Bits, Bytes and Transfer Speeds

Every internet connection is sold in one unit and consumed in another. Your provider speaks megabits per second; your download dialog speaks megabytes. Between those two vocabularies sit two separate gaps — a factor of eight, and a decimal-versus-binary prefix disagreement — and together they explain almost every "my internet is slower than advertised" confusion.

Eight bits make a byte

The first gap is definitional: a byte is eight bits, because in the 1960s that was the smallest chunk of bits that could conveniently encode one character of text. Network engineers measured link capacity in bits — the raw pulses on the wire — while software counts payload in bytes. So the same speed has two names: 100 megabits per second divided by 8 is 12.5 megabytes per second. Nothing is being lost or hidden; it is a unit change, like miles per hour versus kilometres per hour. The divide-by-eight (and multiply-by-eight) rule works at every scale: 8 Mbps is 1 MB/s, 24 Mbps is 3 MB/s, and a 1 Gbps line is 125 MB/s.

Decimal prefixes for bits, binary for bytes

The second gap is subtler. Telecom convention gives bit rates decimal prefixes: 1 Mbps is exactly 1,000,000 bits per second, because transmission engineering counts in powers of ten. But many file managers and download tools count bytes in binary prefixes: a mebibyte (MiB) is 1,048,576 bytes — 1024 × 1024 — about 4.9% larger than a decimal megabyte. Put the two together and a 100 Mbps line is simultaneously 12.5 MB/s (decimal) and about 11.92 MiB/s (binary). Both numbers describe the identical pipe. This is the same decimal/binary schism that separates KB from KiB, told from the speed side of the fence — and the deeper historical why behind two competing measurement traditions is told in Why Two Measurement Systems. If the prefix zoo itself is the puzzle, KB vs KiB Explained walks the storage-side arithmetic.

What your download dialog actually shows

Even after converting units correctly, a real download never quite hits the raw line rate. Every TCP/IP packet carries headers — tens of bytes of addressing per chunk of payload — and the protocol spends some capacity acknowledging traffic in the other direction. Wi-Fi adds contention and retransmissions; the distant server adds its own limits. A healthy connection typically delivers 90–95% of its nominal rate before unit conversion, which is why 100 Mbps landing at 11.2 MiB/s instead of 11.92 is normal, not a fault. Judging an ISP by a single file transfer mixes all of these effects into one number.

Doing it in your head

The everyday arithmetic is friendly. To get from a connection speed to a download rate, divide the bit rate by eight and keep the prefix the same size class: Mbps → MB/s, Gbps → GB/s. 50 Mbps → 6.25 MB/s. 300 Mbps → 37.5 MB/s. 8 Gbps → 1 GB/s. To estimate a download time, divide the file size by the byte rate: a 700 MB file on a 25 Mbps line (3.125 MB/s) takes roughly 224 seconds, or about four minutes. The only place to stay alert is the MB/MiB distinction — if the number on screen is a file manager's, assume binary and expect the slightly smaller figure.

Try it yourself

Open the Data Transfer Rate Converter and verify this article against the arithmetic: enter 100 with From set to Mbps and To set to MB/s to read exactly 12.5, then switch To to MiB/s and watch the same line re-price itself at 11.92. Set 1 Gbps to GB/s for 125, and try a byte-side input — 1 KiB/s converts to 8.192 kbps, which is the binary prefix showing through. For file sizes rather than speeds, the Data Unit Converter handles the storage side, and the Speed Converter covers the km/h-versus-mph flavour of the same discipline.