Pick how many of each activity will run at the same time on your network. We add 25% headroom because real-world TCP overhead, retransmissions, and background updates always eat more than the spec sheet suggests.
Each activity has a typical sustained bitrate. Streaming a single 4K Netflix title averages around 25 Mbps. Two 4K TVs plus a Zoom call already passes 50 Mbps, and that's before background traffic from phones, smart speakers, and OS updates.
TCP doesn't run at line rate continuously, overhead, retransmissions, and the bursty nature of HTTP/2 mean you need ~20-30% more headroom than the raw codec bitrate suggests. Without headroom, video re-buffers when a phone in the background decides to download an app update.
Most ISP plans advertise download speed. Cable and DSL plans often have upload speeds that are 5-10× lower. If you're a streamer, work-from-home worker, or back up to the cloud, look at the upload number too.