Memory Cards FAQ

We demystify all those cryptic figures and symbols on your memory card.

Memory cards are straightforward in use: you just pop them in your camera, format them and you’re off. Trying to make sense of their various figures and symbols, however, is another story.

The situation has become more problematic in recent years as more advanced cards have been inscribed with new terminology to indicate certain aspects of their performance.

More basic cards are thankfully free of many terms, but as cameras get more advanced it becomes even more important to understand whether you’re actually using the right card to do its capabilities justice. Fail to do so and you can end up having your camera’s burst depth cut short or your video recording interrupted, and lots of hanging around waiting for images to be recorded.

To help clear everything up, we’re going to run through all the symbols currently used on common SD-type cards and explain what each one means.

1. Brand 2. Position in range 3. Capacity 4. Type 5. Speed 6. Speed Class 7. UHS Class 8. UHS Bus IF product family 9. Video Speed Class

1. Brand
This is the easy one: the manufacturer of the card. The most common names you will see here are Sandisk and Lexar, although Kingston, Transcend, Samsung, Toshiba and others are also commonly available. You may even have one from the same manufacturer as your camera.

Most people will have a card from one of the first two brands as these are the most popular, but there are perfectly good cards from the others that are often cheaper. As with hard drives, memory cards are typically only made by a handful of companies and simply rebadged by others.

Some brands are known for providing particularly good warranties or image-recovery software with their cards as standard, so you may want to factor these issues in if choosing between brands. Your best bet is to check the manufacturer’s website for full details as to what you get with each.

2. Position in range
This indicates where in a manufacturer’s lineup a card sits. Not all manufacturers have these different classes, but those that do give you a quick idea as to what kind of performance you should be able to expect from a card.

Sandisk, for example, currently has Ultra, Ultra Plus, Extreme, Extreme Plus and Extreme Pro classes for its SD-type memory cards, as well as a more basic one that bears no particular designation. As you step up a class you are likely to see improved transfer speeds (more on this later), and more advanced cards may offer things like protection from water and freezing temperatures too. Naturally, this will be reflected in the asking price.

3. Capasity
All memory cards have a capacity that should be clearly indicated on the card itself. This could be as little as 4GB or 8GB (and even less for older SD cards), or as much as 512GB (at the time of writing). Larger 1TB and 2TB cards will at some point be available too, but frankly, even 512GB is way beyond most people’s needs.

The larger the card the more images and videos you can squeeze on it, although quite what you end up in practice depends on a number of factors. Whether you shoot JPEG images or Raw files, for example, together with what level of compression you use, whether you shoot high-resolution videos and how the camera records these among other things.

Most people tend to go for a card that’s about 16-64GB in size, and these are now very affordable. From the perspective of security it’s a good idea to have a number of smaller cards rather than a single large one, but the convenience of being able to fit weeks’ worth of shoots or video footage onto a single, high-capacity card makes these tempting.

4. Type
Currently, all SD-type cards fall into one of three camps: SD, SDHC and SDXC. They are all the same shape and size, but the type will be indicated clearly on its front.

SD (Secure Digital) cards are still in existence, but there is not much demand for them anymore as they do not offer the kinds of capacities and transfer speeds to do today’s cameras justice. Even if you do manage to find one, you’ll get considerably better value going for an SDHC or SDXC card, so they’re best avoided.

You can still find SD cards but you’re better off going for a newer SDHC or SDXC one

SDHC (Secure Digital High Capacity) cards are those that have a capacity between 4-32GB (inclusive). As they get larger in size they double in capacity: so, you can either go for a 4GB, 8GB, 16GB or 32GB card. If you find an SDHC card with any other capacity – 21GB, for example – you probably need to start shopping elsewhere.

SDXC (Secure Digital Extra Capacity) cards are those that offer anything above this. These are currently 64GB, 128GB, 256GB and 512GB cards, but soon this will increase to even greater capacities.

Most of today’s cameras that take SD media support SD, SDHC and SDXC cards, but older cameras that only support SD cards won’t work with SDHC or SDXC cards, and cameras that only support SD and SDHC varieties will not accept SDXC cards.

5. Speed
This is where things start to get a little confusing.

Most cards have one or more of their transfer speeds written in either MB/s (megabytes per second) or with an ‘x’ to show this as a factor. This tells you how quickly the card can operate – more on this in a second.

The card below is one of a handful that has both. These figures mean exactly the same thing, but it can become confusing when trying to compare cards that aren’t marked with both. This card is a good example of how you do just that; a speed of 150MB/s is equal to 1000x, as a speed of 150kb is equal to 1x. So, a card with a 45MB/s rating is the same as 300x, 90MB/s is the same as 600x and so on.

Some cards show transfer speeds in both MB/s (150MB/s) and as a factor (1000x)

Often there will only be one figure, and in this case it will refer to the read speed. Read speed is how quickly information can be read from the card. This is different to the write speed, which refers to how quickly information can be written to it. So, when you take images on your camera they are written to the card. When you put your card into your computer, images are being read from it.

Read speeds are typically a little bit higher than write speeds, so if you only see one figure it will be this one. After all, a higher figure looks more impressive. Some cards even show both. Either way, you should be able to find what both of these speeds are on the card manufacturer’s website.

This is particularly useful to note if you shoot with a modern camera with a high-resolution sensor, especially

This is particularly useful to note if you shoot with a modern camera with a high-resolution sensor, especially if you capture bursts of images in one go. You may find with slower cards that you can’t shoot images consecutively for as long a period (known as burst depth) or that you’re just having to wait around for your camera to clear these to the card.ively for as long a period (known as burst depth) or that you’re just having to wait around for your camera to clear these to the card.

