Mercury vs Io
Mercury (4,879 km) is 1.34× the size of Io (3,643 km).
Io is 75% of the height of Mercury. The gap between them is 1,236 km (4,055,774 ft).
The comparison in numbers
| Thing | Size | In feet | Measured as |
|---|---|---|---|
| Mercury | 4,879 km | 16,008,530 ft | diameter |
| Io | 3,643 km | 11,952,756 ft | diameter |
What the scale shows
Mercury (4,879 km) is 1.34× the size of Io (3,643 km). The gap between them is 1,236 km (4,055,774 ft). Both are planets & moons, so the figures describe the same kind of thing.
Draw both on one baseline and the smaller one fills 75% of the frame while the larger one runs to the top. The two are close enough that the difference is easy to see side by side.
Read the rest — 8 more sections
What each figure means
The figure for Mercury is its diameter (mean diameter). The figure for Io is its diameter (mean diameter).
Crossing them
At a jet's 900 km/h, Mercury's 4,879 km would take 5.4 hours and Io's 3,643 km would take 4 hours.
In people: Mercury is 2,870,235 adults of 1.7 m tall, Io is 2,143,058.8. By size Mercury is 11th of the 16 planets & moons on this site and Io is 12th, so 1 place separates them in the ranking.
How to read these two figures
Before trusting any comparison, check what each number measures. Mercury is given as a diameter, mean diameter. Io is given as a diameter, mean diameter. Both are diameter figures, so putting them on one baseline is a like-for-like comparison, although the exact individual or convention still matters.
Worlds are listed by diameter. Gas giants are not round, so equatorial diameter is used for them and mean diameter for the rest. All figures are rounded from published mission data.
Where the pair sits among all the sizes here
Across all 86 things on this site, Mercury is number 11 from the top and Io is number 12. No other item falls between them, so in this collection they are direct neighbours.
Because the collection spans 13 mm up to 1,392,700 km, a ratio of 107,130,769,231× from smallest to largest, the position of a pair says a lot: both sit in the upper third of the list.
What the length ratio hides
A length ratio of 1.34× is easy to picture, but area and volume grow faster than length. If two objects had exactly the same shape, Mercury would have about 1.79× the surface or footprint of Io, and about 2.4× the volume. These two are not the same shape, so treat the figures as an upper bound for an imaginary scaled copy, not as a measurement of either one.
The drawing above shows length only. Io is drawn 75% as tall as Mercury, but the equivalent share of footprint would be only 55.7%, which is why the smaller one looks even slighter than the numbers alone suggest.
Time to cross each one
Light travels at 299,792,458 m/s, so it crosses Mercury (4,879 km) in 16 milliseconds and Io (3,643 km) in 12 milliseconds. A jet at 900 km/h would take 5.4 hours for the larger and 4 hours for the smaller.
These times assume a straight line and constant speed, which no real crossing has, but they turn a number in metres into something the body understands. At any fixed speed the travel times keep the same 1.34× ratio as the sizes.
The same figures in other units
Mercury measures 4,879 km, which is 16,008,530 ft or 3,031.9 miles. Io measures 3,643 km, which is 11,952,756 ft or 2,263.78 miles.
The difference between them is 1,236 km (4,055,774 ft). In kilometres the smaller one is 3,643.2 against 4,879.4, so the 1.34× ratio holds in every unit; only the numbers on the ruler change.
Questions people ask about this pair
Which is bigger, Mercury or Io?
Mercury is bigger. At 4,879 km it is 1.34× the size of Io, which is 3,643 km.
How many times does Io fit along Mercury?
Less than twice: Io is 75% of the size of Mercury, so one fits with 1,236 km to spare.
Are the two figures measured the same way?
Yes in kind: both are diameter figures. Mercury is mean diameter; Io is mean diameter.
How does the gap look in people?
Taking an adult of 1.7 m, Mercury is about 2,870,235 people tall and Io is about 2,143,058.8. The gap of 1,236 km is 727,176 people.
Where do the numbers come from?
Each item has its own page that names the measurement and links to the Wikipedia article it was taken from. Some figures are rounded, and animals vary from one individual to the next, so treat them as representative values rather than records for every case.
How the sizes are taken. Each figure says what it measures — height, length, diameter or elevation — and for which individual or convention. Everything is drawn standing upright on one baseline so a single scale holds, including things that are long rather than tall. Values come from the linked Wikipedia article for each item.