Two-body Keplerian assumption. Ignores J2 oblateness, third-body perturbations, drag for low orbits, and relativistic effects. The semi-major axis is from the central body's CENTER (radius from center, not altitude above the surface).
What to do next: For Earth-orbit satellites, use the Earth body; semi-major axis is the radius from Earth's center (r_earth ≈ 6,378 km plus altitude). For solar-system planets, use the Sun and supply the planet's mean orbital distance.