SD stands for Secure Digital. Back in 2000, three electronics makers got together and formed the SD Association so cameras, phones and music players could all share one kind of card, and that group still runs the standard today.
You've probably also seen SDHC or SDXC printed on a card. Those extra letters are about size, and we'll go through each one, then decode the rest of the small print on the label.
What an SD Card Actually Is
Think of it as a tiny flash drive without the USB plug. Pull one out of your camera after a holiday and every photo is still there, because flash memory doesn't need power to hold on to data. Drones use them, and so do dash cams, a lot of laptops and the Nintendo Switch.
The standard comes in three physical sizes. A full size card is roughly the size of a postage stamp at 32 by 24 millimeters. microSD is closer to a fingernail. There was a miniSD in between as well, but you'd struggle to find one in a shop now. Inside, all three speak the same language, and that's the reason a microSD card works in a full size slot as soon as you clip it into an adapter.
The "Secure" part is a leftover from the early 2000s. Music labels wanted copy protection on anything that could hold songs, so the first cards had it built in. Hardly anyone uses that feature today, but the name never changed.
SD, SDHC, SDXC and SDUC: The Capacity Classes
As cards got bigger, the SD Association had to keep adding new classes on top of the original one. The class tells you how big the card can be and which file system it ships with.
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Class
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Full name
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Capacity
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File system
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SD (SDSC)
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Secure Digital Standard Capacity
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Up to 2GB
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FAT12 or FAT16
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SDHC
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Secure Digital High Capacity
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Over 2GB up to 32GB
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FAT32
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SDXC
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Secure Digital eXtended Capacity
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Over 32GB up to 2TB
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exFAT
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SDUC
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Secure Digital Ultra Capacity
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Over 2TB up to 128TB
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exFAT
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Most cards on sale today are SDXC, since anything from 64GB upward falls in that class. SDUC is still rare, and you'll only need it once cards pass 2TB. The class definitions come from the SD Association's card standards page.
Why Older Devices Can't Read SDXC
Compatibility only works in one direction. A camera with an SDXC slot reads SDHC and plain SD cards without complaint. The reverse isn't true, and an SDHC only camera will reject a 64GB SDXC card or ask you to format it.
There are two reasons for this. SDXC cards use the exFAT file system, which many devices made before about 2010 never learned to read, and they also rely on a newer version of the SD host specification. Manufacturers rarely went back and added either one with a firmware update, so an old camera stays an SDHC camera.
It's worth a quick look before you buy a big card for an older camera. Some bodies print the SDXC logo right next to the card door, and the manual usually has a page listing supported cards.
When that page only says SDHC, stick to 32GB or smaller. When you're ready to choose a card, you can compare capacities and speed ratings across Digiera's SD and microSD cards.
How to Read the Symbols on an SD Card
Pick up any card in a shop and you'll see half a dozen little logos crammed onto the front. It looks like a lot, but there are really only six things being shown, and the table below covers all of them.
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Symbol
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Example
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What it tells you
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Capacity logo
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SDHC, SDXC, SDUC
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The capacity class, and so which devices can read the card
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Speed Class
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C inside a circle, with 2, 4, 6 or 10
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Minimum steady write speed in MB/s, the oldest rating
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UHS Speed Class
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A number inside a U: U1 or U3
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U1 writes at 10MB/s or more, and U3 at 30MB/s or more
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Video Speed Class
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A V followed by 6, 10, 30, 60 or 90
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Minimum steady write speed in MB/s for recording video
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Bus interface
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Roman numeral I, II or III, or the word EXPRESS
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How fast the card can talk to the device, if the slot supports it
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Application Performance Class
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A1 or A2
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How quickly the card runs apps on a phone or handheld
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Here's how that works on one of our own cards. Look at the Digiera LS100 SD card and you'll spot U3, V30 and a small Roman numeral I, so you know it never drops below 30MB/s while writing and it talks to the camera over the UHS-I bus. That's enough for steady 4K recording on most cameras.
The Digiera LSU200 UHS-II SD card has a second row of contacts on the back and a Roman numeral II, so it runs much faster in a camera with a UHS-II slot.
FAQs
What does SD stand for in SD card?
SD stands for Secure Digital. The "secure" bit refers to copy protection that music companies wanted on the very first cards, and the SD Association has looked after the standard ever since.
And what does the HC in SDHC mean?
HC is short for High Capacity, which sounded impressive in 2006 when it arrived. Today it covers the smaller end of the market, from just over 2GB up to 32GB, and those cards come formatted as FAT32.
SDHC vs SDXC: what's actually different?
Capacity is the main difference. You can't buy an SDHC card bigger than 32GB, while SDXC starts just above that and goes all the way to 2TB. SDXC also switches to exFAT, and that matters for video, because FAT32 can't hold a single file over 4GB.
Is microSD just a smaller SD card?
They follow the same standard but come in different sizes. A microSD card is much smaller, and you can use it in a full size SD slot by putting it in an adapter.
Can I use an SDXC card in an SDHC camera?
Usually you can't. A camera built only for SDHC can't read the exFAT file system that SDXC cards use, so stick to cards of 32GB or less in that camera.
What does the V30 or U3 on my SD card mean?
Both are minimum write speeds. U3 promises at least 30MB/s, and V30 gives the same 30MB/s guarantee measured for video recording, which covers most 4K cameras.
