How do I measure bandwidth used.

I’m trying to determine how much bandwidth my company uses on avg. We’re running a full T1 (so we’re told) and I would like to know how much of it we are using. How do I determine this? I downloaded DU Meter. It gave me some information but I reallydon’t know how to use it. The information it gave me was Max Transfer rate for Down Load and up loads, Average Transfer rate, and Total data transferred. How do I use these? What is it they tell me in relevance to bandwidth usage. Thanks.