Estimates how long a download takes from the file size and the line speed. Line speeds are quoted in bits and file sizes in bytes, a factor of eight apart. The efficiency is the share of the nominal speed actually achieved.
This tool estimates how long a download will take, given the size of the file and the speed of the line.
Here is the file size in gigabytes, the nominal line speed in Mbps, the efficiency as a percentage, the effective speed and the time in seconds.
Line speeds are quoted in bits and file sizes in bytes. A byte is eight bits, so the two scales stand a factor of eight apart, and a 100 Mbps line actually carries 12.5 MB each second.
The 8000 in the formula converts gigabytes to megabits: one gigabyte is 1000 megabytes, which is 8000 megabits.
A line never delivers its nominal speed. Some of the capacity goes to the protocols themselves, some is lost to congestion at the far end, and a wireless link loses more again to signal conditions. Sixty to eighty per cent is a fair assumption on fibre.
A 5 GB file over a 100 Mbps line at seventy per cent efficiency takes 9 minutes 31 seconds. The effective speed is seven tenths of 12.5 MB/s, or 8.75 MB/s.
The effective rate is 100 × 0.7 = 70 Mbps. Five gigabytes is 40000 megabits, so 40000 ÷ 70 = 571.4 seconds, which is 9 minutes 31 seconds.
Gigabytes here are decimal, with 1 GB equal to 1000 MB. Where software reports sizes in binary units, with 1 GiB equal to 1024 MiB, the two figures differ by about seven per cent.