In order to calculate your total bandwidth available per month, first run a speed test using a testing service such as http://speedtest.xfinity.com/
Download speed is a measurement of how quickly your device receives data from the Internet. Most Internet connections download much faster than they upload. The speed test will measure both your current up- and download speed in Mbits/s.
Bandwidth in TB/month would be the bandwidth cap that your ISP imposes on you, but if you do not have a cap in your contract, you should still calculate your total bandwidth TB/month from the Mbit/s up- and download speed you got from your speed test numbers. To make the conversion from Mbit/s to TB/month, use a Bandwidth converter such as http://www.kylesconverter.com/data-bandwidth/megabits-per-second-to-terabytes-per-month or https://toolstud.io/data/bandwidth.php?compare=network&speed=100&speed_unit=Mbps and plug in the up- and download speeds you got from your speed test before, then sum up the resulting two values in TB/month.