All human bodies fit in 1 km³—but not comfortably

Using a rough average body density close to water, an adult body occupies about 0.06–0.08 m³. For roughly 8 billion people, that is about 0.5–0.6 km³ of body volume—less than 1 km³. But this is tightly packed body matter, with no air gaps, movement, sanitation, or life support. It does not mean people could be “comfortably laid” in such a cube; comfortable living space would require a very different volume.