What is the difference between Mbps and MB/s?

A factor of eight, and it accounts for most of the confusion about why a fast broadband connection produces download speeds that look disappointingly slow.

The two units:

Mbps — megabits per second. Lowercase b for bits. This is how connection speeds are always quoted: broadband packages, Wi-Fi standards, mobile data, Ethernet.

MB/s — megabytes per second. Uppercase B for bytes. This is how file sizes and transfer progress are measured, because files are measured in megabytes and gigabytes.

There are 8 bits in a byte, so to convert a connection speed into download speed, divide by 8.

The arithmetic in practice. A 100 Mbps connection delivers a theoretical maximum of 12.5 MB/s. A 1 Gbps connection gives about 125 MB/s. A 5GB download on a 100 Mbps line takes around seven minutes at best.

Why the real figure is lower still:

Protocol overhead. Packet headers, acknowledgements and error correction consume some capacity — typically a few per cent, more on some links.

The advertised speed is a maximum, and contention, distance from the exchange, and network congestion all reduce it.

Wi-Fi figures are aggregate theoretical maximums across all streams and both directions, and real throughput is commonly a fraction of the number on the box.

The server or service at the other end may be the limiting factor, which is frequently the actual cause of a slow download on a fast connection.

Other devices sharing the connection.

Why the units differ at all. Networking has measured in bits since telecommunications; storage has measured in bytes since computing. Both conventions are entrenched, and marketing has no incentive to quote the smaller-sounding number.

A further wrinkle: storage manufacturers use decimal megabytes (1,000,000 bytes) while operating systems frequently display binary units (1,048,576 bytes), which is a separate reason numbers disagree — and the reason a 1TB drive shows as roughly 931GB.