Understanding bandwidth units
Bits with a small b, bytes with a big B.
Why "100 Mbps internet" downloads at ~12 MB/s, the SI vs IEC distinction, and the 1000 vs 1024 fight that won't die.
The factor of 8.
Network speeds use bits per second (lowercase b). File sizes use bytes (uppercase B). One byte = 8 bits. A 100 Mbps connection therefore downloads at 100/8 = 12.5 MB/s peak, before TCP overhead. The whole decade of "but I'm paying for 100 megabits" confusion comes from this single conversion. Marketing departments love bits because the numbers are bigger.
1 byte = 8 bits ; MB/s = Mbps ÷ 8
SI vs IEC — 1000 vs 1024.
The other fight. SI uses powers of 1000: 1 MB = 1,000,000 bytes. IEC (and many programmers) use powers of 1024: 1 MiB = 1,048,576 bytes. The IEC unit names — KiB, MiB, GiB, TiB — were standardised in 1998 specifically to disambiguate. Hard drives are sold in SI units (a "1 TB" drive is 10¹² bytes). Operating systems often display in IEC (a "1 TB" drive shows as 931 GiB). Both are correct; the unit prefix is the confusion.
A worked download.
A 4 GB game file on a 200 Mbps connection. 200 Mbps ÷ 8 = 25 MB/s theoretical. Assuming 80 % real-world efficiency (TCP overhead, server throttling, etc.), expect ~20 MB/s. 4000 MB / 20 = 200 seconds ≈ 3.3 minutes. The marketed connection speed and the actual download time line up only after the bit/byte conversion and the efficiency haircut.
4GB on 200 Mbps
bps ÷ 8 ÷ efficiency
Marketed bandwidth → real download time.
200 / 8 = 25 MB/s × 0.8 = 20 MB/s ; 4000 / 20 = 200s
= ~3-4 minutes
Streaming bitrate bands.
Spotify: 96-320 kbps. SD video: 1-3 Mbps. 1080p HD: 5-10 Mbps. 4K HDR: 15-40 Mbps. 8K: 80+ Mbps. The numbers come from codec efficiency and target quality; better codecs deliver the same quality at lower bandwidth (AV1 ≈ 30 % less than H.264 for equivalent quality). Household bandwidth budgets — "how many streams can run concurrently" — multiply by family size.
Latency is not bandwidth.
A common confusion. Bandwidth is throughput per second; latency is the round-trip delay. A 1 Gbps fibre link can have 200 ms latency (transcontinental). A 56 kbps modem can have 50 ms latency (local). Bandwidth determines how fast bulk transfers complete; latency determines how snappy interactive applications feel. A game's "ping" is latency, not bandwidth; a download's "speed" is bandwidth, not latency.