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The diameter of Neptune is approximately 49,500 km. This makes Neptune the 4th largest planet in the Solar System, after Jupiter, Saturn and Uranus.
I say approximately because the diameter of Neptune changes depending on where you measure it. Neptune is rotating on its axis, completing a full day once every 16 hours or so. This rapid rotation flattens Neptune out slightly so that the diameter measured from pole to pole is less than the equatorial diameter.
Neptune’s polar diameter is 48,682 km. While its equatorial diameter is 49,528 km. In other words, points on the equator are 423 km more distant from the center of Neptune than the poles.
Want some comparison? The diameter of Neptune is about 3.9 times the diameter of Earth.
We have written many stories about Neptune for Universe Today. Here’s an article about how Neptune’s south pole is the warmest part of the planet.
If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.
We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.