SD cards

I'm often asked by family and friends what SD card will work best in their device. Physically the SD card has changed little in terms of size and shape since it was created in 1999. Based on the MMC card the slightly thicker Secure Digital card measuring 32 x 24 x 2.1mm with an optional write-protect slider has experienced many technology changes over the years (mainly on the inside) in order to meet higher capacity and speed demands. More recent cards feature a second and sometimes a third row of pins as the only external evidence of any change. Yet this little device can be so confusing to consumers with multiple acronyms and characters and numbers written on it's tiny label. For example, do you know what the following means?

SDXC UHS-II 64GB V60 C10 U3 250MB/s

Storage Capacity

The storage capacity of an SD card is limited by the technology in use. The original technology, now known as Standard Capacity or SDSC for short allowed an SD card to have up to 2GBs of memory. When more capacity was required SDHC (Higher Capacity) cards were introduced allowing up to 32GBs of storage space, followed by the SDXC (eXtended Capacity) cards allowing up to 2TBs, and then the SDUC (Ultra Capacity) cards allowing up to 128TBs of storage space.


An SD card's speed is usually reported in terms of megabits-per-second (MB/s) read speed but there are two values available: the read speed and the write speed. Most manufacturers advertise the read speed as it appears more impressive, but most consumers want to know the write speed as this will determine if a particular card will meet their needs. The faster your device needs to write (store) large amounts of data the higher you want the write speed number to be. For example if shooting 4K video or taking high-resolution digital photos and saving as raw images. Slow cards today generally have a write speed of around 60MB/s and faster cards around 700MB/s.

Achieving higher speeds means hitting hardware limits so the Ultra High Speed bus standard was created. This is written as either UHS-I, UHS-II, UHS-III or just I, II or III on the card. The UHS-I standard did not alter the look of the card but UHS-II and UHS-III added a second row of pins. UHS is only available with SDHC, SDXC and SDUC cards.

A further technology was recently introduced to offer higher speeds than that of UHS-III known as SD Express and is compatible with SDHC, SDXC and SDUC cards using the same pin layout as UHS-II/III although some newer cards have introduced a third row of pins. SD Express cards are usually written as EX I on the card.


Not all cards are created equal and while some state that they can read at X speed and write at Y independent testing has shown that this is not always the case. Therefore classes were introduced to represent the minimum speeds a card should achieve in any given situation.

There are several speed classes. The original is represented by a number within the letter C. Then there is the UHS speed class represented by a number within the letter U. Finally there is the video speed class for use with video and is represented by a number next to the letter V. The higher the number the faster the speed baseline. At the time of writing the higher classes are C10, U3 and V90.


Going back to the specification I gave earlier, we had the following:

SDXC UHS-II 64GB V60 C10 U3 250MB/s

We can now interpret this card as an SD card with eXtended Capacity supporting the Ultra High Speed bus version 2, with a 64 gigabyte capacity and meeting the class 10, UHS class 3 and video class 60 minimum speeds with a stated maximum read speed of 250MB/s being achievable, although under what conditions is unknown.


This is a very high-level write-up meant to give you a quick overview of SD card technology in order to help you decide if a particular card meets your needs. You should always check the specifications of your device provided by the manufacturer in relation to the SD card types that are supported. And be careful as there are a lot of fakes out there as well as slow cards purported to being faster than they actually are. Always buy from a trusted source so that you can return the card if it fails to meet the speeds it states it can meet.

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Last updated: 21st April 2022

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