Quick answer: A 5 GB file over a 100 Mbps connection takes 5 × 8,000 = 40,000 megabits ÷ 100 Mbps = 400 seconds, or 6 min 40 s.
How it's calculated
Time = file size in bits ÷ speed in bits per second. File sizes are given in bytes and connection speeds in bits, and one byte is 8 bits, so multiply the size by 8 first.
Network speeds use decimal SI prefixes (1 Mbps = 1,000,000 bits per second, 1 GB = 1,000,000,000 bytes). Some operating systems show sizes in binary units (1 GiB = 1,073,741,824 bytes), about 7% larger than a GB, so choose GiB if your computer reports them.
Real downloads are slower than the plan speed because of protocol overhead, Wi-Fi and the server's own limit. Add 10–20% overhead for a realistic estimate.
Worked examples
- 5 GB at 100 Mbps: 40,000 Mb ÷ 100 = 400 s = 6 min 40 s
- 50 GB game at 500 Mbps: 400,000 Mb ÷ 500 = 800 s = 13 min 20 s
- 700 MB at 10 Mbps: 5,600 Mb ÷ 10 = 560 s = 9 min 20 s
Frequently asked questions
Why does a 100 Mbps connection download only about 12 MB per second?
Because speed is in megabits and the file is in megabytes: 100 ÷ 8 = 12.5 MB/s at most, and a little less after overhead.
What is the difference between GB and GiB?
A GB is 10⁹ bytes; a GiB is 2³⁰ bytes, 7.4% more. Storage makers and networks use GB; Windows reports sizes in GiB but labels them "GB".
Does this work for uploads?
Yes. Use your upload speed, which on many home plans is lower than the download speed.
How long does a 5 GB file take on 100 Mbps?
About 6 minutes 40 seconds in theory. In practice, allow closer to 8 minutes once Wi-Fi and overhead take their cut.
Why is my Wi-Fi slower than my plan?
Walls, distance from the router, older devices and everyone else on the network all eat into it. Plugging in a cable, or moving the router closer, usually makes a big difference.
How much speed do I need for streaming?
Each service publishes its own numbers, but HD usually needs around 5 Mbps per screen and 4K somewhere around 15 to 25. Add up every screen and video call that might be running at once.