File transfer time calculator

The formula

Size is usually given in bytes and link speed in bits per second, so the first step is always the 8× conversion:

Decimal units are used for both, the way vendors specify them: 1 GB = 10⁹ bytes and 1 Gbps = 10⁹ bit/s. Binary units (GiB, TiB) are available in the size selector because operating systems often report sizes in them: 1 TiB is 9.95% larger than 1 TB.

Worked example: 10 GB over 100 Mbps

1. 10 GB × 8 = 80 Gbit.
2. 100 Mbps minus 10% overhead = 90 Mbps effective.
3. 80,000 Mbit / 90 Mbps = 888.9 s ≈ 14 min 49 s.

At full line rate it would take 13 min 20 s; the overhead adds roughly a minute and a half.

Choosing the overhead

The header overhead of a full-size TCP segment is (14 + 4 + 8 + 12 + 20 + 20) bytes of Ethernet framing, preamble, IFG, IP and TCP over 1538 bytes on the wire, about 5.1%. Jumbo frames (9000 MTU) cut it to under 1%.

Why real transfers are often slower

• TCP window limits. A single flow can never exceed window / RTT. Across an ocean, an untuned 64 KiB window caps you at a few Mbps regardless of the link. Check the bandwidth-delay product calculator.
• Storage. A spinning disk or a busy NAS often delivers less than 1 Gbps on its own. Test with fio or dd before blaming the network.
• Single-threaded tools. scp and some SMB clients are bound by one CPU core for encryption. Parallel streams (rclone --transfers, iperf3 -P) help.
• Shaping and shared links. Policers, QoS classes and other users all take a share.

Measure before you plan

# raw TCP throughput between two hosts
iperf3 -s                     # on the receiver
iperf3 -c 10.0.0.2 -t 30 -P 4 # on the sender

# real file copy with progress
rsync -a --info=progress2 /data/ user@host:/backup/

For large migrations, compare the result with a physical option: shipping an encrypted drive can beat a 100 Mbps link once you pass a few terabytes.