Quick answer
It counts orbits, not biology. Divide by the orbital period — multiplying is the common error and inverts the answer.
The calculation
A planet's year is the time it takes to orbit the Sun once. Mars takes about 1.88 Earth years, so your Mars age is your Earth age divided by 1.88 — a lower number, because Martian years are longer.
Mercury goes the other way. Its year is about 0.24 Earth years, so your Mercury age is your Earth age divided by 0.24, which is roughly four times higher. Divide in both cases; the direction of the result follows from whether the period is above or below one.
Use this guide with the right tool: Open the Planetary Age Calculator for an Earth age converted to approximate ages on other planets. If the question shifts, compare it with the Age in Specific Units Calculator or the Decimal Age Calculator.
The orbital periods
Mercury 0.241 Earth years, Venus 0.615, Mars 1.881, Jupiter 11.86, Saturn 29.45, Uranus 84.02, Neptune 164.79. A person of 31 is about 129 on Mercury, 16 on Mars, 2.6 on Jupiter and 0.19 on Neptune.
Neptune's period is long enough that it completed its first full orbit since discovery only in 2011, 165 years after it was first observed. Almost nobody has ever had a first Neptunian birthday.
Days are a separate curiosity
Orbital period and rotation period are independent, and the results are strange. A day on Venus is longer than its year — about 243 Earth days to rotate against 225 to orbit. Mercury rotates three times for every two orbits.
Mars is the outlier in the other direction: its day is 24 hours 37 minutes, so close to Earth's that mission teams working on Mars time drift about forty minutes later each day relative to everyone else.
What it does not mean
Nothing about biology. You would not age faster on Mercury; you would simply count more orbits, because the yardstick is shorter. Time passes at the same rate for you either way.
The one context where elapsed time genuinely differs is relativistic, and the effect at any speed or gravitational field a human has experienced is measured in fractions of a second. It is real, and it is not what this calculation is about.
Worked example
The shape of the calculation stays the same with your own figures. What changes is the source of the rule, so check that before treating the number as settled.
Common mistakes to avoid
- Multiplying by the orbital period. Divide. Multiplying inverts the result for every planet.
- Confusing a planet's day with its year. They are independent. Venus rotates more slowly than it orbits.
- Reading it as biological ageing. It counts orbits. Time passes at the same rate wherever you are.
Use the right calculator
Start with whichever matches the information you already hold. The other two are there for when the question shifts.
Frequently asked questions
Which planet gives the highest number?
Mercury, because its year is shortest — about four times as many years as Earth.
What about Pluto?
Its orbital period is about 248 Earth years, so almost nobody reaches their first Plutonian birthday. It has been classified as a dwarf planet since 2006.
Would I age more slowly on another planet?
No. Relativistic effects exist but are fractions of a second at any human-experienced speed or gravity.