Why your 100 Mbps connection downloads at 12 MB/s
Internet speeds are sold in bits and files are measured in bytes. Here is the factor of eight, what else slows a download, and how long files really take.
You pay for 100 Mbps. You start a download and your browser reports 12 MB per second. Nothing is broken and nobody is cheating you — the two numbers are measuring different things, and the gap between them is one of the most persistent sources of confusion in consumer technology.
Bits and bytes are not the same unit
Network speeds are quoted in megabits per second (Mbps, lowercase b). File sizes are measured in megabytes (MB, uppercase B). One byte is eight bits, so the conversion is simply division by eight.
100 Mbps ÷ 8 = 12.5 MB/s. Your browser is reporting exactly what the connection delivers, in the unit that matches the file. The industry has never unified these because networking historically counted bits on a wire while storage counted bytes in memory, and both conventions stuck.
| Advertised speed | Maximum real transfer rate |
|---|---|
| 10 Mbps | 1.25 MB/s |
| 25 Mbps | 3.13 MB/s |
| 100 Mbps | 12.5 MB/s |
| 500 Mbps | 62.5 MB/s |
| 1,000 Mbps (1 Gbps) | 125 MB/s |
What a download actually takes
Once the units line up the arithmetic is easy — file size in megabits, divided by connection speed:
| File | On 25 Mbps | On 100 Mbps | On 1 Gbps |
|---|---|---|---|
| 1 GB update | 5 min 28 s | 1 min 22 s | 8 s |
| 4 GB film | 21 min 51 s | 5 min 28 s | 33 s |
| 50 GB game | 4 h 33 min | 1 h 8 min | 6 min 50 s |
The jump from 25 to 100 Mbps saves over sixteen minutes on a film. The jump from 100 Mbps to a gigabit saves under five. Speed upgrades follow diminishing returns, and past a certain point the connection stops being the bottleneck at all.
Why you often get less than you pay for
The advertised figure is the capacity of the link to your provider. Everything after that can constrain it:
- The server at the other end. If it limits you to 5 MB/s, a gigabit connection makes no difference whatsoever.
- Wi-Fi. Distance, walls and the 2.4 GHz band routinely cut throughput well below the wired rate. A cable is the fastest fix.
- Sharing. Every device in the house draws from the same link, and providers may share neighbourhood capacity at peak times.
- Your own hardware. An older router or a 100 Mbps Ethernet port caps everything behind it, regardless of the line.
Bandwidth is not latency
These get conflated constantly. Bandwidth is how much data moves per second; latency is how long a single round trip takes. Upgrading bandwidth speeds up large downloads and does almost nothing for a video call or a game, both of which depend on latency.
A useful analogy: bandwidth is how many lanes the motorway has, latency is how long the journey takes. Adding lanes moves more cars per hour but does not shorten anyone's drive. For gaming and calls, ping matters far more than the headline speed.
What speed do you actually need?
Streaming 4K uses roughly 25 Mbps per stream; HD needs about 5. Video calls need 2–4. Ordinary browsing barely registers. Most households are well served by 100 Mbps, and the honest case for gigabit is large downloads and many simultaneous heavy users — not making the web feel faster, which is governed by latency instead.
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