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What do all those microSD and SD card numbers and letters mean?

Deciphering microSD card and SD card storage jargon.
adrian-kingsley-hughes

There are two commonly used storage media that people with Android smartphone or digital cameras might be aware of -- these are the SD card and microSD card.

At first blush it might seem that all SD cards and microSD cards are the same, but a closer will show that they are not.

If you look at the front of any SD or microSD card, you'll see a whole bunch of symbols and specs, and being able to decipher these will tell you a lot about the card's performance.

Here are two different microSD cards:

There are two commonly used storage media that people with Android smartphone or digital cameras might be aware of -- these are the SD card and microSD card.  At first blush it might seem that all SD cards and microSD cards are the same, but a closer will show that they are not all the same. If you look at the front of any SD or microSD card, you'll see a whole bunch of symbols and specs, and being able to decipher these will tell you a lot about the card's performance. Here are two different microSD cards: Xxx

Two different microSD cards

OK, so there's a lot of numbers and specifications on there so let me annotate one card to show you what it all means:

Annotated microSD card

Annotated microSD card

As you can see, there's a lot going on there. Fortunately, memory vendor Kingston comes to our rescue with a breakdown of what most of this means.

Speed classes for SD and microSD cards

Speed classes for SD and microSD cards

Kingston

As you can see, there are a lot of speed ratings, and these refer to the minimum speeds that you should expect from the card when in a device, whether that be a smartphone, camera, or video camera.

What's missing from this list is the Application performance class rating. There are two -- A1 and A2 -- and these give a rating for the minimum IOPS (input-output) access per second for the card. These come into play when using a microSD cards in smartphones or tablets to store data, especially applications.

  • A1: 1500 IOPS minimum random reads, 500 IOPS minimum random writes, 10 MBytes/s minimum sustained sequential writes
  • A2: 4000 IOPS minimum random reads, 2000 IOPS minimum random writes, 10 MBytes/s minimum sustained sequential writes

There's also a difference in the pin contact layout on the backs of the cards between XC I and XC II cards (XC II have more contacts):

XC I (top) and XC II (bottom) microSD cards

XC I (top) and XC II (bottom) microSD cards

Similar ratings also appear on SD cards. 

A selection of SD cards

A selection of SD cards

And again, the contact layout differ between XC I and XC II cards, with XC II having more contacts.

XC II cards (left and center) and XC I (right)

XC II cards (left and center) and XC I (right)

OK, so after all this, the question is what cards should you buy?

First off, always buy a name-brand card from a reputable seller. Not only are there a lot of cheapo no-name cards out there, but there are also a lot of counterfeit knockoffs. 

If you need a microSD card for a smartphone, then at the minimum it needs to conform to the A2 rating. However, I would buy a card that conforms to both XC I and the U3 UHS speed class. 

For drones and action cameras, an XC I microSD card that conforms to the U3 UHS speed class should offer adequate performance. Some devices may support high-specced cards, but make sure that you're getting a speed improvement if you are going to spend the extra money.

What about SD cards for cameras? This is where things get complicated. If you are using your camera to take photos or shooting HD video, then an XC I card with a speed rating of U3 and V30 will be fine for you. 

However, a lot of modern cameras now support the higher-speeds offered by XC II cards, and so spending the extra money might be worthwhile, especially if you are planning on shooting 4K video.

Again, check with the manufacturer to see if higher speeds are supported, otherwise all you are doing is throwing money away.

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