SD Card Speeds: X vs MB/s

Some SD cards have speed ratings marked with the X system rather than the more conventional MB/s. Here’s how to make sense of it.

OM System OM-5 SD Cards. Photo by David Coleman "
Text & Photos By David Coleman
Filed Under: Memory Cards
Topics: SD Cards

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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.

Profile photo of David Coleman | Have Camera Will Travel | Washington DC-based Professional Photographer

Text & Photos by David Coleman

I'm a professional photographer based in Washington DC. Seven continents, up mountains, underwater, and a bunch of places in between. I've been shooting for 30+ years, and my my photos and time-lapse videos have appeared in a bunch of different publications from major newspapers to magazines and books, billboards, TV shows, professional sports stadiums, museums, and even massive architectural scrims covering world-famous buildings while they're being renovated. You can see some of my travel photography here and here.

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