How ISPs Advertise Speed (And Why It's Always in Mbps, Not MB/s)

Every internet advertisement you have ever seen quotes megabits. No provider anywhere markets a plan as '125 MB/s', even though that describes exactly the same connection as the gigabit plan they are selling.

There are two reasons. One is completely legitimate. The other is marketing.

The legitimate reason

Networking hardware has always been rated in bits per second, because a network transmits a serial stream of bits that does not align neatly to byte boundaries.

This is not a convention invented by ISPs. It runs through the entire history of telecommunications: modems were rated in baud and then bits per second, Ethernet standards are 10BASE-T, 100BASE-TX and 1000BASE-T for 10, 100 and 1,000 megabits, and every Wi-Fi standard from 802.11b onwards quotes a megabit figure.

A network engineer measuring a link genuinely cares about bits, because the overhead the link carries — packet headers, framing, error correction — is measured in bits too and is not part of any file. Quoting bytes would mean quoting a number that only becomes meaningful after you subtract things the wire does not know about.

So Mbps is the correct engineering unit. That much is not spin.

The marketing reason

It also produces a number eight times larger, and that is not nothing.

'Up to 1000 Mbps' is a far better billboard than 'up to 125 MB/s'. Both are true. Only one gets printed.

The industry standardising on the larger-sounding unit is a happy coincidence for the industry. It would be naive to think that played no part in it becoming universal in consumer advertising, as opposed to remaining a technical spec.

The words that do the heavy lifting

The unit is only the first layer. The phrasing around it matters more.

"Up to"

This is the important one. 'Up to 500 Mbps' is a ceiling, not a promise. On shared infrastructure such as cable, the speed you get depends on how many neighbours are online. Many regulators now require providers to also publish a realistic peak-time figure — that second number is the one worth comparing.

"Average speed"

Usually defined as the speed achieved by at least 50% of customers at peak time. Better than 'up to', but it still means half of customers get less.

The missing upload figure

Cable and DSL plans are strongly asymmetric. A '500 Mbps' cable plan may have only 20 Mbps of upload — 2.5 MB/s — which is the number that will actually frustrate you when sending a large file. Fibre is usually symmetric. See upload vs download speed.

How to read a plan honestly

1. 1Convert the headline to MB/s. Divide by 8. That is the number you will see in a download.
2. 2Find the upload figure. It is usually in the small print and often ten times smaller.
3. 3Look for the peak-time or average figure, not just the 'up to' number.
4. 4Check the contract length and the post-discount price. A cheaper 300 Mbps plan often beats a discounted 1 Gbps plan that doubles in price at month 13.
5. 5Ask whether you would notice. Above roughly 300 Mbps, extra speed mainly shortens large downloads — see what internet speed do you actually need.

A worked comparison

Two plans, same price. Which is better?

• Plan A: 900 Mbps down / 35 Mbps up, cable, 'up to'.
• Plan B: 500 Mbps down / 500 Mbps up, fibre, symmetric.

Plan A wins the billboard by a wide margin: 112.5 MB/s against 62.5 MB/s. But Plan B uploads at 62.5 MB/s versus Plan A's 4.4 MB/s — fourteen times faster. If you back up to the cloud, send video, or work from home, Plan B is comfortably the better connection despite the smaller headline.

The advertised number tells you about the best case in one direction only. It is the least informative figure on the page.

The takeaway

Mbps is a real standard, not an invention of marketing departments — but its persistence in consumer advertising, where a byte figure would be more useful to the buyer, is not an accident either.

Divide by eight before you compare anything. It costs a second and it turns a marketing number into a number about your actual life.