To really understand how big Neptune really is, you need some kind of comparison. Let’s see how Neptune compares to Earth in every way.
First, let’s just look at pure size. The diameter of Neptune is approximately 49,500 km. This makes Neptune the 4th largest planet in the Solar System. And compared to Earth? Neptune is 3.9 times bigger.
Now mass. The mass of Neptune is 1.02 x 1026 kg. If you wanted to write it out, it would be 102,000,000,000,000,000,000,000,000 kg. Neptune has 17 times as much mass compared to the Earth.
How about volume? The volume of Neptune is 6.3 x 1013 km3. You could fit 57 Earths inside Neptune and still have room to spare.
A day on Earth is 24 hours, but a day on Neptune is 16 hours and 6 minutes. A year on Earth is, um, 1 year obviously, while a year on Neptune is 164.79 years.
Here’s one element that’s actually pretty close. The surface gravity on Neptune (if it actually had a surface that you could stand on) is only 14% stronger than the pull of gravity on Earth. You would have a difficult time noticing if you were standing on the surface of Neptune compared to the surface of Earth.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.