This is more complicated than it seems, too. You can’t just use a single method, as you could have Windows PCs, Macs, Android phones, iPhones and iPads, game consoles, smart TVs, and set-top streaming boxes all connected to your home router. Worse yet, many of these devices — laptops, smartphones, and tablets — won’t only use data on your home network. So you can’t even rely on a data usage meter that shows how much data you’ve downloaded on your laptop, as some of that will have taken place outside of your home on a different Wi-Fi network.
Different operating systems do have different tools that can help. GlassWire is a free and polished network-monitoring tool that will track data usage on all versions of Windows. On Windows 10 and 8, you could also set a specific connection as a “metered” connection and Windows will track data usage for it. This will change how Windows and some applications use the connection, however.