SD cards were launched in 1999, SD stands for Secure Digital. This technology has been growing since then with different formats, capacity and speeds.

Please note that the below are the formats, speeds and capacities for the SD card, the host where the card is connected to must support the same formats, speeds and capacities too.

Formats

When first released the format was the standard SD, over the years new smaller format like miniSD and now microSD used in increasingly smaller and more portable devices.

Capacity

The original capacity was SDSC or Secure Digital Standard Capacity, supporting up to 2 GB capacity

In 2006 the SDHC or Secure Digital High Capacity was introduced, supporting capacities from 2 GB up to 32 GB

In 2009 the SDXC or Secure Digital eXtended Capacity was released, supporting capacities from 32 GB up to 2 TB

In 2018 the SDUC or Secure Digital Ultra Capacity was introduced, supporting capacities from 2 TB up to a staggering 128 TB

Speed

The speed of an SD card is defined by its sequential read or write speed and it’s expressed in Megabyte per seconds or MB/s

The default speed was 12.5 MB/s, increased to HS or High Speed later on to reach 25 MB/s

In more recent times, three types of UHS or Ultra High Speed were introduced

UHS-I supporting speed up to 104 MB/s

UHS-II supporting speed up to 312 MB/s

UHS-III supporting speed up to 624 MB/s

The latest speed introduced is called SD Express supporting up to an impressive 985 MB/s

It is important to note that UHS-II, UHS-III and SD Express come with a second line of pins to support the extra speed, the standard line of pins is for backwards compatibility

Class

The SD Association defines three types of Speed Class ratings: the original Speed Class, UHS Speed Class, and Video Speed Class showing the guaranteed minimum read and write speeds.

Speed Class ratings are 2, 4, 6 and 10 represented by a C symbol with the number inside it showing the speed (2, 4, 6 and 10 MB/s)

UHS Class ratings are 1 and 3 represented by a U symbol with the number inside it showing the speed (10 and 30 MB/s)

Video Speed Class ratings are 6, 10, 30, 60 and 90 represented by a V symbol with the number on the side showing the speed (6, 10, 30, 60 and 90 MB/s)

Application Performance Class is a newly defined class which defines the minimum random IOPS or Input/Output Operations per seconds the card can perform at

They are A1 (1500 IOPS Read and 500 Write) and A2 (4000 IOPS Read and 2000 Write)