Orbital Period Calculator — How Long Is a Year on Other Planets?

A year on Mercury lasts 88 Earth days. A year on Neptune takes 165 Earth years. Orbital period — the time to complete one trip around a star — depends almost entirely on how far out you are. Kepler figured out the math in 1619, and it still holds up perfectly.

Calculate orbital periods for any distance with the orbital period calculator on CalcHub.

Kepler's Third Law

The fundamental relationship: the square of the orbital period is proportional to the cube of the semi-major axis (average orbital distance).

T² ∝ a³

More precisely, for objects orbiting the Sun:

T (years) = √(a³), where a is in AU (astronomical units, 1 AU = Earth-Sun distance)

Mars is at 1.524 AU: T = √(1.524³) = √(3.54) = 1.88 years. That's exactly right — Mars takes 686 days to orbit the Sun.

Planetary Orbital Periods

Planet	Avg. Distance (AU)	Orbital Period	Days per Year
Mercury	0.387	88 days	87.97
Venus	0.723	225 days	224.7
Earth	1.000	365.25 days	365.25
Mars	1.524	1.88 years	686.97
Jupiter	5.203	11.86 years	4331
Saturn	9.537	29.46 years	10,756
Uranus	19.19	84.01 years	30,688
Neptune	30.07	164.8 years	60,182

How to Use the Calculator

1. Enter orbital distance in AU, km, or light-years
2. Enter central body mass (defaults to solar mass for solar system objects)
3. Get orbital period in days, years, or seconds
4. Or enter period to find the required orbital distance

The CalcHub calculator works for any orbit — including artificial satellites around Earth, moons around planets, and exoplanets around other stars.

Satellites and Moons

The same law applies to moons and satellites. The Moon orbits Earth at ~384,400 km with a period of 27.3 days. A satellite in low Earth orbit at ~400 km altitude orbits in about 92 minutes.

The International Space Station orbits at ~408 km, completing roughly 15.5 orbits per day — which is why ISS astronauts experience 15–16 sunrises and sunsets every 24 hours.

Geostationary Orbit

There's one special altitude where orbital period equals exactly 24 hours: geostationary orbit at ~35,786 km above Earth. Satellites here appear stationary in the sky — hence the name. Most communication and weather satellites occupy this orbit. The calculator lets you verify this: input 24-hour period around Earth, get exactly 35,786 km altitude.

Why do outer planets take so much longer to orbit?

Two compounding effects: they have longer paths to travel (circumference scales with radius) AND they move more slowly (orbital velocity decreases with distance). Both factors multiply together, which is why Neptune's year is not just 30× Earth's (its distance ratio) but ~165× Earth's year.

What is a synodic period vs. sidereal period?

Sidereal period is the orbital period measured against the fixed stars — the "true" orbital period. Synodic period is the time between the same apparent position relative to Earth — what determines conjunction cycles. Mars has a sidereal period of 687 days but a synodic period of about 780 days, because Earth is also moving and needs extra time to "lap" Mars (or vice versa).

Can I calculate orbital period for exoplanets?

Yes, as long as you know the orbital semi-major axis and the mass of the host star. The calculator handles non-solar mass central bodies — input stellar mass in solar mass units and proceed as normal. Many discovered exoplanets have their orbital parameters published in databases like the NASA Exoplanet Archive.

Related Calculators

• Planet Weight Calculator — gravity on each planet
• Satellite Orbit Calculator — altitude and velocity for Earth satellites
• Light Year Converter — distances beyond the solar system