Types of SD Cards Explained

There are several different types of SD card, as well as multiple speed rating systems and cryptic codes. Here’s a rundown of what the differences are.

SD Memory Cards
SD Memory Cards
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There are several types of SD cards available. And it’s not always clear what the differences are. The cards come in different physical sizes, different storage capacities, different filesystems, different interfaces, and different speeds. And they have a bunch of cryptic codes and ratings on them.

So here’s a rundown of how to make sense of all that to make sure you get the right SD card for your camera or device.

First, the basics:

  • SD cards are a category of memory card for storing data.
  • They use flash memory for their data storage (as compared with the much older magnetic memory). Flash memory is fast, stable, not susceptible to normal-level magnetic fields, has no moving parts, and can be miniaturized. Flash memory is what’s used in your mobile phone and the SSD drives of newer computers.
  • The SD stands for Secure Digital. The group in charge of setting the standards and promoting the technology is the SD Association. And to be able to use the various labels and ratings—and to be able to rightfully call it an SD card, memory card manufacturers are required to adhere to the SD Association’s published standards.
  • There are actually two different physical sizes (or form factors) of SD cards: full-size SD cards and microSD cards. The ones I’m focusing on here are full-size SD cards (I’ll cover microSD cards separately). A full-size SD card won’t fit in a slot for a microSD card, but you can get cartridge adapters that let you use microSD cards in a full-size SD memory card slot.

SD Card Speed

When talking about the “speed” of SD cards, while they all refer to data transfer speed, there are actually several types of speed to consider.

Some types of speed are more important to particular uses, while others matter more in different types of devices. They all refer to the speed of data transfer, but there’s more than one way for data to be transferred.

For starters, there’s the direction of data transfer. There are speeds for getting data off the SD card, which is referred to as read speed, as well as speeds for getting data onto the card, which is known as write speed. These are often different, and often very different. Most of the time, an SD card has a faster read speed than write speed.

But it breaks down even more. Often the most important of those two speeds is the write speed. This is what determines whether a given SD card can keep up with the requirements of the device it’s being used in. As a good example, if a digital camera is recording high-resolution 4K or 8K video, it’s throwing a lot of data at the SD card to save. If the card can’t keep up, it can lead to errors such as the camera freezing up, the recording stopping unexpectedly, dropped frames, or the camera overheating. All of which is bad. Which is why it’s important to have card with a fast write speed in many of today’s cameras.

But even then, it’s a particular type of write speed that matters for this particular purpose. It’s known as sequential write speed, and it basically means that the card can save a long, constant stream of sequential data (ie. a video footage) without issues.

But for a gaming device, where lots of small bits of data are saved rapidly, what matters more is known as random write speed. And there are cards that are particularly well suited to that kind of use but less well-suited to use in cameras. (More on that below.)

What are the Different Types of SD Memory Cards?

While they are all commonly known as SD cards, there are currently four sub-types of secure digital cards: SD, SDHC, SDXC, and SDUC.

These codes aren’t a performance rating, as such. They’re sub-types defined by the SD Association, and they specify the type of filesystem in combination with a storage capacity range.

The most common that you’re likely to come across at stores are SDHC and SDXC cards. SD cards are older and much less common now. SDUC is a new standard that hasn’t really hit the consumer market in any meaningful way yet. They’re all the same physical dimensions (microSD cards are different; and microSD cards are sometimes mistakenly referred to as macro SD cards).

The SD Association has specified that cards that have a storage capacity of 32GB or smaller will be formatted as FAT32 and they will fall within the SDHC sub-type. Cards that are 64GB and up to 2TB larger will be formatted with exFAT. So, as a practical example, if you see a 32GB SD card in the store, it will always be an SDHC card. And if you see a 512GB card, it will be SDXC.1

SD Cards

SD is often used as the generic type for all these cards, but if you want to be pedantic about it, it actually refers to a specific kind that was the original kind that was released. It applies to cards up to 2GB in storage capacity. They’re formatted with FAT12 and FAT16.

SDHC Cards

(Secure Digital High Capacity) is a design specification that refers to SD cards that are over 2GB and up to 32GB in capacity and formatted with the FAT32 filesystem.

FAT32 supports individual files up to a maximum of 4GB (which is also why many cameras break up their video files into chunks that are 4GB or less in a process known as chaptering).

SDXC Cards

(Secure Digital eXtended Capacity) refers to SD cards with a capacity larger than 32GB and up to 2TB.

