What Is a SD Card?

One of the most popular memory card formats, Secure Digital cards store files across a range of platforms: phones, cameras, music players, computers and even Nintendo consoles and handheld devices. In essence, an SD card works like a USB drive, but the cards come in various form factors, capacities and speeds, and every SD-capable device has its own compatibility limitations.

Memory plug in
Full-size SD cards have a "lock" switch to prevent editing.
credit: Riccardo_Mojana/iStock/Getty Images

What Is a Memory Card?

Put simply, a memory card stores files and can be used to transfer them to and from compatible devices. The technology inside is very different, but in function, an SD card operates like an old floppy disk. Rather than use magnetic storage, SD cards contain technology similar to a USB drive: They write data in flash memory, which uses transistors to retain data even when disconnected from power.

Memory Card Formats

Historically, memory cards came in numerous formats, including SD cards, Memory Sticks, xD-Picture cards and CompactFlash cards, but SD has become the most common memory card format, while many others have grown obsolete. SD cards work across many types of devices produced under numerous brands. SD cards themselves are also sold by many companies -- "SD" itself is not a brand name.

Form Factors

As of publication, SD cards come in three form factors: SD, miniSD and microSD. Other than their physical sizes, the cards are technologically compatible -- with a converter, a microSD or miniSD card works in a full-size SD card slot. Smaller devices, such as phones, often only support microSD or miniSD cards, whereas many PCs have a full-size SD slot.

The term "SD card" can refer specifically to the full-size card or to any card in the SD family, including miniSD and microSD cards.

Memory Capacity

Just like USB drives, SD cards come in a range of capacities, from small cards with a few hundred megabytes to multi-gigabyte cards. Unlike USB drives, SD cards are split into three tiers based on their capacity: SD, SDHC and SDXC. The original tier covers cards up to 2GB, SDHC (High Capacity) cards run from 2GB up to 32GB and SDXC (Extended Capacity) reaches up to a potential limit of 2 terabytes. As of publication, however, the largest SDXC card in production holds 512GB.

Not all devices support all SD card capacities. Even many new electronics only support cards up through SDHC, so check the devices you own before buying the highest capacity card on the shelf.

Card Speeds

The last grouping for SD cards concerns their speed. Faster cards read and write data more quickly, significantly reducing the time it takes to move large files. Many devices specify a minimum compatible card speed, and faster cards improve performance. In a camera, for example, a slow card causes a longer delay between shots, as you have to wait for the card to finish writing your last picture.

SD cards specify speed by speed class, indicated by a number inside a circle. A class 2 card reads and writes at 2MB per second, a class 4 at 4 Mbps, and so on, up to class 10. Cards marketed as Ultra High Speed bear a UHS speed class marker, a number inside the letter U. UHS class 1 cards run at 10Mbps and class 3 cards run at 30 Mbps.

In practice, a SD card might run a bit faster or slower than its class indicates, especially when only transferring a single file.