To convert centimeters to millimeters, you multiply the number of centimeters by 10, since there are 10 millimeters in every centimeter.

So, for 128 cm:

\[ 128 \, \text{cm} \times 10 \, \text{mm/cm} = 1280 \, \text{mm} \]

Therefore, \( 128 \, \text{cm} = 1280 \, \text{mm} \).