They’re formatted in the exFAT filesystem, which allows truly massive individual filesizes.2

Standard consumer SD cards haven’t hit the upper storage of 2TB limit; the largest on the market currently are 1TB cards.

SDUC Cards

The SD Association has also created a newer specification known as SDUC, for Secure Digital Ultra Capacity. It has its own host technology, protocols, and drivers, but in terms of card storage capacity, it’s designed to cover cards ranging from 2TB up to 128TB. SDUC cards will only work with devices that have SDUC compatibility specifically included, so you can’t just put an SDUC card in a camera that doesn’t specifically support SDUC (which, for now, is none of them).

They’re formatted with the exFAT file system.

But you won’t find any of those cards in the wild just yet, so, for now, at least, you don’t really need to be concerned about accidentally getting one of those.

Compatibility

If you’re buying an SD card, it’s a good idea to check the requirements of the camera or device you’re putting it in. Some older cameras will only support SDHC cards and can’t work with SDXC card. And some much older devices will only work with SD. But if a device supports SDXC, you can use SDXC, SDHC, and SD in it (but not SDUC)—the system is designed to be backward-compatible like that.

SD Card Speed Classes in Digital Cameras

And on the topic of compatibility, it’s also important to get an SD card that’s fast enough for your device. Using a fast SD card won’t help you take better photos or videos or improve the image quality, but it can enable you to use all of the camera’s features without running into problems.

In addition to storage capacity and filesystem, SD cards are also defined by their speed. With many devices, being able to get a lot of data on or off the card quickly is a crucial function that the memory card has to be able to keep up with.

So the SD Association has created several types of speed classes to help consumers know whether a particular card should be fast enough for a particular device. These refer to a minimum write speed that the cards can meet. They’re often faster than that minimum threshold; sometimes much faster.

There are two different categories of speed classes that apply to SD cards, and they apply to different types of writing speed. For devices like cameras and music players, the relevant speed class is video speed class—the current iteration of this class is written with a V followed by a number (eg. V30 or V90).

For computing devices such as gaming devices, more important is the A speed, which is written in the form of A1 or A2 (more on that below).

Notes on SD Card Speed Ratings

There are some things worth noting when it comes to SD card speed ratings. The fastest SD cards on the market currently are UHS-II cards that carry a V90 video speed class rating.

Speed Ratings: Megabytes vs. Megabits

The traditional way to refer to the speed that data can be written to and downloaded from memory cards is with MB/s or MBps. It stands for megabytes per second, and the “B” is always in uppercase. When you’re looking at SD card speed ratings, you’re nearly always working in megabytes per second (MBps or MB/s).

Confusingly, there’s a similar metric: Mb/s or Mbps. It stands for megabits per second, and the “b” is always lower case. Mbps is typically used when talking about video recording bitrates.

They’re not the same thing. There are 8 bits in a byte, so to get from megabits per second to megabytes per second, you multiply by 8. So 80MB/s is the same as 640Mb/s. Here’s a handy conversion calculator.

X Rating vs MB/s. Some manufacturers use a more cryptic x rating in place of MB/s. Lexar, in particular, has long used this system. It comes from the old way of measuring the speed of CD-ROM drives when the standard speed of a CD-ROM drive was 150KB/s. Each x, therefore, equals 150KB/s.

Here’s a quick cheat sheet for converting some of the common x ratings to MB/s:

x ratingMB/s
100x15
120x18
133x20
150x23
200x30
400x60
600x90
633x95
1000x150
1400x210
2000x300
2500x375
2933x440
3000x450
3400x510
3500x525
3600x540
4000x600
4500x675
5000x750

SD Card Video Speed Class

SD cards are given a speed class rating that refers to its category for writing data, with each category describing a real-world video recording use. These apply the same way to SDXC and SDHC cards.

The most recent types of speed class start with a U or V, as in U3 or V90. Some cards might carry both a U and V rating (and a Class 10 as well, for that matter), in which case you can focus on the higher, which would be the V rating.

Where things get a little complicated, though, is that these speed ratings don’t necessarily reflect the absolute speed of the card. It’s more a “fit-for-use” declaration than a measurement.

