UHS-I vs UHS-II on microSD & SD Cards

Wondering what the difference is between UHS-I and UHS-II with SD and microSD cards? Here's an explanation.

If you’ve looked at buying an SD or microSD card lately, you might have run into some new codes: specifically, UHS-II.

It refers to something called ultra high-speed bus, which in turn refers to the interface between the card and the device. So far, there is UHS-I and UHS-II. There is a specification of UHS-III, but I’ve not seen any cards actually turn up using that yet.

UHS-II is the newer, and potentially faster, system but adoption is still not widespread. And a UHS-II isn’t necessarily guaranteed to be faster than every UHS-I card in practice. I’ve put quite a through microSD and SD cards through speed tests, and the UHS-II cards aren’t necessarily faster, at least for now.


The UHS-I and UHS-II specifications are designed to be backward compatible. That means that you can use a UHS-II card in a device that’s compatible with UHS-I and vice versa. But if you do so, you’ll be limited to the lower spec, in this case UHS-I. That is the general intention, at least. But it gets a little more complicated, because there are different implementations of UHS-I that have different speeds. Some cards fall back to a slower UHS-I spec; better cards fall back gracefully to a UHS-I spec known as SDR104. Unfortunately, that particular part of the card’s capabilities is rarely specified in either the packaging or the technical specs.

But in many cases, it frankly doesn’t matter which you get. But that’s simply because the cameras and devices that can take full advantage of UHS-II are only just starting to become available.

And some that are UHS-II compatible can be a bit quirky. For instance, the FujiFilm X-Pro 2 camera has two SD slots. But only one uses UHS-II. So if you put a UHS-II SD card in the slot that has the UHS-I interface, you’ll be limited to UHS-I speeds.

But if you have a device that does use the UHS-II interface–and the technical specs or instruction manual will say so if it does–then there’s a good chance you’ll get better performance out of a UHS-II card.

One common device where this can become an issue is with memory card readers. Most of the card readers currently available support UHS-I cards. UHS-II cards will work, but they’ll be limited to UHS-I speeds. There are some card readers that do support UHS-II, such as the Lexar SR2 reader or the SanDisk Extreme Pro reader, but you can’t assume that every card reader does.

How to Tell the Difference Between UHS-I and UHS-II

There are two ways to tell the difference. The first is obvious: look at the label or packaging. They’ll 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 incorrectly written as a number 1, like UHS-1.

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

U1 vs U3 vs V30

The UHS-II code isn’t the only new code you’ll see on some microSD and SD cards. There’s also a new code that relates to suitability for recording high-resolution video. U1 and U3 have been around for a while; the new addition is V30.

All of these are speed classes aimed at recording high-resolution video. And they’re a separate designation from the UHS-I vs UHS-II one. I have more information on microSD speed classes here.

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View Comments

  • Hi David,
    Thanks. Simple, concise information.

    Q. I can't find any mention of capacity limit for a SONY FDR-AX700 camcorder (even in the manufacturers site and manual). I suspect these limits are not so prevalent nowadays but do you know if these matter?

    • Hi Paul. As a newer camera, the FDR-AX700 takes SDXC card (as confirmed by the specs), so you should be good for of the current cards, at least up to 1TB.

  • Hey,

    Thanks for your fantastic post.

    The Lexar Professional 1667x 128GB SDXC UHS-II Card seems like a really great card in terms of cost:performance and I'm wondering whether I should just buy two of these cards for my a7r iii for both slots even though my camera won't make the most of its spec for both slots.

    Is it worth buying a UHS-1 card that will probably cost a similar price but at a lower spec when I could just buy two of the Lexars? Will I get higher performance from the UHS-1 card filling the appropriate UHS-1 slot instead?


    • There’s no benefit to buying UHS-I cards unless they’re cheaper. If it was me, if the price is about the same for the same storage capacity, I’d just get two of the UHS-II cards, with the understanding that there won’t be any in-camera benefit in the a7r III’s second slot.

    • I don’t know that specific camera, but the spec is designed to be compatible each way (ie. backwards, whether from the camera or card). So I’d be surprised if there was any problem using it. You just wouldn’t get any of the extra benefit of the UHS-II when using it in the camera (you might when connected to your computer, depending on your card reader).

    • For just regular single-shot shooting, no, not really. If you're using the a7Riii's high-framerate burst shooting, then the UHS-II will be better.

  • I've got one of those cameras and have used UHS-II cards in it to check. They work in it fine, but the camera's own interface appears to have a UHS-I bus interface, so you won't get any of the extra benefit from the UHS-II.

    • The OM-D EM1 Mark II has one UHS-I slot and one UHS-II slot. If you put a UHS-II card in the UHS-I slot, it will slow down to UHS-I speeds.