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.
| SD | SDHC | SDXC | SDUC | ||
|---|---|---|---|---|---|
| Logo SD Cards |  |
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|
| Logo miniSD Cards |  |
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| Logo microSD Cards |  |
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|
| 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
| Min writing speed | Speed Class | Video format | |||||
|---|---|---|---|---|---|---|---|
| Speed Class | UHS Speed Class | Video Speed Class | SD | HD / Full HD | 4K | 8K | |
| 2 MB/s |  |
N/A | N/A | Yes | No | No | No |
| 4 MB/s |  |
N/A | N/A | Yes | |||
| 6 MB/s |  |
N/A |  |
Yes | |||
| 10 MB/s |  |
 |
 |
||||
| 30 MB/s | N/A |  |
 |
Yes | |||
| 60 MB/s | N/A | N/A |  |
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| 90 MB/s | N/A | N/A |  |
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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.
| 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 |
| UHS- I | SDHC, SDXC and SDUC |  |
50MB/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 |
