Recently I was asked this question while I was helping my friends to select appropriate microSD card for their phone. I found the question funny, but my friend didn’t. This made me realize how convoluted and confusing those “hieroglyphic” characters can be for most people who did not make it their mission to study and decypher them. That is why we decided to help decipher those characters and help you identify the card you need.

What are SD cards?

Secure Digital, officially abbreviated as SD, is a proprietary non-volatile memory card format developed by the SD Card Association (SDA) for use in portable devices. SD Card was introduced in 1999 by joint efforts between SanDisk, Panasonic and Toshiba as an extension of MultiMediaCards (MMC), and since it has become the industry standard. The companies also formed the a non-profit organization SD Association (SDA) in 2000 to promote and create SD Card standards that enforce compliance with its specifications to assure users of compatibility.

There are three dimensions card variants SD Card, miniSD card and microSD card.

These are their Dimensions:

  • Standard: 32.0×24.0×2.1 mm (1.260×0.945×0.083 in), 1,612.8 mm3 (0.09842 in3)
  • Mini: 21.5×20.0×1.4 mm (0.846×0.787×0.055 in), 602 mm3 (0.0367 in3)
  • Micro: 15.0×11.0×1.0 mm (0.591×0.433×0.039 in), 165 mm3 (0.0101 in3)


SD and microSD cards are more widely used.

What are the characters on the cards?

We will use image below as an example of the characters you can find on SD cards and then we will explain what do they stand for.


Brand and model – Most of the time you will see manufacturer Logo and model of the card, although not always.

Memory Capacity – Basically the storage capacity of the card.

Max Read Speed – Sometimes Max Read speed (MB per second) is included on the card. On some cards there is x-rating value displayed instead, this is based on the original data transfer speed of CD-ROMs (150 KB/s).  This shows how fast the data read is from the memory card under ideal circumstances.

Card Type and Capacity Limit – Also known as families capacity limits – see comparison table below.

Logo SD Cards SD Logo SDHC Logo SDXC Logo SDUC Logo
Logo miniSD Cards miniSD Logo miniSDHC Logo
Logo microSD Cards microSD Logo microSDHC Logo microSDXC Logo microSDUC Logo
Capacity Min 128 MB more than 2GB more than 32GB more than 2 TB
Max up to 2 GB up to 32 GB up to 2 TB up to 128 TB
Typical FS FAT16 FAT32 FAT32 / exFAT exFAT

Speed Class, UHS Class Speed and Video Speed Class

Comparison of Speed Class ratings
Min writing speed Speed Class Video format
Speed Class UHS Speed Class Video Speed Class SD HD / Full HD 4K 8K
2 MB/s Class 2 N/A N/A Yes No No No
4 MB/s Class 4 N/A N/A Yes
6 MB/s Class 6 N/A V6 Yes
10 MB/s Class 10  U1 V10
30 MB/s N/A  U3 Yes
60 MB/s N/A N/A V60
90 MB/s N/A N/A V90


Application Performance Class – This is a newly defined standard from the SDA which not only define sequential Reading Speeds but also mandates a minimum IOPS (Input/output operations per second) for reading and writing. A2 class cards require host driver support as they use command queuing and write caching to achieve their higher speeds. If used in an unsupported host, they might even be slower than the A1 cards.

Comparison of Application Performance Class ratings
Name Min random IOPS Min sustained sequential writing
Read Write
1500 IOPS 500 IOPS 10 MB/s
4000 IOPS 2000 IOPS


Bus Interface – Initial SD bus speed of 12.5MB/s is the Default Mode, then a 25MB/s High Speed Mode was introduced to support digital cameras. As higher performance levels were needed to support new and faster devices, the SD Association introduced faster speed bus interfaces: UHS-I, UHS-II, UHS-III and SD Express. UHS-I provides faster bus speed using just one row of pins. And UHS-II, UHS-III and SD Express have ability to provide even faster speeds than UHS-I by using two lanes for data transfer via two rows of pins. SD Express offers the fastest data transfer rates up to 985MB/s using PCIe Gen.3 interface and NVMe application protocol.

Bus Interface Card Type Bus Mark Bus Speed
Default Speed SD, SDHC, SDXC and SDUC 12.5MB/s
High Speed SD, SDHC, SDXC and SDUC 25MB/s
(SDR50, DDR50)
104MB/s (SDR104)
UHS- II SDHC, SDXC and SDUC 156MB/s Full Duplex
312MB/s Half Duplex
UHS- III SDHC, SDXC and SDUC 312MB/s Full Duplex
624MB/s Full Duplex
SD Express SDHC, SDXC and SDUC 985MB/s
PCIe Gen.3