Orbital Period Calculator
How long an orbit takes and how fast the orbiting body moves — Kepler's third law.
Kepler's third law says the square of the orbital period is proportional to the cube of the semi-major axis.
How the orbital period calculator works
Kepler's third law says the square of the orbital period is proportional to the cube of the semi-major axis. It falls straight out of Newtonian gravity, and it means orbital period depends only on the central mass and the orbit's size — not on the mass of the orbiting object.
That last point is worth sitting with. A grain of dust and a space station at the same altitude orbit in exactly the same time. It is the same reason everything falls at the same rate in a vacuum.
Geostationary orbit is the case everyone meets: the altitude at which the period is exactly one sidereal day, 35,786 km above the equator, so a satellite appears to hang still.
Formula: T = 2π√(a³/GM); v = √(GM/a)
Worked examples
| Inputs | Orbital period | Note |
|---|---|---|
| The ISS at 400 km | 92.41 minutes | about 92.6 minutes |
| Geostationary orbit | 23.93 hours | one sidereal day |
| Low Mars orbit | 118.04 minutes | about 2 hours |
FAQFrequently asked questions
How long does the ISS take to orbit Earth?
About 92.6 minutes at 400 km, so roughly 15.5 orbits a day.
What is geostationary orbit?
35,786 km above the equator, where the orbital period equals one sidereal day so the satellite appears stationary.
Does a heavier satellite orbit more slowly?
No. The orbiting mass cancels out entirely — a dust grain and a space station at the same altitude have the same period.
What is Kepler's third law?
Period squared is proportional to semi-major axis cubed, with the constant set by the central mass.
Why does the ISS need reboosting?
At 400 km there is still enough atmosphere to cause drag, which lowers the orbit by a couple of kilometres a month.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G and the speed of light
- IAU 2015 Resolution B3 — nominal solar and planetary conversion constants — the astronomical unit, solar mass and planetary radii
- Royal Observatory Greenwich — the historic home of the prime meridian
Last checked: September 2026. Constants are CODATA 2018 (G, c) and IAU 2015 nominal values (solar and planetary parameters).