Understanding SD Memory Cards

microSD / microSDHC / microSDXC

microSD memory cards were born due to SD cards being too big for mobile phones. Initially developed by SanDisk, microSD was later on embraced and standardized by the SD Card Association, which announced the form factor in 2005. The initial microSD memory cards were slow and their capacities were limited to 2 GB due to the FAT16 file system, just like SD cards. However, the SD Card Association was quick to release the next generation SDHC cards that lifted those limits. And with the introduction of SDXC that took advantage of the exFAT file system, it became possible to make cards larger than 32 GB in size, with much faster read and write speeds. microSD cards quickly gained popularity among portable device manufacturers for their small size – at just 11.0 (W) x 15.0 (L) x 1.0mm (H), these cards are the smallest memory cards available on the market today and they have been embraced by many mobile phone and tablet manufacturers. Quick Summary: microSD is a popular format for small portable devices, so if you have a phone or a compact camera that utilizes microSD cards, you will need to buy microSDHC or microSDXC cards.

Memory Card Speeds

Memory cards can also vary greatly in speed, or how fast they can read and write information. Unfortunately, this is where things can get quickly confusing, because speed ratings and how they are marked vary greatly by memory card type. 

SD Memory Card Speed Classes

The SD Card Association came up with a way to define SD card speed through something called “Speed Class”, which defines minimum sequential writing speed a memory card can provide. In addition to that, there is also bus speed, which is typically defined as something like “UHS”, which shows the theoretical maximum a card can provide over the bus. There are also UHS Speed Class and Video Speed Class specifications, which define minimum sequential write speeds even further. Let’s start by looking into different bus interfaces and their limits. Below is a short table that summarizes different bus interfaces and their potential bus speeds:

Bus Inteface Compatible Memory Cards Maximum Bus Speed
High Speed SD, SDHC, SDXC 25 MB/sec

As you can see, there is a big difference between High Speed, UHS-I, UHS-II and UHS-III cards in terms of maximum bus speed. While the original cards were pretty much capped at 25 MB/sec, the UHS-I bus interface lifted that to 104 MB/sec and the newer UHS-II bus was able to triple that potential at 312 MB/sec. The newest UHS-III standard is very new, but it enables insane theoretical speeds of up to 624 MB/sec.

It is important to understand that cards with a faster bus speed also require a memory card reader / writer that can support that bus speed. For example, if you purchase a memory card with a UHS-II bus interface, the memory card slot on your camera must also be UHS-II compatible, or you will experience all kinds of performance and reliability issues. The same goes for a memory card reader on your computer – it also must be UHS-II compatible in order to support the higher speeds.


Besides the bus interface, you might also find other speed classes that are marked on SD memory cards. Let’s go over those now:

Minimum Sequential Write Speed Speed Class UHS Speed Class Video Speed Class
2 MB/sec   
Class 2 (C2)
4 MB/sec   
Class 4 (C4)
6 MB/sec   
Class 6 (C6)
Class 6 (V6)
10 MB/sec   
Class 10 (C10)
Class 1 (U1)
Class 10 (V10)
30 MB/sec     
Class 3 (U3)
Class 30 (V30)
60 MB/sec       
Class 60 (V60)
90 MB/sec       
Class 90 (V90)

This all sounds pretty confusing doesn’t it? Well, those are the markings you will commonly find on many SDHC and SDXC memory cards out there. Let’s take a look at a real memory card and see if we can make some sense from all the information provided on it:

  1. Maximum Read Speed – This is the maximum sequential read speed the memory card is capable of in Mega Bytes per second (MB/sec). Please note that write speeds are rarely ever published on memory cards and you will need to find that information in memory card manual or listed specifications. In this case, the maximum read speed of the SD card is 300 MB/sec.
  2. Type of SD Memory Card – You should also be able to locate the proprietary SD card logos on memory card labels that indicate whether the card is of SD, SDHC or SDXC type. In this particular case, it is an SDXC memory card.
  3. UHS Bus Speed – UHS bus speed is also often published directly on memory card labels. If it is a UHS-I card, you will just see roman numeral one (I), whereas if it is a UHS-II card, you will see roman numeral two (II), as in the case of the above card.
  4. SD Speed Class – This number indicates what SD Speed Class card it is, per table above. As in the above case, all modern SD cards should be rated at 10 minimum, which guarantees minimum sequential write speed of 10 MB/sec.
  5. UHS Speed Class – Aside from UHS bus speed, you will also typically find a UHS speed class label. In this particular example, I can see that the card is rated at minimum 30 MB/sec write speed, thanks to this U3 label.
  6. Memory Card Capacity – The capacity of the memory card is typically displayed in large numbers. As can be seen here, this memory card has a total capacity of 128 GB.
  7. A1: The Application Performance Class 1 (A1) was defined by SD Physical 5.1 specification. Not only for storing maps, pictures, videos, music, dictionary and documents, it also enables the user to be freed from sluggish for editing and updating data.

    A2: The Application Performance Class 2 (A2) is defined by SD Physical 6.0 specification. It makes SD memory card much higher performance than the A1 performance by using functions of Command Queuing and Cache.

Now that you can read SD memory cards.



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