The divide by 8 rule
A byte is 8 bits, so a speed in megabits per second divided by 8 gives megabytes per second. A lowercase b means bits and an uppercase B means bytes, though plenty of apps write both as MBps or Mbps, so read the context. Internet plans, Wi-Fi and Ethernet quote bits. File copies, downloads and SSD specs quote bytes.
Common speeds:
- 100 Mbps: 12.5 MB/s
- 300 Mbps: 37.5 MB/s
- 500 Mbps: 62.5 MB/s
- 1 Gbps: 125 MB/s
- 2.5 Gbps: 312.5 MB/s
- 10 Gbps: 1.25 GB/s
Going the other way, a SATA SSD that reads 550 MB/s is moving 4,400 Mbps.
Why a download never matches the plan
A 1 Gbps plan is 125 MB/s on paper. The plan counts every bit on the line, including the headers around your data, while a download window shows only the data. Even on a wired gigabit link that costs a few percent, and Wi-Fi gives up much more: Cisco expects a good test to show about 65% to 70% of the reported link rate.
The server on the other end can be the limit too, and so can the drive you're saving to. When a download shows a steady speed well under the plan, test with a second site before you blame your connection.
Binary megabytes
Some software divides by 1,048,576 instead of 1,000,000 and still labels the result MB, as Windows does for storage sizes. The correct name for the binary unit is the mebibyte, MiB.
At 1 Gbps that turns 125 MB/s into 119.2 MiB/s. The same bytes get a different label. Sizes shrink the same way: 100 GB is 93.1 GiB.
What it means for a big download
A 100 GB game takes 2h 13m 20s at 100 Mbps, 44m 26s at 300 Mbps and 13m 20s at 1 Gbps, before any overhead. A wired link adds about 6% to those waits for headers, as the Gigabit Ethernet page shows. Wi-Fi adds more than that.
The pages below work out other plan and game sizes, so you can see the wait before you buy a faster plan.