Working out how fast an SD card is is often more confusing than it should be.
Part of it is due to SD card speeds being measured in MB/s (megabytes per second) while the camera’s data transfer requirements (such as video bitrate) are measured in Mb/s (megabits per second). I go into more depth on that here.
As if it wasn’t confusing enough already, there’s a related complication. Some manufacturers use a more cryptic x rating in place of MB/s. Lexar, in particular, has long used this system.
It comes from the old way of measuring the speed of CD-ROM drives when the standard speed of a CD-ROM drive was 150KB/s. Each x, therefore, equals 150KB/s.
Calculator for Converting X to MB/s
Nevertheless, you can still find it used on some SD cards. So I’ve created this quick calculator for converting some of the common memory card x ratings to MB/s:
Why I Don’t Use X-Speed Ratings
The X-rating might have been sort of useful back in the day, but the speed of early CD-ROM drives is no longer a useful reference mark.
Thankfully, it’s fading away. But it has been taking a surprisingly long time to die, and you can still see it on some SD cards and other memory cards.
But because I not only don’t see it as useful but also just an unnecessary layer of confusion, I don’t use the X-speed rating system when testing the speed of SD cards. Instead I stick with the better convention of MB/s.