Put another way, a card that has a V30 rating isn’t necessarily faster than one that has a U3 rating. That’s because to display the rating on the card, the manufacturers have to have their cards certified for that rating. It also involves revising packaging and marketing materials, which is an expensive process. So not every manufacturer will go to that trouble and expense right away. The upshot is that it’s not as simple as just looking at a card with a V30 or even V60 rating and knowing that it’s faster than one that only carries a U3 rating. It might be, but that’s not specifically what the rating system is indicating.

V90. The V-class (for video) is a new designation created for cards that are designed to work with the speeds required for 4K and 8K video.

Memory cards in the V90 class are rated to support a minimum sequential write speed of 90MB/sec. Their primary market is for cameras that shoot 8K video, and for now, those are pretty rare, but the class provides plenty of headroom as those cameras and high-bitrate video recording become more common.

V60 is applied to cards that support a minimum sequential write speed of 60MB/sec. They’re aimed primarily at cameras that record 4K and 8K video. Some of the newer cameras that record with ultra-high bitrates, like the Panasonic GH5, require V60 or above. With other 4K cameras that record at lower bitrates, you might get away with a V30 card.

V30 is applied to cards that support a minimum sequential write speed of 30MB/sec. These are designed to support at least full HD video and some 4K video cameras.

U3 is designed to support 4K video recording at a sustained video capture rate of 30MB/s. This class overlaps with the newer V30 class.

U1 is designed to support real-time broadcasts and HD video (720p and 1080p) with a minimum serial write speed of 10 MB/s. This overlaps with the newer V10 class.

Class 10 is designed to support 1080p recording at a minimum (but again, not at all framerates) with a minimum serial write speed of 10 MB/s.

Classes 2, 4, and 6. Class 2 supports SD video recording with a minimum serial write speed of 2MB/s. Classes 4 and 6 are designed to support from 720p and 1080p video (but not all framerates) with a minimum serial write speed of 4 MB/s and 6 MB/s, respectively. Most newer cameras need cards faster than these, so memory cards in these speed classes aren’t as commonly available now.

For practical purposes, the most common currently are Class 10, U1, U3, V30, V60, and V90, with V60 and V90 being the current fastest classes. It is technically still possible to find older, slower cards that are Class 2, 4, or 6, but most modern cameras are likely to work better with at least Class 10 cards, especially for recording video.

What is the A Rating (e.g., A1 and A2)?

A1 and A2 ratings have started appearing on more SD cards. These are part of a new type of speed specification that’s relevant for running apps from the memory card. The A rating is a separate specification, not one in the same sequence. So it’s possible for a card to have both A1 and V30 ratings, for example.

Up to this point, the primary market for memory cards has been for storing media like videos or photos. But increasingly, devices can run apps directly from a memory card. And that requires a different type of interaction with the space on the memory card—specifically, random read and write speeds and small chunks of data are placed wherever on the card there’s space for them.

So the new A specification (it stands for for App Performance) incorporates the random read/write speed. If you’re buying a memory card for a camera, it’s not especially relevant because what’s requires on those is sequential speed. So if you’re recording video (or shooting photos), the speed classes you want to look for are the ones starting with V (e.g., V30 or V60) or U (U1 or U3). But if you’re buying a memory card for a gaming device like a Nintendo Switch or a smartphone or some other kind of device that runs apps and uses SD or microSD memory cards, the A rating is designed as a guide for what cards are best suited to that kind of use.

I have more on these ratings here.

UHS-I vs. UHS-II

Most of the SD cards now on the market have a feature called ultra-high-speed bus, which refers to the interface between the card and the device. It’s not a rating system–it’s a type of interface technology.

So far, there are UHS-I and UHS-II SD cards available. UHS-I supports a maximum bus speed of 104Mb/s, while UHS-II supports up to 312MB/s. There is also a spec for UHS-III, maxing out at 624MB/s, but none of those cards have become available in the consumer market so far (and would need a UHS-III device to take advantage of it).

The product labeling for cards with this technology will have either UHS-I or UHS-II, or sometimes just I or II. Technically, it should be Roman numerals, but you’ll sometimes see it listed with a number 1, like UHS-1 (even some manufacturers make this mistake).

You can also easily tell them apart by looking at the cards themselves. UHS-I cards have a single row of pins on the back. UHS-II cards have two rows of pins. In this example, both of these are SanDisk Extreme Pro cards, just different generations.

UHS-I SD Card vs UHS-II SD Card

As with many aspects of memory cards, it’s not cut and dried. UHS-II (and UHS-III) allows for higher potential speeds, but in practice, there’s no guarantee that a UHS-II SD card will be faster than a UHS-I SD card, because there are other factors that come into play.

