Secure Digital (SD) is the standard for portable non-volatile (flash) memory. If you have a digital camera, video camera, smartphone or eReader, chances are you use SD cards. Really, this is a boon to consumers, since it saves us from having to buy all sorts of adapters, converters and other proprietary gizmos in order to transfer our photos from our cameras to our computers to our phones, etc. Without SD, we’d be stuck in a never ending cycle of compatibility issues.
However, despite the convenience of the SD memory card standard, there is still plenty of room for confusion. For example, let’s try to understand just one type of SD card:
16 GB SDHC Class 4 60x Speed Flash Memory Card
Okay lets try another, take a look at this picture of a different SD card and try to figure out whether or not it will work in your camera:
Or try to tell me why the card pictured–which is a 64 GB SanDisk Ultra SDXC Class 4 15 MB/s*–costs $189 while a 16 GB Kodak High-Speed SDHC Class 4 60x memory card costs $46. Here’s a hint: It’s not just the brand, and only about half of those terms are important to you, the consumer.
Yeah. Confusing. Not to mention that asterisk there, which could mean anything (in this case, it means that 15 MB/s is the manufacturer’s speed rating and doesn’t have any bearing on the official Class Rating–I’ll explain in a sec).
But, there is some method to all this madness, and it’s overseen by a group called the SD Association, whose raison d’etre is to ensure that you and I understand translate what all these acronyms, numbers and symbols all mean. Really, it’s not all that hard to understand—there are just three things about a card you need to know: its physical size, its memory capacity and its speed.
In this groovy guide, we’ll show you how to decode all of this information for any given SD card.
In a hurry? Click here to read the crib notes version of the information below.
What is the SD Association?
Back in 2000, three big technology manufacturers—Panasonic, SanDisk and Toshiba—got together to form the SD Association. The goal of this industry group is to set industry standards and encourage the development for devices that use secure digital technology. Today, there are over 1,000 members of the SD Association. These are the guys that make sure that (A) each device knows how to handle an SD card and (B) you, the consumer, know how to choose an SD card. This is important because, as you’ve noticed, there are many different companies that manufacture SD cards. Lexar, SanDisk, Kingston, Kodak, PNY, Panasonic, V7, Centon—just to name a few. And there are even more manufacturers that create products that use SD cards.
The SD Association makes sure that the SD cards that Lexar sells meet the same functionality and performance standards as the SD cards sold by Kingston, etc. so you know what you’re getting when you shop for SD cards. Instead of trying to understand the terms that each manufacturer uses, all you have to learn are the terms and differentiators laid out by the SD Association, which are explained below:
SD Card Size: SD, miniSD and microSD
|SD Card Size||Typically used in||Types of Cards|
digital audio recorders,
|SD, SDHC, SDXC|
|miniSD||Digital cameras, older PDAs, computers,
some digital musical instruments
personal media players (slotMusic, MP3 players, etc.)
|SD, SDHC, SDXC|
SD card size is the easiest attribute to figure out (or at least it should be). There are three sizes: standard (biggest), mini (medium) and micro (smallest). If it’s anything other than a standard SD card, the size will be tacked on to the front of it. For example, microSD or miniSDXC or just plain SDHC. There are obvious compatibility issues between the different sizes of cards—if the card won’t fit in the slot, then it won’t work. Practically all smartphones that take SD cards use microSD, while laptops, camcorders and digital cameras usually use the standard size SD card. miniSD devices are becoming less common, and in fact, SDXC doesn’t even come in the mini size, which means they will likely be phased out completely.
There are adapters that let you bump up one or two sizes so you can insert a microSD card into a miniSD or standard SD slot or a miniSD card into a standard SD card slot. Usually, when you buy a microSD card it comes with at least one adapter.
SD Card Capacity
|SD||2 GB and below *|
|SDHC||up to 32 GB|
|SDXC||32 GB to 2 TB|
*There are some 4 GB SD cards, but the official SD Association standard for SD type cards is a max capacity of 2 GB.
There are three different types of SD cards: Secure Digital Standard Capacity (SDSC, or just SD), Secure Digital High Capacity (SDHC) and Secure Digital eXtended Capacity (SDXC). You don’t have to memorize the capacity ranges for each family of SD cards, however, since every SD card for sale will have the capacity listed or labeled. But it is important to check if your device is compatible with the type of SD card you’re buying. Older SD host products may not be compatible with SDHC cards, and likely won’t be compatible with SDXC cards.
So, while it may seem like a good idea to get the biggest card you can find, you should really be shopping for the biggest compatible SD card you can find. The SD Association made up this chart to help you understand compatibility issues—though, really, all you need to know is that SDHC and SDXC cards are backwards compatible but SD hosts (i.e. cameras, card readers, smartphones) are not forwards compatible. Check the manual or look for the SDXC or SDHC logo on your device to determine its compatibility.
