A kilometer consists of 1,000,000 millimeters.
This conversion comes from the fact that a kilometer is equal to 1,000 meters, and each meter contains 1,000 millimeters. Therefore, to find out how many millimeters are in one kilometer, you multiply the number of meters in a kilometer by the number of millimeters in a meter:
1 kilometer = 1,000 meters x 1,000 millimeters/meter = 1,000,000 millimeters.
So, whenever you need to convert kilometers to millimeters, just remember that there are 1,000,000 millimeters in a kilometer.