The density of Neptune is 1.638 g/cm3.
Just to give you some comparison, the density of water is 1 g/sm3. In other words, if you had a bathtub big enough, Neptune would sink into it. This is different for Saturn which has a density of less than 1. While Neptune sinks, Saturn would float. Of course, both planets are much less dense than Earth, with a density of 5.51 g/cm3.
Want to calculate the density of Neptune on your own? It’s pretty simple math. Just take the mass of Neptune, and divide it by its volume.
The mass of Neptune is 1.0243×1026 kg, and the volume of Neptune is 6.254×1013 km3. Divide the two, and convert to grams per cubic centimeter, and you’ll get the density of Neptune: 1.638 g/cm3.
We have written many stories about Neptune on Universe Today. Here’s an article about how there might be liquid oceans deep within Neptune. And here’s an article with cool videos of Neptune captured by Hubble.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.