Memory card marketing is littered with acronyms and jargon, but one of the most frustrating is the “x-rating” (like 1066x or 2000x) printed on the front of some SD and CompactFlash cards.
It looks impressive, but it makes it incredibly difficult to compare cards side-by-side when you’re trying to figure out if a card can actually handle your camera’s video bitrates.
To make comparing apples-to-apples easier, I put together this simple converter. Just select the x-rating printed on your card to see exactly what that translates to in standard Megabytes per second (MB/s).
Edit any field and the others update. 1 byte = 8 bits; an X-rating is a multiple of 150 KB/s (the original CD-ROM transfer speed), so 1066× ≈ 160 MB/s.
Why do cards still use X-ratings?
To understand why this rating exists, we have to step back in time. Because it’s really not relevant to anything used these days.
The “x” is actually an old multiplier used to measure the speed of legacy CD-ROM drives, where the standard baseline speed was 150 KB/s.1
Therefore, 1x = 150 KB/s.
Memory card manufacturers adopted this multiplier early on because, quite frankly, the math produces big, impressive-sounding numbers. A card labeled “1066x” sounds incredibly fast to a buyer in a store, but when you do the math (1066 * 150 KB/s), it translates to a much more terrestrial 160 MB/s.
The marketing trap: Read vs. write speeds
While most modern SD cards and CFexpress cards have thankfully transitioned to printing the MB/s directly on the label, you still have to be careful when navigating these marketing specs.
Whether it’s an x-rating or a prominently displayed MB/s number, manufacturers almost always print the peak read speed on the front of the card. Read speed is how fast data can be pulled off the card when you are sitting at your desk transferring files to your computer.
However, when you are actually shooting, the camera needs to push data onto the card. This relies entirely on the card’s sustained write speed, which is usually significantly slower than the read speed printed on the box. If a card boasts a massive x-rating but lacks a minimum sustained write guarantee (like a V90 logo on an SD card or a VPG400 logo on a CFexpress card), it can still easily bottleneck your camera during video recording or fast photo bursts.
Notes & References:
- Original base CD-ROM data transfer rate standard: 150 Kilobytes per second [↩︎]
