489 (four hundred eighty nine) millimeters are equal to 48.9 (fourty eight point nine) cenimeters. Here's an easy tip for questions like these: 10 millimeters=1 centimeter.

because the metric system is based on power of ten. for example, ten millimeters equals a centimeter, ten centimeters equals a decimeter, ten decimeters equals a meter, and so and so forth.