There are hundreds of free tools to do this. All you need is Google (and search for
internet monitoring or something like that)
But you also need to know what your connection type is, as not all tools you will find are capable of monitoring all types of connections. The most (free) tools are capable of monitoring modem connections, this means that if you use LAN to connect to a router those tools can't monitor it. But there are also some which can monitor it though, only a bit harder to find.
What you also can try is to go to your ISP's website. Most ISP's provide a page with your monthly stats of your download/upload volume.
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