How long to download 5 GB over a 100 Mbps line?

Question

File sizes are quoted in bytes; connection speeds in bits per second. A 5 GB file downloads over a 100 Mbps link. Solve in your head. Roughly how long does the download take? Aim: under 30 seconds.

Accepted Answer

Convert the file to bits by multiplying by 8: 5 	ext{ GB} = 5 	imes 8 = 40 	ext{ Gigabits} = 40{,}000 	ext{ Megabits.} Divide by the rate: \frac{40{,}000 	ext{ Mb}}{100 	ext{ Mb/s}} = 400 	ext{ seconds} \approx 6.7 	ext{ minutes.} In practice, protocol overhead makes it a bit longer, so call it 7-plus minutes. Takeaway. The trap is the factor of 8: bytes to bits. Whenever a size is in bytes and a rate in bits, multiply the size by 8 before dividing. A handy landmark, a 100 Mbps line moves about 12.5 MB per second.

How long to download 5 GB over a 100 Mbps line?

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