In detail
Twitch streams are live, which changes how data adds up: viewers often keep a stream running in the background for three, four or more hours at a time. Quality is chosen by the streamer's settings and your player — 'Source' or 'Auto' usually means the highest quality available, up to 1080p at 60 frames per second.
Because most Twitch broadcasts are limited to around 6–8 Mbps, a 1080p60 stream uses roughly 2.7–3.6 GB per hour, 720p about 1.5–2 GB and 480p roughly 0.6–0.7 GB per hour. Our calculator uses 3.5 GB per hour for Full HD, which reflects a high-bitrate stream.
Watching four hours a day in 1080p adds up to around 400 GB a month. If you watch on mobile data or a hotspot, lowering the player's quality to 480p cuts usage by roughly 80% and still keeps chat and gameplay readable on a phone screen.
Audio-only mode, available in the Twitch mobile app, is a good option for podcasts, music streams and Just Chatting when you don't need the picture — it uses a fraction of the data of video. Clips and VODs use the same data per minute as a live stream at the same quality.
The calculator above is pre-filled with three hours of Twitch in Full HD. Add the other services you use and the number of devices to see your total monthly data.