One is the design of the card. Another is that it matters what you’re putting the card in. It’s important to note that taking advantage of the potential speed increases of UHS-II requires that both the card and the host (e.g., the device, such as a card reader or camera or phone) support it. Putting a UHS-II card in a device that only supports UHS-I will work, but it will automatically fall back to UHS-I speeds. Ditto on putting a UHS-I card in a UHS-II-enabled device.

SD vs microSD vs CompactFlash vs CFAST 2.0 vs XQD 2.0

Some cameras come with multiple memory card slots, so you can choose which to use (or use both).

The speed of SD cards has come along in leaps and bounds in the past few years. At the moment, the fastest SD cards currently available on the consumer market are faster than the fastest microSD cards, faster than the fastest CompactFlash cards3, and slower than CFexpress/XQD and CFAST cards. So if your camera or device has multiple slots, you will typically get faster performance out of a CFexpress/CFAST slot than the SD slot (but those cards are also usually significantly more expensive).

Mini SD/miniSD vs Micro SD/microSD

You might come across mentions of mini SD cards. Most of the time, they’re actually referring to microSD—people sometimes use “mini SD” loosely to refer to the cards that are smaller than regular SD cards.

As a technical matter, there is such a thing as a miniSD card spec—it’s one of the three defined form factors that the SD Association has specced. But in practice, miniSD has been overtaken by the microSD form factor, and in nearly all cases—except the most highly specialized uses—what people mean when they’re looking for a mini SD card is actually a microSD card. You generally won’t find miniSD cards for sale in shops or online, and you’ll be hard-pressed to come across any devices that use them specifically.

MicroSDHC and microSDXC refer to cards of the same physical size. That distinction refers to the filesystem on board and is also related to the storage capacity of the card. There’s more information on that above.

Related Technologies

SD cards don’t exist in a memory card technology vacuum, of course. It’s a highly competitive space, with technological improvements coming rapidly and much jostling for dominance. Here are some of the upcoming and competing technologies related to SD cards.

SD Express

The SD Association is regularly tweaking specifications to allow for faster and better performance from SD and microSD cards. It makes sense for its own sake, of course, but also in the context of the growing rise of the CompactFlash Association’s CFexpress formats, some of which (specifically, CFexpress Type-A) are explicitly taking aim at replacing SD.

In May 2020, the SD Association released an SD Express 8.0 specification with a dramatic increase in potential transfer speeds, allowing for transfer speeds of up to nearly 4000MB/second. It’s designed to be available on SHDC, SDXC, and SDUC cards. But for now, it’s just a spec and hasn’t yet been implemented in any consumer cards or devices.

SD vs CFexpress Type-A

There are new types of memory cards developed by the CompactFlash Association known as CFexpress. There are currently three types of CFexpress cards: CFexpress Type A, CFexpress Type B, and CFexpress Type C. Each type is a different physical size and has its own characteristics.

CFexpress A cards are essentially the same physical dimensions and pin layout as SD cards. So a CFexpress A card will fit in an SD slot, and vice versa. CFexpress A cards are also much faster and more expensive than SD cards.

But fitting in the slot is not the same thing as being compatible. There are some devices, such as the the Sony a1 mirrorless camera and a handful of memory card readers, that have slots that read both CFexpress A and SD cards using the same slot.

But in most devices, just because the cards will physically fit doesn’t mean it’s compatible. The device has to be specifically compatible with CFexpress A for those cards to work.

Best Types of SD Cards For . . .

Best SD Card Types for Cameras

Not all cameras can work with all SD cards. Some quite old digital cameras, for exampe, can’t work with SDXC cards. So always make sure in the camera’s specs or manual that SDXC is mentioned. And some cameras, of course, use different types of memory cards other than SD.

As a general rule of thumb, cameras work best with cards that have a fast-enough sequential write speed. That’s most true when shooting video, but it can also be an issue when shooting continuous burst mode photos or even time lapse with short intervals. So look for the rating system that’s designed specifically for that kind of speed. Most cards on the market these days come with these rating systems on them. In increasing speed, it should have: U1, U3, V30, V60, or V90.

Most current model cameras have a UHS-I SD card slot, but more high-end cameras are now starting to use UHS-II.

