The honest answer is less than most people expect for one stream and more than most people expect for a household. Here are figures you can plan with.
Per stream, by resolution
Standard definition: 3–5 Mbps. Comfortable on almost any modern connection, and a sensible fallback on mobile data.
High definition: 10–15 Mbps. Where most live television sits. This is the figure worth planning around.
4K: 25 Mbps or more. Varies considerably with how the stream is encoded, and this is where a marginal connection stops coping.
These are working figures with some headroom, not minimums. A stream may technically play below them while leaving nothing spare for the inevitable fluctuation.
Multiply by connections
This is the step people skip. Each simultaneous stream needs its own share. Three HD streams at once is 30 to 45 Mbps of sustained throughput, before anything else in the house uses the connection.
And something else always does. A game console downloading an update, a phone backing up photos, someone on a video call — these compete directly with your streams. A connection sized exactly to your streams will buffer the moment anything else happens.
Stability matters more than headline speed
A rock-solid 40 Mbps is better for streaming than a 200 Mbps connection that collapses for a few seconds every couple of minutes. Video playback is a continuous demand; it does not care about your peak, it cares about the floor.
This is why a speed test can look excellent while streaming is poor. The test measures a short burst under ideal conditions. Streaming measures the worst ten seconds of every hour.
Test in the right place
Run the test on the device you actually watch on, connected the way it is normally connected. A laptop next to the router tells you what the router can do, not what the streaming stick behind the television receives — and the difference between the two is frequently more than half.
Most devices have a browser or a speed-test app. If yours has neither, stand where it sits, with your phone on the same Wi-Fi band, and test there instead.
Upload speed is almost irrelevant
Streaming is a download. A connection with 100 Mbps down and 10 Mbps up is perfectly fine. Upload only matters if somebody in the house is doing video calls or uploading large files while you watch, in which case it competes for the same line.
If you are short
Lower the quality setting inside the player — most offer a choice, and a well-encoded HD stream on a 42-inch television is hard to tell from 4K at normal viewing distance. Use a cable where you can. Stagger heavy downloads to overnight. And be realistic about how many simultaneous connections your line will carry: buying a five-connection plan on a connection that struggles with two will not end well.