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What Is Memory Card Class?

Tom from Pennsylvania writes:

I use SD cards in my camera and MP3 players and I see them advertised quite often in your ads. The question I have is that they will refer to different classes. Can you give some information as to the differences in class? Thanks – Tom


The class of memory card refers to the speed at which information can be read and written to the memory card. Some cameras and devices support high-speed cards, while others will only take advantage of speeds up to a certain point. While using a faster card than the device supports won’t damage it, you won’t see any improvement over a slower-speed card.

Here is a breakdown of the various speeds:  (The transfer rate is rated in megabytes per second (MB/sec) and the X rating that appears on some cards is based on a CD-ROMs transfer speed of 150 KB/sec).

Class 2: Minimum of  2 MB/sec or 14X+

Class 4: Minimum of 4 MB/sec or 28X+

Class 6: Minimum of 6 MB/sec or 40X+

Class 10: Minimum of 10 MB/sec or 66X+

Class 10 UHS-I: Speed ratings vary from 30 to 95 MB/sec or 200X+

Don’t underestimate the value of a higher-speed memory card, even if your camera doesn’t support the higher speeds, because your memory card reader may. USB 3.0 card readers support can take full advantage of UHS-I 95 MB/sec cards, while USB 2.0 readers can reach speeds in the 30 MB/sec range.