Best Types of SD Card for Gaming Devices

Gaming devices write to their memory cards in a different way to cameras. Something like a Nintendo Switch, for example, uses a lot of small chunks of data that need to be written and read quickly. So rather than one long stream, it’s tiny bites.

The rating system for this kind of use starts with an A (eg. A1 or A2). Only newer SD cards will have this on them. There are also cards without this rating that will probably work well, but the presence of this rating is specifically designed to offer a guarantee that it’s fit for this type of use.

In some cases, there are specific cards for this. SanDisk, for example, markets an SD card specifically for the Nintendo Switch (and yet it doesn’t carry the A rating).

Best Types of SD Card for Action Cameras

Most action cameras are tiny, and to save space they usually use microSD cards. And most are geared toward recording high-bitrate and high-resolution video, such as 4K or 5K. So you’ll want a card that has fast sequential write speed, which will often mean looking for a card that carries a V30, V60, or V90 rating.

Best Types of SD Cards for Storing & Archiving Data

SD cards are not designed for long-term storage of data, so I don’t recommend using one as a primary copy. But if you have a backup and are using one, say, for a portable music collection of MP3s or for video files to watch on a flight or photos to give to someone else, the speed of the card is pretty much irrelevant. With a slow card, it might take a bit longer to copy the files to the card and download them again, but for storage uses that’s not usually too much of an inconvenience.

Best SD Card Types for Dashcams & Surveillance Cameras

Dashcams and security cameras usually record on loop. That is, they’re continuously writing over the previous data, and they do it constantly.

SD cards will eventually fail after large numbers of data operations, but there are types of SD cards that are designed for this type of constant use that offer extra protection against failure. Look for the word “endurance” in them somewhere, like this one from Samsung or this one from SanDisk.

General Recommendations for Using SD Cards

Here are some general tips for buying and using SD cards.

  • There are plenty of counterfeit memory cards in the marketplace, especially when you’re buying online. Buying from a reputable retailer helps minimize the risk of getting a fake.
  • SD cards are surprisingly strong and rugged and are advertised as waterproof, crushproof, freezeproof, and dustproof. There are even some that go to extra lengths to be ruggedized against physical damage. But having said that, they’re not completely indestructible. And being so small, they’re all-too-easy to misplace when they’re not inside a device. They’re also not designed for long-term archival storage of photos and videos. It’s good practice to download the data as soon as practical and get it backed up securely.
  • SD cards are complicated electronic products. And while modern manufacturing–at least, from the larger, more reputable memory card manufacturers–is remarkably reliable, there’s still a small percentage of electronic products end up being faulty from the manufacturing process. So it’s good practice to test your card before using it in a mission-critical application. Better yet, have a spare (or, better yet, spares) on hand as a backup.
  • Always format the card in the camera rather than with your computer, and do it fairly regularly. This keeps the memory card’s filesystem set up the way the camera wants it and reduces the risk of formatting problems.
  • If it’s not possible to format the card in the camera or device–of if that’s not what you want to do, you can also format cards using a computer. But there are some things to know when formatting SD cards to minimize the risks of your camera having problems with them. So I’ve put together guides on how to format SD cards on Mac and how to use the free SD Card Formatter app for Windows or Mac. For even greater flexibility, you can also use the built-in tools of your computer’s operating system. The risk with diverging from the standard, though, is that you might run into compatibility issues when you insert the card into the device you plan on using it in.
  • During sustained, data transfer, such as when recording high-bitrate recording, memory cards can get very hot. That’s usually normal, but it does mean you should add any labels, adhesive, or glue that could melt or burn or cause the card to get stuck in the camera.
  • You can use microSD cards in an SD card slot by using a microSD-to-SD adapter. Most of them are UHS-I, but you can occasionally find one that’s UHS-II.
NOTES:

  1. Technically, it’s possible to use a computer to format, say, a 32GB microSD card with exFAT or a 128GB card with FAT32. But doing so goes against the SD Association specifications, can cause problems in some cameras, and they’ll be overwritten to the appropriate standard next time you format the card in the camera. In general, I don’t recommend it. That said, if you really want to do it, I’ve put together a guide to using the official SD card formatter
  2. The theoretical maximum file size on exFAT is 16 exbibytes, but in practice it’s limited by the smaller 128 pebibytes partition limit. Either way, these are many orders of magnitude larger than any consumer device is going to produce. 
  3. This is true of the cards themselves, but some devices can read much more quickly from CompactFlash slots than SD slots. 

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