Calculate how long a transfer takes from the amount of data and the line speed. Data sizes are usually given in bytes (B) and line speeds in bits per second (bps), so the key step is multiplying by 8 to work in bits. You can also enter the transmission efficiency, the share of the line actually usable after headers, retransmissions and so on.
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Result
Transfer time
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Data size (bits)
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Effective speed (line speed × efficiency)
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Line speed needed to finish in that time
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Propagation delay (the time for the signal to arrive) and time spent waiting for TCP acknowledgments are not included. When small pieces of data are exchanged many times over a line with a long round-trip time (RTT), the delay has a large effect.
See it as a graph
Other inputs stay at their current values while the chosen input varies from 1/4 to 4 times its current value. The red dot is the current value.
The basics of transfer time
Transfer time is the data size in bits divided by the effective speed in bits per second. File sizes are normally given in bytes and line speeds in bits per second (bps), so first multiply the data size by 8 to convert it to bits. For example, sending a 100 MB file over a 100 Mbps line means sending 100 × 10⁶ × 8 = 8 × 10⁸ bits at 10⁸ bits per second, which takes 8 seconds. Mixing up bytes and bits makes the answer off by a factor of 8.
Transmission efficiency
In real communication, headers and control information are sent along with the data, and data with errors is sent again, so not all of the line speed is available for data. The share that can be used for data is the transmission efficiency, and the effective speed is the line speed × the transmission efficiency. In the example “1 GB over 100 Mbps at 80% efficiency”, 8 × 10⁹ bits are sent at 80 Mbps, which takes 100 seconds. With TCP/IP over Ethernet, the IP and TCP headers take 40 bytes of each 1500-byte packet, and the Ethernet header and inter-frame gap add more, so the efficiency is about 95%.
KB versus KiB
The k, M and G of communication speeds always step by 1000. The KB, MB and GB of data sizes, however, have been used both for steps of 1000 and for steps of 1024, so steps of 1024 are written KiB, MiB and GiB to tell them apart. 10 GiB is about 10.7 × 10⁹ bytes, so it takes about 7% longer than a calculation that treats it as 10 GB. Exam questions usually define the unit in the question, for example 1 MB = 10⁶ bytes, so follow that.
Latency and throughput
Transfer-time calculations often consider only the time needed to send the data out, but in practice propagation delay and time spent waiting for replies are added. If small pieces of data are sent one at a time and each waits for a reply, the round-trip time (RTT) dominates and a faster line hardly shortens the transfer. Sending large amounts of data together gives a throughput close to the line speed.
Link speeds are usually given in bits per second (bps) and file sizes in bytes (B), so first align the units. One byte is 8 bits, so 100 MB = 100 × 8 = 800 megabits. Sending this at 100 Mbps takes 800 ÷ 100 = 8 seconds. In practice it takes longer because of overhead such as headers and the waiting introduced by TCP's control mechanisms.
Each frame carries 1500 − 20 − 20 = 1460 bytes of application data. On the wire, however, each frame takes the time of 1500 + 18 + 20 = 1538 bytes. The efficiency is therefore 1460 ÷ 1538 ≈ 0.949, and the throughput limit is 1000 × 0.949 ≈ 949 Mbps. Using the TCP timestamp option adds 12 bytes to the header and lowers efficiency slightly further. The smaller the packets, the larger the share of headers and other overhead, and the lower the efficiency.
Each request takes one round trip, 50 ms, from sending the request until the response starts to arrive, plus the time to receive 10 KB = 80000 bits at 100 Mbps: 80000 ÷ (100 × 10^6) = 0.8 ms. The total is (50 + 0.8) × 100 = 5080 ms ≈ 5.1 s, almost all of it waiting for round trips. Even with ten times the bandwidth, 1 Gbps, it only drops to (50 + 0.08) × 100 ≈ 5.0 s. Light in optical fiber travels about 200 km per millisecond, so distance sets a lower bound on RTT. To speed up this kind of processing, batch requests, send several requests in parallel without waiting for responses, or place servers close to users.
FAQ
How long does a 1 GB file take over a 1 Gbps line?
With 100% transmission efficiency and 1 GB = 10⁹ bytes, it takes 8 × 10⁹ bits ÷ 10⁹ bits per second = 8 seconds. At 80% efficiency it takes 10 seconds.
What is the difference between Mbps and MB/s?
Mbps means megabits per second and MB/s means megabytes per second, so 1 MB/s = 8 Mbps. Download speeds are often shown in MB/s and contracted line speeds in Mbps, so be careful.
What transmission efficiency should I assume?
In exams it is given in the question. In practice, TCP over a wired LAN often exceeds 90%, while on wireless LANs it can drop a lot depending on signal conditions and congestion.
How does this appear on exams?
IT certification exams such as Japan's Fundamental Information Technology Engineer Examination often ask for the transfer time from the data size, line speed and transmission efficiency, or for the line speed needed to finish within a given time. Watch the byte-to-bit conversion and the unit prefixes (k, M, G).