The download time formula
File sizes are measured in bytes but connection speeds in bits per second, and one byte is eight bits. So you multiply the size by 8 to get bits, then divide by the speed:
seconds = ( file MB × 8 ) ÷ speed Mbps
Convert GB to MB by multiplying by 1000 first. The calculator also shows an estimate with about 15% overhead added, because protocol headers, server throttling and Wi-Fi rarely let you hit the theoretical maximum.
Worked example
A 1000 MB (1 GB) file on a 100 Mbps connection:
Bits: 1000 × 8 = 8000 megabits.
Time: 8000 ÷ 100 = 80 seconds (1 min 20 s).
With overhead: ≈ 92 seconds in practice.
Mbps vs MBps: a 100 Mbps line moves at most ~12.5 megabytes per second. Always divide advertised speed by 8 before comparing it to a file size.
What affects real download time
The ideal figure assumes a perfect, dedicated connection. In reality your speed is shared with other devices, capped by the server, reduced by Wi-Fi distance and trimmed by TCP/IP overhead. Treat the estimate as a best case and expect actual downloads to run 10 to 20 percent longer.