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Whether for a phone, a camera, or some other gadget, shopping for microSD cards seems like a pretty simple thing to do, right?

Yet there’s a lot more to them than you might realise, and it’s surprisingly easy to wind up falling into a number of traps: overpaying, terrible performance, or the card not working at all.


Part of the reason is the number of specs related to memory cards 10 Things To Know About Digital Camera Memory Cards 10 Things To Know About Digital Camera Memory Cards Read More . So let’s take a look at how to read these specs, how to determine which one is right for you, and which mistakes to avoid.

1) Buying Incompatible Cards

When you talk about microSD cards, you are mostly talking about the form factor. All microSD cards fit into all microSD card slots, but they won’t all work. There are three different card formats, as well as different standards, that determine compatibility.

The three formats, which you’re probably already familiar with are SD, SDHC, and SDXC (or microSD, microSDHC, and microSDXC, but both micro and full-size cards are based on the same spec). Each format is defined in the SD specification, but they don’t work in the same ways.


As a result, the formats are not backwards compatible, and you cannot use newer cards in hardware that only supports older formats.

sd formats

The differences between the three formats are significant:

  • microSD: Has a capacity up to 2 GB, and can be used in any microSD slot.
  • microSDHC: Has a capacity of more than 2 GB and up to 32 GB, and can be used in hardware that supports either SDHC and SDXC.
  • microSDXC: Has a capacity of more than 32GB and up to 2 TB (although at the time of writing this, 512 GB is the largest available card), and can only be used in devices that support SDXC.

In addition to checking that a card’s format is compatible with your hardware, you need to check a few other details, too.

Physical Size

First, hardware that supports microSDXC slots won’t automatically support every size of card in this format. The HTC One M9, for example, officially supports cards up to 128 GB and might not work with anything larger.

sandisk ultraplus

And if you’re planning to use your microSD card with your PC at any point — for example, to move files on and off — you also need to ensure your PC supports the file system From FAT To NTFS To ZFS: File Systems Demystified [MakeUseOf Explains] From FAT To NTFS To ZFS: File Systems Demystified [MakeUseOf Explains] Do you really know what your hard drive does whenever you read a file from it or write one to it? Our hard drives can now store massive amounts of data, and that massive space... Read More that the card is formatted with. MicroSDXC cards use the exFAT system by default. Windows has supported it for over a decade, but OS X only since version 10.6.5 (Snow Leopard).

Ultra High Speed

The SDHC and SDXC formats can support the Ultra High Speed (UHS) bus interface — faster circuitry that enables data to move at a quicker rate. The two versions of UHS are UHS-I (with bus speeds of up to 104 MBps) and UHS-II (up to 312 MBps).

In order to benefit from the increased performance of UHS, it needs to be supported by your hardware. UHS memory cards will work in older slots but with a reduced bus speed of 25 MBps.

2) Choosing the Wrong Speed

Identifying the speed of a microSD card is just as complicated as deciphering formats and compatibility. There are four ways to show how fast a card is, and it’s not uncommon for manufacturers to use all of them.

class icons

Speed Class

The Speed Class shows the minimum write speed of a memory card in megabytes per second. There are four Speed Classes defined as follows:

  • Class 2: At least 2 MBps.
  • Class 4: At least 4 MBps.
  • Class 6: At least 6 MBps.
  • Class 10: At least 10 MBps.

Showing base level performance helps you to identify whether a card is suitable for a specific task, but because it makes no comment on maximum speeds, it’s technically possible for a Class 2 card to be faster than a Class 6 card. Class 10 cards should be noticeably faster, though, as they have a bus speed of 25 MBps (compared to 12.5 MBps on Class 2 to Class 6 cards).

uhs speed

UHS Speed Class

UHS Speed Class shows the minimum write speed for microSD cards that support the UHS-I and UHS-II bus speeds. We’re listing it as a separate category because some manufacturers list both classes on their cards. The two UHS Speed classes are:

  • U1: At least 10 MBps.
  • U3: At least 30 MBps.

Rated Speed

While it’s generally safe to assume that a higher Speed Class correlates to faster all-around performance, and UHS cards faster still, some manufacturers also quote a maximum speed for their products.

These speeds are shown in megabytes per second and help you pick out the absolute fastest cards. The speeds are based on manufacturer tests, however, so they may represent a best case scenario rather than real world performance.

kingston speed

In practice, there are other external factors that will affect read and write speeds. If you’re copying files to your PC, for instance, your PC’s specs — and even the USB cable you’re using — will play a role.

Relative Speed

The other way manufacturers show the speed of their cards is a throwback to the old CD writing days No DVD Drive on Your Tablet or Notebook? Use an Old Laptop Drive Instead! No DVD Drive on Your Tablet or Notebook? Use an Old Laptop Drive Instead! More and more Windows computers are shipping without optical drives, and this is a phenomenon that exists beyond the tablet PC. The lack of a DVD drive might prevent you from installing your favourite apps... Read More .

