Bits, Bytes and Why Downloads Look Slow
The single most common confusion in networking: connection speeds are quoted in bits per second, file sizes in bytes. Eight bits make one byte, so a 100 Mbps connection transfers at most 12.5 MB per second.
MB/s = Mbps ÷ 8 Download time (s) = File size (MB) × 8 ÷ Speed (Mbps)
Note the capitalisation: a lowercase b means bits, an uppercase B means bytes. Mbps and MBps differ by a factor of eight, and internet providers always advertise the larger-sounding one.
Speed to Real Transfer Rate
| Connection | MB/s (theoretical) | 1 GB file | 5 GB file | 50 GB game |
|---|---|---|---|---|
| 10 Mbps | 1.25 | 13 min | 67 min | 11 h |
| 25 Mbps | 3.13 | 5.3 min | 27 min | 4.4 h |
| 50 Mbps | 6.25 | 2.7 min | 13 min | 2.2 h |
| 100 Mbps | 12.5 | 80 s | 6.7 min | 67 min |
| 250 Mbps | 31.3 | 32 s | 2.7 min | 27 min |
| 500 Mbps | 62.5 | 16 s | 80 s | 13 min |
| 1 Gbps | 125 | 8 s | 40 s | 6.7 min |
Why You Never Get the Advertised Speed
- Protocol overhead. TCP/IP headers, acknowledgements and retransmissions consume roughly 5–15% of raw throughput.
- Wi-Fi losses. Wireless throughput is typically 40–60% of the link rate your device reports, and falls sharply with distance and interference.
- The other end. A download is limited by the slower of the two ends. A server capping you at 5 MB/s makes your gigabit connection irrelevant.
- Shared capacity. Cable and mobile networks share bandwidth between subscribers, so evening speeds fall.
- Device limits. An old laptop's Wi-Fi card or a mechanical hard disk can become the bottleneck before the network does.
Planning at about 85% of the advertised figure is realistic on a wired connection, and 50–60% on Wi-Fi.
Streaming Data Use
| Activity | Speed needed | Data per hour |
|---|---|---|
| Music streaming | 0.5 Mbps | ~0.07 GB |
| SD video (480p) | 3 Mbps | ~1 GB |
| HD video (1080p) | 5–8 Mbps | ~3 GB |
| 4K video | 15–25 Mbps | ~7 GB |
| Video call (HD) | 2–4 Mbps | ~1.5 GB |
| Online gaming | 3–6 Mbps | ~0.1 GB |
| Web browsing | 1–5 Mbps | ~0.15 GB |
Online gaming needs very little bandwidth but is highly sensitive to latency — the round-trip delay. A 1 Gbps connection with 90 ms ping plays worse than a 25 Mbps connection with 15 ms.
How Much Speed Does a Household Need?
| Household | Recommended |
|---|---|
| One person, browsing and SD video | 25 Mbps |
| Two people, HD streaming | 50–100 Mbps |
| Family, several HD streams and video calls | 150–300 Mbps |
| Multiple 4K streams, large downloads, working from home | 500 Mbps–1 Gbps |
Beyond about 300 Mbps most households see no difference in day-to-day use. Upload speed and latency usually matter more than headline download figures, particularly for video calls and cloud backup.
Decimal and Binary Units
| Decimal (SI) | Bytes | Binary (IEC) | Bytes |
|---|---|---|---|
| 1 KB | 1,000 | 1 KiB | 1,024 |
| 1 MB | 1,000,000 | 1 MiB | 1,048,576 |
| 1 GB | 109 | 1 GiB | 1,073,741,824 |
| 1 TB | 1012 | 1 TiB | 1,099,511,627,776 |
Networking and storage manufacturers use the decimal form; operating systems have traditionally reported the binary form. This is why a 1 TB drive shows as about 931 GB in Windows — nothing is missing, the two are counting differently.
Frequently Asked Questions
How long to download a 50 GB game on 100 Mbps?
About 67 minutes at the theoretical rate, and closer to 80 minutes in practice.
Is 100 Mbps fast?
Comfortably enough for a household running several HD streams and video calls. It becomes limiting only with multiple 4K streams or frequent very large downloads.
Why is my Wi-Fi slower than my plan?
Wi-Fi throughput is typically half the link rate, and falls with distance, walls and interference. A wired test is the only way to measure what your connection actually delivers.
Does upload speed matter?
Increasingly. Video calls, cloud backup, working from home and sending large files all depend on it, and many cable plans provide only a tenth of the download figure.