Venus's mass is about what fraction of Earth's mass?

Venus’s mass is about 0.815 times Earth’s mass. In other words, Venus contains roughly 81.5% of Earth’s mass, making it the closest planet to Earth in both size and mass.

The similarity is striking: Venus has about 95% of Earth’s diameter and about 90% of Earth’s surface area. Its mass is lower because its average density is slightly lower than Earth’s, approximately 5.24 grams per cubic centimetre compared with Earth’s 5.51.

The answer is a fraction of Earth’s mass, not a mass measured in Earths as a physical unit. Thus 1.000 Earths would describe Earth itself, while 0.107 Earths is roughly Mars’s mass. The value 5.97 Earths is not a plausible comparison for Venus.

Despite its similar mass, Venus is very different from Earth at the surface. Its atmosphere is dominated by carbon dioxide, surface pressure is about 93 times Earth’s, and temperatures reach roughly 465 °C. Similar planetary heft did not produce similar modern environments.