You pay for a 100 Mbps connection. A speed test confirms 100 Mbps. Then a game download runs at 11.8 MB/s, and it feels like something is broken. Nothing is. Two units are involved, and they differ by a factor of eight before anything else gets in the way.
Bits and bytes
A bit is a single 1 or 0. A byte is 8 bits. Network speeds have always been quoted in bits per second, because networks send data one bit at a time down a wire or through the air. File sizes are quoted in bytes, because that’s how storage is organized.
The abbreviations differ only by capitalization:
- Mb or Mbit is a megabit (1,000,000 bits). Speeds are written Mbps or Mb/s.
- MB is a megabyte (1,000,000 bytes). Speeds are written MB/s.
To convert, divide by 8:
- 100 Mbps = 12.5 MB/s
- 300 Mbps = 37.5 MB/s
- 1000 Mbps = 125 MB/s
- 1 MB/s = 8 Mbps
Internet providers, speed test sites and routers use bits. Web browsers, game launchers and operating systems usually show bytes. The Mbps to MB/s converter and its reverse, MB/s to Mbps, do the arithmetic both ways.
Why you never quite reach the full figure
Dividing by 8 gives the theoretical maximum. On a real connection, part of every second is spent on the envelopes that carry your data, not the data itself.
Take wired Ethernet with a standard 1,500-byte packet. Each packet carries TCP and IP headers (40 bytes) and travels in an Ethernet frame with its own header, checksum, preamble and a mandatory gap before the next frame (38 bytes in all). Out of every 1,538 bytes on the wire, 1,460 are your file: about 94.9%.
So the best realistic download on a 100 Mbps wired connection is about 94.9% of 12.5 MB/s, or roughly 11.87 MB/s. A download running at 11.5 to 11.8 MB/s on that plan is getting essentially everything it paid for.
Other things reduce it further:
- Wi-Fi. A wireless link spends airtime on acknowledgments and retransmissions and shares the channel with every nearby device and network. Losing a third or more of the link rate is normal, and walls and distance cost more.
- The server. A busy download server, or one far away, may send slower than your connection can receive.
- Other devices. A video stream or cloud backup elsewhere in the house takes a share.
- VPNs. Encryption and an extra hop add both overhead and distance.
- Storage and ports. On gigabit connections the bottleneck can be the device itself: a USB 2.0 stick or an older hard drive can’t always write at 125 MB/s.
MB/s or MiB/s?
There’s a second, smaller trap. Some download managers and torrent clients display speeds in binary units: MiB/s, where 1 MiB is 1,048,576 bytes. On those, a perfect 100 Mbps connection shows 11.92 MiB/s, and the realistic maximum is about 11.3 MiB/s. The difference between MB and MiB is explained in why a 1 TB drive shows 931 GB.
How long will a download take?
Divide the file size in bits by the speed in bits per second. A 50 GB game is 400,000 megabits (50 × 1,000 × 8). At 100 Mbps:
- In theory: 400,000 ÷ 100 = 4,000 seconds, or 1 hour 6 minutes 40 seconds.
- With the Ethernet overhead above: about 4,214 seconds, roughly 1 hour 10 minutes.
- Over average Wi-Fi: plan on longer still.
A 4 GB movie at 50 Mbps takes 640 seconds (10 minutes 40 seconds) in theory. The download time calculator does this for any size and speed and lets you allow for overhead.
Upload is usually slower
Many cable and DSL plans are asymmetric: a 300 Mbps download might come with 20 Mbps upload, which is only 2.5 MB/s. That’s why sending a large video to the cloud can take many times longer than downloading it. Fiber plans are more often symmetric. When a speed test shows two numbers, the smaller one is usually upload.
If you’re uploading recordings, the video file size calculator estimates how large footage is at a given bitrate, which tells you how long an upload will take.
Choosing a plan
The useful question isn’t “how many Mbps?” but “how many MB do I move, and how often?”
- Streaming: a 4K stream needs about 15 to 25 Mbps. A 100 Mbps plan covers several at once.
- Large downloads: a 100 GB game takes about 2 hours 13 minutes at 100 Mbps and about 13 minutes at 1 Gbps, in theory. If you download big files often, speed matters.
- Working from home: video calls need only a few Mbps each, but upload speed matters more than people expect.
Once you can read both units, a plan’s headline number becomes easy to translate into what you’ll actually see on screen: divide by 8, then take off about 5% for a wired connection and more for Wi-Fi.