Why your age changes from planet to planet
When we say someone is 36, we mean they have travelled around the Sun 36 times with the Earth. Every other planet circles the Sun at its own pace, so the same stretch of time adds up to a very different number of “years” depending on where you count it. Mercury, the closest planet, finishes a lap in just 88 Earth days, which is why anyone over a year old has already had several Mercury birthdays. Neptune, far out at the edge of the Solar System, needs almost 165 Earth years for a single orbit, so nobody alive today has reached their first Neptune birthday.
Your age on a planet is simply the time you have been alive divided by the length of that planet’s year. The calculator above does that division for all eight planets, using the time since your birthday down to the day, and then works out the date of your next whole birthday on each one.
How we calculate it
We start from the exact number of days you have been alive, counting leap days, and divide by each planet’s orbital period measured in Earth days. The periods come from NASA’s planetary fact sheet and use the sidereal year, the time a planet takes to return to the same position relative to the distant stars. For the Earth that is 365.256 days, a few minutes longer than the calendar year, which is why your Earth age on this page can differ from your everyday age by a fraction of a day.
| Planet | One year in Earth time | Example age |
|---|---|---|
| Mercury | 88 days | 151.7 |
| Venus | 225 days | 59.4 |
| Earth | 365.26 days | 36.5 |
| Mars | 1.88 years | 19.4 |
| Jupiter | 11.86 years | 3.1 |
| Saturn | 29.45 years | 1.2 |
| Uranus | 84.02 years | 0.44 |
| Neptune | 164.79 years | 0.22 |
Worked example. Someone born on 12 April 1990 has been alive for 13,350 days on 30 October 2026. Mars takes 687 Earth days to orbit, and 13,350 ÷ 687 = 19.4, so they are about 19 and a half in Martian years. Their 20th Mars birthday falls on 23 November 2027.
Birthdays you can actually celebrate
Because Mercury and Venus are so quick, they give you plenty of extra birthdays. A Mercury birthday comes round roughly every three months, and a Venus birthday a little more than once every seven months. Mars is the fun one for most people: its year is almost two Earth years long, so a Martian birthday is rare enough to feel special but frequent enough to plan for. Jupiter birthdays arrive about once every 12 years, which makes them good markers for big life chapters, and very few people ever see a second Saturn birthday.
If you like the idea, add your next Mars birthday to your calendar with the button in the calculator. It is a good excuse for a small celebration that nobody else will be expecting.
Questions people ask
Why am I younger on Mars?
A Martian year lasts about 1.88 Earth years, so fewer of them fit into your lifetime. Your body is the same age everywhere; only the unit of measurement changes.
Has anyone had a birthday on Neptune?
No. One Neptune year lasts about 164.8 Earth years, longer than any recorded human life. Neptune itself has only completed one full orbit since it was discovered in 1846.
Does a day on another planet change the result?
No. This page measures years, the time to circle the Sun. A planet’s day, the time to spin once, is a different thing: a day on Venus is longer than its year.
Is Pluto included?
Pluto has been classed as a dwarf planet since 2006, so it is not in the main list. For the curious, one Pluto year is about 248 Earth years.
Sources. NASA Goddard Space Flight Center, Planetary Fact Sheet (sidereal orbit periods); Halley’s Comet return date from NASA/JPL. Calculations run in your browser; nothing you enter is sent anywhere.