How the download time calculator works
The 8× megabit trap
ISPs advertise in megabits per second (Mbps); file systems measure in megabytes (MB). Eight megabits carry one megabyte, so a "300 Mbps" connection moves 37.5 MB/s — and half the wrong download estimates on earth come from dividing by the advertised number directly. The calculator converts honestly and labels both.
Why the advertised speed is theoretical
Advertised speeds are layer-1 maximums: the wire's raw capacity, shared, best-case, to a perfect server. Real transfers pay protocol overhead (~5% for TCP/IP), Wi-Fi penalties (walls, contention — 30–50% losses are normal), and the far end's server capacity. Planning on 70–95% of advertised is the honest range, wired vs Wi-Fi.
The bottleneck principle
Transfers run at the SLOWEST link: your connection, the server's uplink, the CDN's congestion, the old router in the middle. A 300 Mbps plan downloading from a saturated server delivers 20 Mbps — the math above prices your end; the server prices the rest. Speed tests measure the pipe; transfers measure the whole path.