Why does a 100 Mbps connection download at only about 12 MB/s?

Question

Weirdo · Accepted Answer

Divide Mbps by 8 to get MB/s. Mbps is megabits per second, the MB/s in a download window is megabytes per second, and 1 byte is 8 bits, so 100 Mbps tops out around 12.5 MB/s. Seeing about 12 MB/s is normal.

Speedtest's support page spells it out: Mbps means megabits per second, the ISP industry standard, which is why speed tests and broadband plans use it. File sizes on your computer are measured in bytes, so download tools usually show kB/s or MB/s. Bytes are capitalized (B) and bits are lowercase (b) precisely to keep them apart. Eight bits make one byte, so it takes 8 Mbps to equal 1 MB/s.

Wikipedia's data-rate table lists 1 Mbit/s as 125,000 bytes per second (0.125 MB/s) and 1 Gbit/s as 125 MB/s. That makes 100 Mbps about 12.5 MB/s at most, 300 Mbps about 37.5 MB/s, and 500 Mbps about 62.5 MB/s. Wikipedia also notes that an advertised Mbps figure usually means the maximum achievable bandwidth, not a guarantee. Some apps use binary MiB/s instead, where 100 Mbps is roughly 11.9 MiB/s, slightly smaller again.

If a real download lands far below the divide-by-8 number, the unit isn't the cause any more: Wi‑Fi signal, other devices using the connection, or a throttled server can all pull it down. Speedtest also warns to make sure two numbers use the same unit before comparing them, otherwise the gap looks much bigger than it is.

Why does a 100 Mbps connection download at only about 12 MB/s?

Lowercase b is bits, uppercase B is bytes

The conversion: divide by 8

Only suspect something else if it's slower than the math