Speed Class Rating vs. X-Speed Rating
A note about bandwidth units: the standard for measuring SD card speed is in megabytes per second, which is notated as MB/s or MBps, with a big B. One MB/s is equal to 8 megabits per second, or Mb/s and Mbps with a lowercase b. For a good explanation of bits vs. bytes, etc., check out our tutorial on how to Understand Your Internet Connection and Test Its Speed.
In the totally groovy How Memory is Made movie that we featured last Friday, there’s a very quick factoid that reads: “Every speed-rated memory card is tested to assure that it meets our speed specifications.” Speed-rated? Specifications? Sounds nice, but what does it mean? The next shot shows a CompactFlash card rolling off the assembly line with a “600x” speed-rating printed on the front.
The x-speed rating is also sometimes used for SD cards. The x-rating is a leftover from CD-ROM speed ratings, which is why it’s such a ridiculously high number (kind of like horsepower and car engines). Each x equals 150 kB/s, so 600x has a transfer rate of 90,000 kB/s, or about 90 MB/s. The problem with the x-rating, however, is that it’s a manufacturer rating. It’s almost on par with a cereal company claiming it’s product is 100% more delicious—you have to take the figure with a grain of salt.
This is why the SD Association devised the Speed Class Rating. When an SD card has a speed class rating, such as Class 2, Class 4, Class 6 or Class 10, it’s much more meaningful than an x-speed rating because in order to label an SD card using the Speed Class Rating system, you must meet the SD Association’s standards. In other words, rather than being self-congratulatory fluff, a Speed Class Rating is an industry standard, much like the USDA Organic seal or Fair Trade Certified logo.
The Speed Class refers to the minimum transfer speed of the card, which almost always refers to the write speed of the card, since reading from a card is always faster than writing to a card. This differs from the x-speed rating, which usually lists the maximum read speed. When you see a Speed Class rating on a card, it’s a guarantee from the SD Association that that card will perform at that speed or faster. So, if Lexar makes a card that claims to be a Class 10 and it only transfers at 1 MB/s, then the SD Association will send out its goons to punch Lexar’s VP of marketing in the stomach. (Not really, but there are consequences for misusing the Speed Class rating.)
Just to reiterate, there are currently five numeric Speed Classes:
- Class 0 – No rating. These cards were manufactured before the Speed Class Rating system was put in place.
- Class 2 – 2 MB/s and up
- Class 4 – 4 MB/s and up
- Class 6 – 6 MB/s and up
- Class 10 – 10 MB/s and up
UHS-I and UHS-II
Although Class 10 is the highest class in the numeric Speed Class rating, a Class 10 is not the fastest SD card money can buy. Aside from the normal Speed Class set of SD cards is another class, known as the UHS Speed Class. here are three different buses for SD cards. This is the interface by which devices read/write to and from the card. There is the normal bus (default speed), the high speed bus and the ultra high speed bus. Normal bus and high speed bus cards fall into the Speed Class range. Cards that use the ultra high speed (UHS) bus call into the UHS Speed Class.
Currently, you can only buy UHS-I SD cards, which have write speeds up to 104 MB/s. UHS-II SD cards will theoretically have write speeds up to 312 MB/s. The UHS Speed Class logo is a U with a number in it, like this:
In spite of UHS-I being faster overall than Class 10 SD cards, UHS-I are only guaranteed to have a minimum write speed of 10 MB/s, though they will almost always be much faster when used with a UHS-I device. Some UHS-I cards are backwards compatible and will work with Class 2, Class 4 or Class 6 devices, but you obviously won’t get the same speed as you would with a UHS-I device.
On a side note, one nice perk about Speed Class difference is that it’s readable by most devices. Each device has a recommended Speed Class, so if your camcorder prefers a Class 10 card and you insert a Class 2 card, you’ll get a warning letting you know that you may not get great performance from your device because your card is too slow.
Which Card to Use?
Speed Class has a big impact on price, so it’s unwise to go overboard and buy a faster card than you need. Your best bet is to check your device’s manual to find the recommended card speed. It’s never wise to choose a slower card than recommended, and choosing a faster card may not give you a performance boost, since your device may not support faster card speeds. In general, for HD video recording, you’ll need at least a Class 4 card. Class 2 cards and slower are fine for SD video recording and backing up or transferring files.
