SD vs. SDHC vs. SDXC vs. SDUC Memory Cards

These days, SD is shorthand for the entire category and form factor. But there are actual several categories of SD cards that are important when it comes to choosing the right type for your camera. Here’s an explanation of what they are and what to look for.

Olympus OM-1 SD Cards. Photo by David Coleman "
Text & Photos By David Coleman
Last Revised & Updated:
Filed Under: Memory Cards
Topics: SD Cards

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Buying SD cards can be confusing. You first need to make sense of your camera’s requirements, and camera manufacturers don’t always make that as straightforward as it could be.

Then you’ve got to wade through a bunch of codes and acronyms on the SD cards themselves. Some are relevant to cameras, and some aren’t.

One of the several codes you’ll see on SD cards is either SDHC or SDXC. In rarer cases, you might also see SDUC.

None of these is directly related to speed or performance (there are separate codes for those). They more directly relate to storage sizes and filesystems. And while there is some cross-compatibility, they’re not necessarily interchangeable.

Full-Size SD vs microSD

The SD Association designs and controls the specs for both SD and microSD form factors. 

First came what are now known as full-size SD cards. They’re the ones that measure around 1 1/4 inches high and almost an inch wide (32mm x 24mm). They’re the ones most often used in the current generation of cameras.  

Later came the much smaller microSD format. That’s often used in action cameras and smartphones. 

And when it comes to technologies like SDXC and SDHC, they apply to both full-size SD cards and microSD cards.

SD (Secure Digital)

Initially, SD was a very specific spec that was initially launched in 1999 through a collaboration of Panasonic, SanDisk, and Toshiba aiming to build on and improve the MultiMediaCards (MMC) standard with extra features and more industry support.

The MMC format did have some advantages: it was thinner (which also made it more fragile) and cheaper to make. But SD had advantages that won out:

  • it’s physically stronger, making it more rugged
  • its asymmetrical design means it can’t be inserted the wrong way
  • and it has better support for digital rights management (aka copy protection).

Over time, as the SD format gained popularity, they also became faster and with larger storage capacities.

SD spec cards are formatted with FAT12 or FAT16.

SDHC (Secure Digital High Capacity) 

SDHC memory cards were the game-changers that first offered more than the traditional 2GB storage limit. Introduced in 2006, they were designed to meet the demand for higher storage capacity, providing between 4GB to 32GB of available space.

SDHC cards use the FAT32 file system. On the plus side, FAT32 is very widely compatible. But a significant drawback of FAT32 is that the maximum size of any individual file is 4GB. That’s fine for photos and low resolution video, but it doesn’t go far when shooting high-resolution 4K or 8K video, which can eat up 4GB in a matter of a few minutes. And that’s why cameras will break up long videos into shorter chunks of under 4GB–a process known as chaptering.

They were initially rated with the Speed Class rating system, which includes categories Class 2, 4, 6, to 10. Newer SDHC cards may also have the UHS Speed Class rating of U1 or U3. And while it’s quite possible for SDHC cards to also carry the newest Video Speed Class rating system (eg. V30), in practice, many memory card manufacturers have chosen to make 64GB (and therefore SDXC) cards the smallest they offer in their newer, faster cards. 

SDXC (Secure Digital eXtended Capacity) 

Moving forward, SDXC memory cards cam as an upgrade in 2009.

The major improvement of SDXC over SDHC comes in terms of storage capacity. SDXC can support a maximum storage capacity from 32GB all the way up to 2TB.

A key innovation with SDXC cards is that, instead of using the FAT32 filesystem, they use exFAT. That still means good compatibility across devices, but one of the key features that’s relevant for cameras is that exFAT supports much larger individual files, to the extent that it’s no longer a practical consideration.

SDUC (Secure Digital Ultra-Capacity) 

Both SDHC and SDXC SD cards have been in very wide use for many years now.

The next emerging upgrade is known as SDUC (Secure Digital Ultra-Capacity). They look much like existing cards, but there are some key differences, and it’s not a case of just choosing one or the other interchangeably.

The SDUC memory card was announced in 2018, but it is not currently being used in any consumer cameras on the market.

Crucially, SDUC supports a massive upgrade in storage capacities, starting a 2TB and going all the way up to 128TB.

They’re again using the exFAT filesystem, so there’s no practical limit on the size of individual files.

Worth noting: It’s worth noting that the storage limits of specs are, at first, theoretical, and it doesn’t necessarily mean that there are–or will ever be–cards that max out the spec. As an example, at the time of writing, there are still no 2TB SDXC cards available on the consumer market. 

Comparison Table

Spec IntroducedStorage Capacity RangeFilesystem
SD (Secure Digital) 1999128MB to 2GBFAT12/FAT16
SDHC (Secure Digital High Capacity)20062GB to 32GBFAT32
SDXC (Secure Digital eXtended Capacity)200932GB to 2TBexFAT
SDUC (Secure Digital Ultra Capacity)20182TB to 128TBexFAT

What to Look For

The development of the SD card format has generally aimed at a principle of backward compatibility. But it’s by no means a perfect system, and there are some key compatibility issues worth considering:

  • SDUC cards require host devices that specifically support SDUC. You can’t put a SDUC card in a current-generation camera and expect it to work, for example.
  • SDXC cards require host devices that specifically support SDXC. There are some older cameras you can come across that will only support SDHC; you can’t use an SDXC card in those.
  • If a camera supports SDXC, you can also use SDHC cards.

More to Explore on SD Cards

The issue of SD vs SDHC vs SDXC vs SDUC is only one aspect of choosing between SD memory cards.

Another crucial factor is the card’s speed. Or, more specifically, it’s write speed. The SD Association has come up with some categories designed to make that choice easier. The current generation of those ratings is known as Video Speed Classes.

As if that wasn’t complicated enough already, the SD Association has also launched a new up-and-coming format known as SD Express. Again, they’re not really interchangeable, but they offer huge leaps ahead not just in storage space (because they use SDUC), but also in speed. I have more on the emerging memory card format of SD Express separately.

And the speeds of SD cards vary widely. For a number of years, I’ve been running my own independent speed tests on SD and microSD cards and posting the results on this site.

Profile photo of David Coleman | Have Camera Will Travel | Washington DC-based Professional Photographer

Text & Photos by David Coleman

I'm a professional photographer based in Washington DC. Seven continents, up mountains, underwater, and a bunch of places in between. I've been shooting for 30+ years, and my my photos and time-lapse videos have appeared in a bunch of different publications from major newspapers to magazines and books, billboards, TV shows, professional sports stadiums, museums, and even massive architectural scrims covering world-famous buildings while they're being renovated. You can see some of my travel photography here and here.

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