This downloads a real ~20MB file from this site's own server, in your browser, and reads it in small streamed chunks rather than waiting for the whole thing — that's what makes the live chart possible. Every ~150ms it records how many bytes arrived since the last checkpoint, converts that to megabits per second, and plots it. The instantaneous line is genuinely noisy — that's real network jitter, not a rendering artefact — so a rolling moving average is layered on top to show the trend a single number would hide. Latency is measured the same honest way: several small round-trip requests to this site, timed directly.
A measured 120 Mbps download comfortably covers 4 simultaneous Netflix 4K streams (25 Mbps each, per Netflix's own published minimum) with headroom to spare, downloads a 15GB 4K movie in about 17 minutes, and pulls down a modern ~100GB GTA V install in a little over 1 hour 51 minutes — assuming the connection holds that speed the whole time, which real connections rarely do perfectly.
Every speed test measures against its own server, over its own route through the internet — different providers, different distances, different momentary congestion. None of them is "wrong"; they're measuring slightly different paths. Run a test a few times, at different times of day, for a realistic picture.
A genuine upload test needs a server willing to receive a large file and time how long that took — this site is static files only, with nothing to send data to. Rather than fake a live number, this tool applies your connection type's typical download:upload ratio, clearly labelled as an estimate.
Real networks don't deliver data at a perfectly constant rate — TCP slow-start, other traffic on your line, Wi-Fi interference and router buffering all cause genuine second-to-second variation. The moving average exists specifically to show the underlying trend through that noise.
Because that's the honest ceiling — real downloads from services like Netflix or Steam also depend on that specific server's load, your Wi-Fi (not just your ISP plan), and other devices sharing your connection at the same time. Treat these numbers as a best case, not a guarantee.