Neptune’s mean diameter is about 49,200–49,300 km, which is roughly 3.9 times Earth’s diameter. In simpler terms, you could line up almost four Earths across Neptune’s face.

Quick size facts

  • Mean radius: about 24,600 km.
  • Equatorial radius: about 24,764 km.
  • Polar radius: about 24,341 km.
  • Volume: about 6.25 × 10¹³ km³, around 58 times Earth’s volume.

How that compares to Earth

  • Diameter: ≈3.9–4 times Earth’s.
  • Mass: about 17 times Earth’s mass.
  • “How many Earths inside?”: about 57–58 Earths could fit in Neptune by volume.

Shape and appearance

Neptune isn’t a perfect sphere; it is slightly squashed at the poles and bulged at the equator (an oblate spheroid) because of its rotation. This rotation plus its icy-gas composition gives it the look of a large, deep-blue globe in space.

If Earth were a basketball, Neptune would be close to a beach ball in size — noticeably larger, but not as huge as Jupiter or Saturn.