The original transfer rate for CDs was 150 KBps. As drives developed, they would advertise themselves as being 2x, 4x, 16x, and so on, showing how many times faster than 150 KBps they were.

MicroSD cards are also often labelled in this way. When a card is described as 100x, it means 100 x 150 KBps, which is 15 MBps. That speed is, again, under ideal lab conditions.

3) Wrong Cards For the Task

When buying a microSD card, it’s important to pick one that is right for its intended use How To Choose The Right SD Card For The Job How To Choose The Right SD Card For The Job SD cards aren't all about storage! In fact, there are several other factors to consider, and when purchasing your cards, you should make yourself aware of them. That said, SD cards aren't all created equal,... Read More . This means finding a card that is large enough and fast enough — not necessarily the largest and fastest card out there. High capacity UHS-II U3 cards often still have a price premium and you won’t always notice the benefits they offer.

If you’re using a microSD card to increase the storage on your smartphone, then it makes sense to pick the highest capacity you can. At the same time, speed won’t be a major priority since you won’t be transferring large files back and forth frequently.

The exception would be if you’re using a newer phone that can shoot 4K video Live Photos & 4K: Why You Shouldn’t Buy a 16GB iPhone 6s Live Photos & 4K: Why You Shouldn’t Buy a 16GB iPhone 6s If you want to take advantage of the new iPhone 6s camera, you'll probably want to avoid picking the 16GB storage option. Read More . Just as with a 4K video camera (like the GoPro Buying a GoPro or Action Camera: 7 Useful Things You Need To Know Buying a GoPro or Action Camera: 7 Useful Things You Need To Know If you're into skiing, cycling or surfing, then an action camera is the perfect way to record your adventures. Read More ), large capacity and high speed are musts.


Panasonic recommends UHS Speed Class 3 (U3) for shooting 4K video. For full HD video, it suggests Class 10 or Class 6 at a push. If your card’s write speed is too slow, it will result in dropped frames and produce stuttering video.

For photography, some users prefer several smaller cards to a single large one so they minimize the risk of losing all of their photos if a card corrupts. If you’re shooting RAW, where files might be 20 MB or more Budding Photographer? Here's Why You Should Be Shooting Raw Budding Photographer? Here's Why You Should Be Shooting Raw Every dSLR, prosumer and even some high-end compact cameras have the ability to save raw image files. This isn't just a higher quality of image, it's a gift from the photography gods. Read More , you’ll benefit from having U1 or U3 speeds (but they require at least SDHC format).

lexar microsd

And in case you’re wondering, there’s no difference between a full-size SD card and a microSD card in an SD adapter. If your camera only has an SD slot, you can still use a microSD card in it.

4) Buying Fake Cards

It sounds like an obvious thing to avoid, but sadly, buying fake memory cards is incredibly easy.

If you find a good deal on branded memory cards from a non-reputable seller, then there is a real risk that it may be counterfeit. In fact, a few years ago a SanDisk engineer reportedly stated that as a many as a third of all SanDisk-branded cards were fakes. It’s unlikely that that number has declined since.

The buying guides on Ebay include a page on spotting counterfeits due to how common they are. Amazon Warehouse sellers have been accused, too. If you’re buying from a source you’re unsure about, check the reviews first.


Counterfeit cards are configured to report the capacity that is printed on the packaging, but actually contain far less. You won’t notice this until the card fills up unexpectedly quickly.

Use the utility H2testw for Windows, or F3 for Mac and Linux, to check that the cards you already own are genuine.

5) Cheaping Out on Brands

We’ve all owned flash memory cards that have stopped working for no apparent reason. While reliability is generally excellent, microSD cards do fail Hard Drives, SSDs, Flash Drives: How Long Will Your Storage Media Last? Hard Drives, SSDs, Flash Drives: How Long Will Your Storage Media Last? How long will hard drives, SSDs, flash drives continue to work, and how long will they store your data if you use them for archiving? Read More , and when they do, they’ll take all your data with them.

For this reason, buying cards from big brands is always better than buying no-name cards for cheaper. You can expect better performance, greater levels of reliability, as well as more robustness, with cards routinely protected against shock, water, and even airport X-rays.


You also get things like a lifetime warranty (where your card will be replaced if it fails) and access to image recovery software (to retrieve data from a damaged or corrupted card). Manufacturers such as Lexar and SanDisk offer this as standard.

What’s Your Perfect MicroSD Card?

There are many things to consider, but once you understand things like speed, specs, and all of the other issues, it becomes much easier to narrow down your options and make a choice.

As a general rule, it’s always a good idea to buy specific cards for specific tasks. This way you can guarantee the best combination of speed and capacity for your hardware to give you the best possible overall performance.

What do you look for when buying microSD cards? Have you ever encountered compatibility problems or fakes? Let us know in the comments.

Image Credits: Sandisk Ultraplus via, Lexar cards via

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