Bandwidth Time Transfer Calculator

The relationship between payload, link rate, and elapsed time has one degree of freedom, so fixing any two determines the third. That is why the same arithmetic answers three different engineering questions: how long a transfer will take on a known link, what link rate a fixed maintenance window demands, and how much data can move in a given window. The arithmetic is trivial; the units are where errors occur. Storage is conventionally counted in binary multiples, so a gigabyte here means 1,073,741,824 bytes, while circuit rates are decimal, so 1 Mbps means exactly 1,000,000 bits per second. Combined with the factor of 8 between bits and bytes, a 1 GB file is 8,589,934,592 bits and needs about 86 seconds of a perfectly clean 100 Mbps link. No link is perfectly clean, which is what the overhead factor represents. On Ethernet, each 1,500-byte maximum payload carries 18 bytes of frame header and check sequence, an 8-byte preamble and start delimiter, and a 12-byte interframe gap, so roughly 2.4 percent of line rate is unavailable before any higher layer is considered; IPv4 adds 20 bytes and TCP another 20 or more per segment. A 5 to 15 percent derating is a reasonable default for large sequential transfers over wired links. Two limits sit outside this model. Latency caps a single TCP stream through the bandwidth-delay product, so a long-haul path can leave a fast link mostly idle without window scaling or parallel streams. And transfers of many small files are dominated by per-file setup, where effective throughput can fall 40 percent or more below the sequential figure.

The theory came first. Harry Nyquist established in 1928 the maximum symbol rate a bandwidth-limited channel can carry, Ralph Hartley formalised information as a quantity the same year, and Claude Shannon unified the results in A Mathematical Theory of Communication, published in the Bell System Technical Journal in 1948, defining channel capacity in bits per second as a function of bandwidth and signal-to-noise ratio. Shannon also gave the bit its standing as the unit of information, which is why transmission has been measured in bits per second ever since. The practical framing dates to local networking. Robert Metcalfe and David Boggs built Ethernet at Xerox PARC in 1973, and the IEEE ratified it as 802.3 in 1983, fixing the frame format whose fixed overhead still shapes achievable throughput; the 1,500-byte maximum transmission unit and 10, 100, and 1000 Mbps rate tiers all come from that lineage. Vint Cerf and Robert Kahn specified TCP in 1974, and its acknowledgement and window mechanics, extended by the window-scaling option in RFC 1323 in 1992, determine how much of a link a single flow can actually claim over a long path. The unit ambiguity was addressed in 1999, when the International Electrotechnical Commission issued IEC 60027-2 defining kibibyte, mebibyte, and gibibyte for binary multiples, though the older mixed convention that this calculator has to reconcile remains in everyday use.