Most SD cards come pre-formatted on an MBR partition scheme. SD cards will usually be FAT 16*, SDHC cards will be FAT32 and SDXC cards are usually exFAT. You don’t really need to understand this, since practically all systems can read/write to FAT file systems. But you are free to reformat your SD card to almost any file system and partition scheme you’d like. SD cards are also bootable, meaning you can install an entire operating system on it and use it for system recovery.
*FAT file systems don’t support files larger than 4 GB. This isn’t an issue, since SDSC cards don’t come in denominations larger than 4 GB. But if for some reason you choose to reformat an SDXC to FAT for compatibility issues, you should know that you can’t save files larger than 4 GB onto it. If you need to store files larger than 4 GB, consider reformatting your SD card to NTFS.
Note that you can use any disk utility to format SD cards, such as Gparted or the disk utilities built-in to Windows or OS X. But the SD Association recommends using their own SD Formatter to avoid formatting the “Protected Area” that exists on some SD cards that incorporate security features.
Write Protect Tab
Standard size SD cards sometimes have a write protect tab with a little white slider. When the white slider is pushed out to cover the tab, then the card is writeable. When it’s slid back, the card is write-protected, meaning it’s read-only. miniSD and microSD cards do not have write protect tabs. Also, note that not all devices will recognize a write protect tab. But if you’re having trouble writing to your SD card, you might want to double-check that this write protect tab isn’t pushed back. You’ll only usually encounter SD cards that are permanently write-protected when you buy media pre-loaded onto an SD card.
Mixing and Matching SD Cards
It makes the most economical sense to buy SD cards that are compatible with all of your devices. Not only will it make transferring files to and from your computer/smartphone/camera easier, it’ll also save you from confusion while rifling through your laptop or camera bag. To that end, the SD Association put together a delightfully bloop and blip filled animated video that helps you understand the best way to mix and match SD cards.
Okay, so that’s probably more than you ever wanted to know about SD cards. But, it’s important to know lest you find yourself plunking down $100 on an SD card only to discover it doesn’t work in your camera. If all of this went over your head, then the takeaway piece of advice you should remember is to always read the manual for your device and see what type of card they recommend. Using the card that the manufacturer of your camera or recorder intended will almost always give you the best performance, even if it’s not the most state-of-the-art card on the market.
Good luck, and have a groovy time shopping for SD cards as an informed consumer!
P.S. In case you’d like to apply this knowledge to the real life cards mentioned in the beginning, here’s the breakdown:
16 GB SDHC Class 4 60x Speed Flash Memory Card
The 16 GB is self-explanatory–and in order to be at this capacity, it has to be an SDHC card. Remember: SDHC cards are between 2 GB and 32 GB. Interestingly, this card includes both the Speed Class rating and the x-speed class rating. You probably want to ignore the x-speed class rating, because this isn’t regulated by the SD Association. As a Class 4 card, it has a minimum write speed of 4 MB/s (provided that your device can also write that fastreading from the card (i.e. when you are transferring photos to your computer). Again, the most important thing to note about this card is its Speed Class rating and the fact that it’s an SDHC card–are these compatible with your device?
64 GB SanDisk Ultra SDXC Class 4 15 MB/s*
Again, this one is interesting because it shows the Speed Class Rating along with the MB/s. The Ultra and 15 MB/s* are not official ratings approved by the SD Association–these are terms that are mostly meaningful to the manufacturer. In this case, the asterisk is meant to indicate that 15 MB/s is the maximum transfer speed, meaning it’s about as authoritative and useful to you as a consumer as the x-speed class rating. Pay attention to the SDXC notation, though, since this means it may not be compatible in older devices.
Okay, I recognize that all this information may just confuse things further. So, here is the short and to the point version:
There are three physical sizes. From largest to smallest, they are standard SD, miniSD and microSD. You cannot use a larger card in a slot designed for a smaller card for obvious reasons. You can, however, use an adapter to put a smaller card into a larger slot–for example, microSD to standard SD.
There are three SD card types that correspond to their capacity. SDSC, or just SD, can hold up to 2 GB. SDHC can hold up to 32 GB. SDXC can hold up to 2 TB (though the biggest card you’ll find today is only about 64 GB–and cost $4500). Be careful: some older devices may not be able to read/write to SDHC or SDXC cards. Check your manual.
When buying an SD card, it’s important to get the right Speed Class for your device. The Speed Class Rating is a standardized measurement of the card’s minimum transfer speed (i.e. its write speed) and it’s enforced by the SD Association. The higher the class, the faster the card. So, pay attention to the Class (i.e. Class 2, Class 4) rather than the x-speed class rating (i.e. 60x) or the MB/s rating. Check your camera or recorder’s manual to find out the best Speed Class for your device.
Edit: Thanks to commenters over on Reddit for pointing out some typos and other issues in this article. It’s since been updated to tighten it up a bit.