How long to download 100 GB at 25 Mbps

Thirty-two thousand seconds, and the four divisions behind it

100 GB is 100,000 MB. Each MB carries 8 megabits, so the file places 800,000 megabits on the wire. A 25 Mbps plan moves 25 megabits every second, and 800,000 divided by 25 is 32,000 seconds.

The time on the wall comes from that one number. 32,000 seconds is 533.33 minutes, which is 8 hours 53 minutes 20 seconds. The odd 20 seconds survive because 32,000 does not sit evenly on 60, and 533 on 60 leaves a third of a minute behind. Expect the last byte at 8:53:20, not at a round 9 hours.

The units run in powers of ten, as the NIST page on SI units sets out, so a gigabyte is 1,000,000,000 bytes. The chain uses 8,000 for that reason, not 8,192.

The overnight window fits, with six minutes to spare

Most of us start a file this size before bed, so the question worth asking is whether one night holds it. A night from 10:00 pm to 7:00 am is 32,400 seconds. The transfer needs 32,000 of them, which leaves 400 seconds, or 6 minutes 40 seconds, at the end.

Start at 10:00 pm and the last byte arrives at 6:53:20 am. Add 3 percent overhead for retransmits and headers, and the job runs 32,960 seconds, which is 9 hours 9 minutes 20 seconds. That finish is 7:09:20 am, nine minutes and 20 seconds after the window closed. Nothing failed, the night was just a little too short.

A 10 hour night is the safer plan: it holds the run with 3,040 seconds left even after overhead.

A second file doubles the clock to 17 hours 46 minutes

One line serves one queue. Two 100 GB files do not run in parallel at 12.5 Mbps each; they run one after another, or they share the rate and take the same total time. Either way the bytes add up: 1,600,000 megabits at 25 Mbps is 64,000 seconds, or 17 hours 46 minutes 40 seconds. That is two nights, not one.

Two nights for two files only works if each night runs at least 8 hours 53 minutes and the second file starts on time. A queue that stalls at 70 percent of the night leaves a partial file, and the next night starts by finishing it.

Order the queue by deadline, not by size. A 20 GB file takes 6,400 seconds, or 1 hour 46 minutes 40 seconds, and moving it to the front costs the big file nothing in total time. The alternative, three files at once from one browser, is slower on average: each connection gets a slice, the total stays at 32,000 seconds per 100 GB, and a dropped connection loses more.

One HD stream drops the line to 20 Mbps

The plan number covers the entire home, not this one file. The FCC speed advice places high definition video at 5 to 8 Mbps, a 4K stream at 25 Mbps, and HD video conferencing at 6 Mbps. Take the low end of the HD figure and 20 Mbps remain for the download.

At 20 Mbps the same 800,000 megabits need 40,000 seconds, or 11 hours 6 minutes 40 seconds. That is 2 hours 13 minutes 20 seconds longer than a line with nothing else on it, and it no longer fits a nine hour night. Push the line to 14 Mbps and the finish moves to 57,143 seconds, 15 hours 52 minutes 23 seconds. In the calculator that case is one HD stream, with the delivered share set to 70 percent.

The bottom line is the one to remember. The FCC figure is 25 Mbps for one 4K stream, which is the entire plan, so a home watching 4K at 8 pm is not downloading at 8 pm.

What DSL and fixed wireless hand you in practice

25 Mbps sits at the top of the FCC's 12 to 25 Mbps group, which suits two to four devices doing email and browsing. It is also the number printed on a lot of DSL and fixed wireless plans, and those two technologies rarely deliver the label at 8 pm.

DSL is the clearer case, because the plan has two different speeds. A 25/3 line gives 25 Mbps down and 3 Mbps up, over a copper pair that runs back to the local exchange. Downloading is fine at 32,000 seconds. Uploading the same 100 GB takes 800,000 divided by 3, which is 266,667 seconds, or 74 hours 4 minutes 27 seconds. A folder sync to the cloud would take three days.

Fixed wireless fails in a different way: every subscriber on the tower shares the same airtime, and the carrier oversubscribes it. A 25 Mbps link that tests at 25 Mbps at 9 am often tests at 15 to 20 Mbps at 8 pm, when the neighborhood is home. The breakdown below tracks what a lower delivered share does to the landing time.

A rooftop antenna changes the picture more than any plan upgrade. Line of sight to the mast decides the evening rate, and a mast two miles away across open ground beats one a short distance away behind a hill.

At 75 percent the job needs 42,667 seconds, which is 11 hours 51 minutes 7 seconds and no longer fits any single night. That is the case for starting on a Friday evening instead of a week night, so one bad hour does not push the finish into a morning when the line is needed.

Five minutes twenty seconds per gigabyte

With the rate fixed, time is a straight line in size. One gigabyte costs 320 seconds at 25 Mbps, which is 5 minutes 20 seconds. 10 GB cost 3,200 seconds, 53 minutes 20 seconds. 50 GB cost 16,000 seconds, 4 hours 26 minutes 40 seconds.

Turned around, 11.25 GB an hour is a daily budget. A 24 hour day at 11.25 GB an hour carries 270 GB. A nine hour night carries 101.25 GB, which is why one 100 GB file uses almost the entire night and nothing else fits beside it.

Set the run up before you go to bed

An 8 hour transfer is decided in the first 5 minutes. These steps catch the failures that happen on a home line.

1. Free space. Reserve 105 GB, the file plus a little for the temporary copy and the log.
2. Use a cable if you can. The FCC home network tips are blunt about it: the quick band reaches a shorter distance, and a wire beats both.
3. Pick a downloader that resumes. A file that starts over after 80 percent throws away 25,600 seconds, 7 hours 6 minutes 40 seconds.
4. Turn off the sleep timer. A computer that naps after 30 minutes can stall a job that the disk was still happy to write.
5. Plug it in. Battery saver quiets the radio on some machines, and a machine that shuts down at 20 percent ends the run early.
6. Start at 10 pm, not midnight. A 10 pm start ends at 6:53:20 am, which leaves the morning free even when the run slips by a few minutes.
7. Look at the progress bar at 10 minutes. Expect 1,875 MB by then, because 600 seconds at 3.125 MB/s is 1,875 MB. A reading near 937 MB puts the line at 12.5 Mbps and the finish is closer to 5 pm the next day.

None of this speeds the plan up. It keeps an 8 hour job from turning into a nine hour one, which decides whether the file is ready at breakfast or still running when everyone gets home.