MB/s vs Mbps: why your downloads feel slow

You pay for a "50Mb" line, you start a download, and the figure you're watching sits at roughly an eighth of what you were promised. That eighth trips people up all the time, and it's almost always a units thing rather than a fault on the line. Once someone shows you the trick, you won't be caught out by it again.

There are two measurements hiding behind that little "b", and they sit one capital letter apart.

Mbps is megabits per second. It's how ISPs (internet service providers, the firm you pay for your connection) advertise a line. That lowercase "b" is the tell: it means bit, the smallest unit going, a single 1 or 0.

MB/s (you'll also see it written MBps) is megabytes per second. It's the figure your browser, your download manager, or a file copy tends to show you while it's running. The capital "B" means byte.

And a byte is 8 bits. So the advertised number and the download number are measuring the same thing in different currencies, and the exchange rate is 8 to 1. Take the Mbps figure, divide by 8, and you're roughly at the MB/s you'll see in practice.

So what does that look like in real numbers?

A "100Mb" line, on a good day, gets you somewhere around 12MB/s when you're pulling a file down. A "50Mb" line, about 6MB/s. I say "around" because there's a bit of overhead on any connection, so you tend to land a touch under the tidy divide-by-8 figure, but it's close: 100 divided by 8 is 12.5, and that's your ballpark.

That's the whole of it, really. A "b" and a "B" are eight times apart, and it's an easy one to misread. Once you know which unit you're looking at, that "slow" download usually makes perfect sense: the line's sold to you in bits, and your download's counting in bytes.