Here's how to calculate the diameter of Neptune in millimeters:

* Neptune's diameter in kilometers: 49,528 km

* Conversion factor: 1 kilometer = 1,000,000 millimeters

Calculation:

49,528 km * 1,000,000 mm/km = 49,528,000,000 millimeters

Therefore, Neptune's diameter is approximately 49,528,000,000 millimeters.