Transfer time calculator: how long a file or backup takes over a link

Enter a size, a link speed and an efficiency, and the calculator tells you how long the copy will take: a nightly backup to another site, a VM image across the LAN, a disk image pulled over a VPN. Add a deadline in hours and it works the other way too, giving the link speed you would need to finish in time.

How it works

Sizes are in bytes and links are in bits per second, so the first step is always a multiplication by 8. Efficiency then trims the link to what really gets through after protocol headers, retransmissions and everything else that shares the path.

bits            = size in bytes * 8
effective rate  = link speed * efficiency
time (s)        = bits / effective rate
link needed     = bits / (deadline in hours * 3600 * efficiency)

The units are fixed:

Sizes come in both decimal and binary units. Link speeds are always decimal, the way vendors and providers quote them. Efficiency is a percentage from 1 to 100; anything outside that range shows an error. The time is rounded to the nearest second and shown as hours:minutes:seconds, with a day count in front when it passes 24 hours, so 119.6 seconds reads 00:02:00. Under a minute it shows seconds with one decimal.

Worked example

The calculator opens with 250 GB over a 500 Mbit/s link at 90 % efficiency, with a 6-hour deadline:

bits            = 250 * 1,000,000,000 * 8   = 2,000,000,000,000
effective rate  = 500,000,000 * 0.90        = 450,000,000 bit/s
time            = 2,000,000,000,000 / 450,000,000 = 4,444.4 s
link for 6 h    = 2,000,000,000,000 / (21,600 * 0.90) = 102,880,658 bit/s

So the backup takes about an hour and a quarter, and a link of roughly 103 Mbit/s, kept as clean as this one, would still make a 6-hour window.

Now change the unit from GB to GiB and leave the number at 250. The data grows to 268.435 GB, the time to 01:19:32, and the link for 6 hours to 110.5 Mbit/s. That 7 % gap is the most common reason two people get different answers for the same job.

Limits and gotchas

• Check which unit your size is in. Windows Explorer shows binary sizes with a "GB" label, and ls -lh and du -h print binary units too, while df -H, drive labels and most backup reports use decimal. If the number came from a file manager, GiB is often the right choice.
• Megabits are not megabytes. 500 Mbit/s moves 62.5 MB per second at best. A download dialog showing 55 MB/s on that link is already at 88 % efficiency.
• Efficiency on a clean wired LAN tops out in the low to mid 90s. A full-size 1,500-byte Ethernet frame carries 1,448 to 1,460 bytes of TCP payload in 1,538 bytes of wire time once headers, preamble and the gap between frames are counted, which caps a single stream near 94 to 95 %. VPN encapsulation, Wi-Fi and busy shared links push it lower.
• Long-distance TCP has its own ceiling: one stream cannot move more than its window per round trip. A 64 KiB window over an 80 ms path is capped near 6.5 Mbit/s whatever the link speed. Modern stacks scale the window (RFC 7323), but a middlebox that strips the option brings the cap back. Lower the efficiency for transfers across long paths.
• The link is rarely the only bottleneck. Disk read speed at the source, encryption or compression on a small CPU, and write speed at the target all cap the rate. Deduplicating backup software moves fewer bytes than the backup size, so time it from its own transfer counts.
• Check the slowest direction. An offsite backup over a broadband line is limited by the upload speed, often a fraction of the download figure on the contract.
• The link-needed figure assumes the whole link is free. If daytime traffic shares it, add that load on top, or measure what the port already carries with the counter rate calculator and the method in interface counters explained.

Once you know the restore time for your largest system, write it into the recovery steps. A runbook that says "the restore takes about 75 minutes over the 500 Mbit/s link" saves someone from guessing at 3 a.m.

Questions

How long does 1 TB take over a gigabit link?

About 2 hours 28 minutes at 90 % efficiency (8,889 seconds). At a perfect 100 % it would be 8,000 seconds, 2 hours 13 minutes 20 seconds.

Why is my copy slower than the link speed?

Bits versus bytes is the first suspect: divide the link speed by 8 to get bytes per second. After that come protocol overhead, disk speed at either end, CPU spent on encryption, and TCP limits on high-latency paths.

What is the difference between GB and GiB?

A GB is 1,000,000,000 bytes, a GiB is 1,073,741,824 bytes (2^30). The gap is about 7.4 % at this size and about 10 % between TB and TiB.

What efficiency should I enter?

90 to 95 % for a wired LAN with nothing else on it, 70 to 85 % for a VPN or a busy shared link, and lower for Wi-Fi or long-distance single-stream TCP. If you have measured a real transfer, use that.