Kepler's Third Law Calculator
Orbital period from semi-major axis (or vice-versa) for any central mass, with solar-system shortcuts.
How to use the Kepler's Third Law Calculator
- Enter or select: Central mass.
- Fill in what you know of: Semi-major axis, Orbital period — leave the unknown empty.
- The results (Semi-major axis, Period, Mean orbital speed) are calculated instantly as you type.
- Use “Copy link” to share the calculation with the same input values.
Formula
- P² = a³ / M (P in years, a in AU, M in solar masses)
- T² = 4π²a³ / (G·M)