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Orbital Period Calculator

Orbital period and speed from the semi-major axis using Kepler's third law, for any central mass.

Enter values

AU for the Sun, kilometres from the centre for Earth

Results update as you type.

Result

Orbital period

365.2027 days

In years
0.999871
In seconds
31,553,514.1
Orbital speed
29.7891 km/s
Circumference of the orbit
939,951,143 km

About the Orbital Period Calculator

Orbital period and speed from the semi-major axis using Kepler's third law, for any central mass.

Also known as: kepler third law, orbit time calculator, satellite orbital period, how long is an orbit.

How to use this calculator

  1. Enter orbiting, semi-major axis.
  2. The result updates instantly as you type — press Calculate (or Enter) at any time.
  3. Review the breakdown shown under the main result.
  4. Use Copy result or Share to save the answer or send a link with your inputs.

Formula

T = 2π√(a³ / GM) · Kepler: T² ∝ a³

Worked example

Using the values pre-filled in the calculator above:

  • Orbiting: The Sun
  • Semi-major axis = 1

the calculator returns:

  • Orbital period: 365.2027 days
  • In years: 0.999871
  • In seconds: 31,553,514.1
  • Orbital speed: 29.7891 km/s