One of the more confusing aspects when talking about the speed of SD cards is the difference between megabytes and megabits. There are 8 bits in a byte, so to get from megabits per second to megabytes per second, you simply multiply by 8.
That’s a source of confusion for anything to do with computer storage space, but it takes on an extra layer when it comes to the speed of SD cards. And that’s partly because you start adding the element of time (i.e., the speed is measuring the amount of data transferred over a specifc duration (usually a second)), and partly because different aspects conventionally use one or the other.
What I mean is this: when you’re looking to buy a fast SD card, you’re most likely looking for something that’s fast enough to keep up with the camera’s video bitrate or burst photo shooting. But in that case, you’ll be dealing with both MB/s and Mb/s.
Here’s why . . .
The speed of SD cards is conventionally measured in MB/s (megabytes per second, or MBps, with an uppercase “B”). But video bitrates are conventionally measured in Mb/s (megabits per second, or Mbps, with a lowercase “b”).
Making it even more complicated is that most devices can’t actually transfer data at the maximum theoretical speed. There’s overhead from hardware and software that gets in the way and imposes a speed cost.
The upshot is that it’s not as simple as a direct comparison or even just multiplying or dividing by 8.
This is precisely the reason I conduct my independent SD card speed tests that are designed for real-world results rather than the theoretical speeds claimed by SD card manufacturers.