Estimate data used streaming in standard definition.
Standard definition streaming at around 1 Mbps costs roughly 0.44 GB an hour — about a fifth of 1080p and a twentieth of 4K. That makes it the setting that turns a small allowance into genuinely useful viewing time: more than two hours per gigabyte. The per-device breakdown matters because SD is most often chosen in households where several people watch simultaneously on a constrained connection.
Standard definition data across devices
GB per hour = Mbps x 3600 / (8 x 1024); period GB = GB per hour x hours per day x days x devices; hours per GB = 1 / GB per hour
About 0.3-0.7 GB depending on the service. Netflix quotes roughly 1 GB an hour for its standard setting, which is closer to 576p, while true 480p on well-optimised services comes in under 0.5 GB.
On a large screen, yes — the difference from 1080p is obvious. On a phone or a small tablet at normal viewing distance it is much harder to see, which is why SD is a sensible default for mobile viewing specifically.
Yes. Streams are independent, so three devices at 1 Mbps consume 3 Mbps and three times the data. Even the same programme watched in two rooms is transferred twice.