"Mercury is the smallest planet in the Solar System (4,878 km diameter). Even three outer satellites are equal or larger than Mercury – Callisto (4,818 km diameter), Ganymede (another satellite of Jupiter) at 5,468 km diameter is significantly larger and the Saturnian's Satellite Titan (5,150 km diameter) is also larger. Mercury is only 4,878 km in diameter, or about one-third the diameter of the earth. Its volume is only about 6% that of the Earth, so it would take almost 18 Mercurys to make one Earth."