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There are a few ways that you can do it. Probably the best is to go log in to your ISP's customer portal (or whatever they call it) and see if there is a bandwidth usage total for your account. Most of the major ISPs have it on there somewhere, but you may have to poke around to find it. Many routers also have traffic monitoring and will allow you to keep an eye on your usage. There are also programs that you can install on your computer,such as Freemeter for Windows or SurplusMeter for Macs, but for me this is the least useful, even at home, because there are so many connected devices floating around my house.