(BWRate/1000)*60*60*24*30/1024/8 = GB Equiv Throughput

ex:

(512k/1000)*60*60*24*30/1024/8 = 162GB of Data Transfer

This is based on a sustained bandwidth of 512k (1/2MB)

Example Use:
MRTG Calculation on a switch port showing the BW rate, to determine the GB Equivalent amount of Data Transfer.