BDP (bytes) = total_available_bandwidth (KBytes/sec) x round_trip_time (ms)
The total available bandwidth would be the slowest link in the communication path. Meaning if a 10G link is connecting to a 1G link on the other end, the slowest speed would be 1000KB/sec.
A good method to determine the average round trip time, is to make use of a ping test to the remote system. This should display an average round trip time.