How big is Neptune?
For this comparison, Neptune measures 49,528 km in diameter (30,775.3 mi). Equatorial diameter from NASA’s planet-size guide.
Similar-looking planets still have different measured diameters.
Put the measurement in perspective
The recorded diameter is 3.883 times the diameter of Earth (12,756 km). This is a ratio of the labelled linear dimensions. It does not describe weight, area or volume.
Use the interactive comparison to see both drawings on one shared scale. You can choose another reference or enter your own height. If the sizes are very far apart, the smaller object may be almost invisible; an intermediate reference makes the progression easier to inspect.
Measurement source
NASA Science
The note above explains the specific value selected. Drawings are schematic; the stated measurement determines the scale. Report a measurement issue.
Check the convention
Diameter excludes orbital distance. Check whether the source gives equatorial, mean or rounded diameter before comparing it with another table.
Read our methodology for the selection and scoring rules. Practise these dimensions in the space size guessing game.