6. Speed Class
For some time now, SDHC and SDXC cards have been marked with a figure inside an almost-complete circle. These figures are either 2, 4, 6, or 10, and they refer to the Speed Class of the card.

What this figure tells you is the minimum sustained write speed of the card in MB/s. In other words, this is how quickly the card guarantees to have information written to it continuously. This is useful for those capturing videos, where data needs to be recorded to the card without any interruption for prolonged periods of time.

This card has a Speed Class 10 rating

A Speed Class 2 card guarantees a minimum sustained write speed of 2MB/s, a Speed Class 4 card guarantees a minimum sustained write speed of 4MB/s, and so on. Bear in mind that this is the minimum guaranteed speed, not the set constant speed.

These figures don’t sound very good in comparison with those mentioned earlier, but video is recorded in a different way to still images and the demands are not quite the same. But which do you need? The SD Association reckons that a card with a Class 4 rating is good enough for Full HD video, but that you should ideally opt for a Class 6 or Class 10 card. This does also depends on frame rate, however, with higher frame rates requiring faster cards. When you start to shoot 4K video you need something more capable – more on this in a second.

7. UHS speed class
There are currently two UHS speed classes: UHS Speed Class 1 and UHS Speed Class 3. The way this is written on a card is with the number 1 or 3 inside the letter U.

This one is fairly easy to understand: UHS Speed Class 1 cards have a minimum write speed of 10MB/s, while UHS Speed Class 3 ups this to 30MB/s. Again, this is one for those capturing video, who need to know their footage will be recorded steadily and without issues.

These are only found on SDHC and SDXC cards, rather than older SD types. You can still use these cards in older cameras that don’t support the UHS standard, but you won’t realise the same speed benefits.

8. UHS Bus IF product family
Not to be confused with the U1 and U3 markings described above, there are currently three UHS Bus IF categories: UHS-I, UHS-II and UHS-III. On the card, these are simply marked with a Roman numeral.

This figure relates to the card’s ‘bus interface’, which plays a crucial role in determining transfer speeds. UHS-I cards have a maximum bus speed of 104MB/s, while UHS-II cards have a maximum bus speed of 312MB/s. UHS-III cards, meanwhile, double this to 624MB/s, but they are not available yet.

Why is this important? A faster card will help your camera to have a longer burst depth and will write images in less time. As such, this factor is particularly important for sports, action or wildlife photographers.

It will also mean you can transfer images and videos from the card to a computer in less time, providing you’re using a card reader that supports this technology. Right now, it’s a particular concern to those shooting VR and 360degree footage, or for any other data-intensive recording.

Left: UHS-I Right: UHS-II
SanDisk SDHC UHS-II UHS Speed Class 3 Speed Class 10 transfer speed 300 MB/s

UHS-II and UHS-III cards are easily recognizable for their two rows of contacts on the rear side, whereas UHS-I cards only have one.

To make sure you will benefit from UHS-I, UHS-II or UHS-III cards, you should check your cameras specification list. Next to the type of memory card your camera supports, it will usually state whether support is provided for one or more of the UHS formats. Bear in mind that cameras designed with two card slots may not support UHS equally in each. As a general rule, the primary slot will be the more capable one, although some are now equally matched.


These cards are backwards compatible, which means that UHS-III and UHS-II cards can be used in devices that only support UHS-I (or don’t even support UHS at all). You just won’t get the same speed benefits of them in these.

9. Video Speed Class
Right now, there are five Video Speed Classes: V6, V10, V30, V60 and V90. Much like Speed Class described above, each figure corresponds with a minimum sustained write speed in MB/s. So, the V6 card has a minimum sustained write speed of 6MB/s, the V10 has a 10MB/s speed and so on.

Video Speed Class 6

This relatively recently designation was designed to keep up with the demands of video capture on modern cameras. Again, which one you need depends on exactly how it is you’re shooting video, but the SD Association recommends V6, V10 and V30 cards for Full HD video; V30 and V60 for 4K video; and V60 and V90 for 8K video. That’s not to say you can’t use a V90 card for Full HD video, just that it’s not required to do so. Essentially, the rule is that higher-rated cards are designed for higher-resolution video footage.

What about CompactFlash cards?
CompactFlash cards don’t have the same UHS and video designations as SDHC and SDXC cards, but things like capacity and speed are typically marked in the same way. They do, however, sometimes have a couple of icons that you won’t find on SD-type media.

UDMA and VPG markings (bottom left) are found on CompactFlash cards but not SDHC and SDXC cards.

One of these is UDMA. This stands for Ultra Direct Memory Access, a technology that has been used by CompactFlash cards for some time now. This tends to have a number next to it, and this guides you on the performance of the card. The most recent types offer UDMA mode 7, simply written as UDMA 7, which has a rating of 166MB/s. The older UDMA mode 6 has a rating of 133 MB/s, although it’s quite common to just see UDMA with no figure next to it.

The other icon exclusive to CompactFlash cards is the Video Performance Guarantee (VPG) speed, which shows a number inside a small clapper board icon. Although this appears slightly different to the Video Speed Class marking on SDXC cards, the principle is the same: the number simply tells you the minimum sustained write speed in MB/s.


The best thing to do is to see what your camera’s manufacturer recommends you use with your camera, as it knows your specific model better than anyone else. This will be in the manual, often detailed with the exactly same icons that you see on the card itself.


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Jonny Mikalsen

I am an enthusiastic amateur photographer with a big passion for photography who loves to take pictures. I’m always aiming to learn new photographic skills and would like to spend more time out in the field or on a photo shoot with my camera.

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