THERE’S NO PLACE LIKE HOME
Earth
Our blue, splashy home! Oceans cover about 71% of Earth’s surface. It’s the only world we know of with living things.
- Mean diameter
- 12,742 km
- One year
- 365 Earth days
2 · DRAG, WONDER, MAKE A GUESS
How big is Mars?
Drag the ruler to resize Mars.
YOUR GUESS · 75%9,557 km
75%of Earth’s width
Same baseline, same scale. We’re measuring width through the center.
3 · Ready for your discovery?
No rush. Every guess is a chance to learn.
LITTLE EXPLORERS. BIG DISCOVERIES.
Size It Up Space Game
Two friendly planets. One big question.
Guess the space size. Discover Earth and Mars.
Earth + MarsPlay, wonder, learnFree to explore
01 · THE FIRST MISSION
Meet Size It Up Space Game
Earth and Mars can each fill a photograph. Put them side by side at the same scale and you can see how much their widths differ. In Size It Up, one planet is your ruler while you resize the other to match your best guess.
Start with Earth and Mars in either order. More planets will join the comparison in a future update.
- Size challenge
- Make a guess, then reveal the real diameter and see how close you got.
- Explore sizes
- See both planets to scale right away and swap which one you use as a reference.
02 · READ, RESIZE, REVEAL
How to play in three steps
Pick a planet to guess
Choose Earth or Mars. The other planet stays on the left and sets the scale. Its width is 100%.
Resize the planet on the right
Drag the ruler to make a rough guess. Use + and − or the ruler’s arrow keys to adjust by one percentage point at a time.
Check your guess
See the two planets at their real relative sizes. Compare your guess with the answer and use your match score to guide another try.
03 · THE SCIENCE OF THE RULER
Measure across the center
Diameter is the full width from one edge to the opposite edge through the center. Radius measures from the center to one edge, so the diameter is twice as long.
Earth and Mars are slightly wider at their equators. The game doubles JPL’s mean radius to get the diameter of a sphere with the same volume as each planet. Switching between kilometers and miles keeps the comparison in the same proportion.
Earth12,742 km
Mars6,779 km
GET TO KNOW YOUR CREW
Two worlds full of wonder.
OUR RUSTY RED NEIGHBOR
Mars
Why so red? Iron minerals in Mars’s rocks and dust have rusted. This rocky explorer’s dream also has two little moons!
- Mean diameter
- 6,779 km
- One year
- 687 Earth days
04 · BACK TO THE CHALLENGE
Three ways to sharpen your next guess
Start with a simple ratio
Picture the target at half, the same, or twice the reference width. Decide which looks closest, then use that mental picture to make your first guess.
Use the guide line
Both planets rest on the same baseline. When the top of your planet meets the dashed line, the two disks are equally wide. Use that 100% mark to judge smaller or larger guesses.
Try the pair in reverse
Guess Mars with Earth as your reference, then guess Earth with Mars as your reference. The percentage changes because you’re measuring in a different planet’s widths.
A CURIOUS MIND IS ALWAYS WELCOME
Big questions from little explorers.
A free online Size It Up game for a quick play, a family science moment, or a little classroom discovery.
How do I guess the space size of a planet?
Use the planet on the left as your reference. Drag the vertical ruler up to grow the other planet and down to shrink it. Your guessed diameter updates above the planet. You can also tap the plus and minus buttons, or use the arrow keys when the ruler is focused. Check your guess to reveal both worlds at their real relative sizes.
What does this size comparison space game measure?
You compare diameter: the width through the center of each planet. This first mission features Earth and Mars. It does not compare their mass or the distance between them. Switch to Explore sizes to see the true scale immediately and swap which world is your reference.
Is this a free online Size It Up game?
Yes! Play directly in your browser on a phone, tablet, or computer. There is no download or account to create. Guess, reveal, and explore at your own pace. The smiling planets are cartoons, while their diameter measurements come from NASA/JPL.
REAL SCIENCE BEHIND THE SMILES
Made for wonder. Grounded in facts.
Our planets have a little personality. Their measurements come from NASA and JPL.
NASA/JPL planetary measurements (opens in a new tab)NASA’s Earth facts (opens in a new tab)NASA’s Mars facts (opens in a new tab)
For grown-ups: measurements & game notes
Earth’s mean diameter is 12,742.0168 km and Mars’s is 6,779 km. Size ratios use the unrounded values; displayed diameters are rounded. Years use JPL’s sidereal orbital periods, converted with 365.25 days per Julian year.
A size match score is 100 × the smaller of (guessed ratio ÷ actual ratio) and (actual ratio ÷ guessed ratio), rounded to the nearest percent. A guess twice or half as wide as the true planet earns a 50% match.
The illustrations are cartoon diagrams. In Explore sizes and after checking a guess, both planet disks share a scale. During a guess, the target’s size is deliberately adjustable. The gap between planets does not represent distance in space. Sources checked November 5, 2026.
Stay curious, little space explorer.