Compute instant throughput

Question

I have to write a program that must print the instant throughput for a network interface. I can see every single packet that is transiting over that interface and, if I wanted, deeply inspect that packet. I just want to show the instant throughput but I don't know how to do it. I thought to add into a counter variable the dimension of the packet and to show it every second before clearing but I think it is too much an euristic approach. How can I do to get a real instant thoughput?