Convert data transfer rate

About the units

• Bits per second (bit/s) – The bit per second is the basic unit of data transmission, from which every contract speed is built. The prefixes are decimal: according to NIST, telecommunications engineers always mean a million bits per second by a megabit per second, though some local network designers have counted in binary. The International Telecommunication Union writes the unit bit/s, as in the modem standard V.90 at up to 56,000 bit/s. The trap is the abbreviations: “bps” and “Bps” differ by one capital letter but by a factor of eight.
• Kilobits per second (kbit/s) – A kilobit per second is 1,000 bits per second, 125 bytes per second. It was the unit of the modem era: the ITU standard V.90 of 1998 gave up to 56 kilobits per second downstream and 33.6 upstream over the telephone network. There are two traps. Kilo here always means 1,000, never 1,024. And 56 kilobits per second is 7 kilobytes per second, not 56 – the modem downloaded a megabyte in just under two and a half minutes.
• Megabits per second (Mbit/s) – Megabits per second is the unit broadband, wifi and mobile networks are sold in: a million bits per second, 125,000 bytes per second. The EU open internet regulation requires the contract to explain the minimum, normally available, maximum and advertised speed, but names no unit – the megabit is trade practice. The trap is the factor of eight: 100 megabits per second gives 12.5 megabytes per second in the download window. Networking always counts decimally, while file sizes are often shown in binary.
• Gigabits per second (Gbit/s) – Gigabits per second is a thousand million bits per second, 125 megabytes per second or 1,000 megabits per second. The unit measures fibre connections, data centre networks and Ethernet standards. A 5 GB film takes 40 seconds in theory. Two traps. A gigabit line does not give gigabytes per second – there is the factor of eight. And real throughput is always below the link speed, because the protocols take their share; that is why EU rules separate maximum from normally available speed.
• Bytes per second (B/s) – The byte per second is eight bits per second and the basic unit for how fast files move. Files are measured in bytes, so programs that copy and download show their speed in bytes per second with prefixes, while internet contracts count bits. The relation is exactly a factor of eight: a modem at 56,000 bit/s delivers at most 7,000 bytes per second. The trap is the symbols, where B means byte and b bit. Programs may also count kilo and mega in binary.
• Kilobytes per second (kB/s) – A kilobyte per second is 1,000 bytes per second, 8,000 bits per second. The unit belongs to slow transfers: the modem standard V.90 gave 56 kilobits per second, which is at most 7 kilobytes per second. The trap is the capital letter and the binary prefix. Many programs write K when they mean 1,024, and GNU coreutils notes that tradition and POSIX use k to mean kibibyte. So “100 KB/s” may be 102,400 bytes per second, 2.4% more than 100 kB/s.
• Megabytes per second (MB/s) – Megabytes per second is a million bytes per second, eight million bits per second. It is the unit in the browser’s download window, in file transfers and on the markings of memory cards and SSDs, while networks and subscriptions are counted in megabits per second. The relation is exact: 12.5 megabytes per second is 100 megabits per second, because a byte is eight bits. The trap is that factor the other way round – 12 MB/s on a 100 Mbit/s line is what the contract promised.
• Gigabytes per second (GB/s) – A gigabyte per second is a thousand million bytes per second, 8 gigabits per second. The industry body USB-IF rates USB4 at up to 40 or 80 gigabits per second, which is 5 or 10 gigabytes per second. A 50-gigabyte file therefore takes 10 seconds in theory over 40 gigabits per second. The trap is that makers write Gbps for bits and GB/s for bytes, and the two look almost alike. On top of that, GB/s in a program may mean gibibytes per second, 7.4% more.