If it’s been a while since you’ve bought portable flash memory, you might be surprised by the broad availability and affordability of high-speed and large-capacity microSD and SD cards. Commonly used to expand the storage in devices ranging from phones to drones and cameras, microSD cards are more frequently purchased than any other SD form factors, although full-sized cards remain popular among professional camera owners.

With this guide, we’ll break down what all the different codes and ratings mean, and offer the best choices ..

SD Card Basics: Size & Storage

All SD cards (short for Secure Digital), regardless of their size, use one or two small NAND flash memory chips — similar to those found in USB memory sticks and SSDs — and a tiny processor to manage the flow of data and instructions.

There are 3 standards for the dimensions, and they are incompatible with each other. In other words, a miniSD card reader won’t work with microSD cards (unless you use an adapter):

  • Standard SD cards: 1.26 x 0.94 x 0.083 to 0.055 inches (32 x 24 x 2.1-1.4 mm)
  • miniSD cards: 0.85 x 0.79 x 0.055 inches (21.5 x 20 x 1.4 mm)
  • microSD cards: 0.56 x 0.43 x 0.039 inches (15 x 11 x 1 mm)

Standard SD cards all come with a small locking toggle, that enables/disables the ability to write or delete data on the card; however, mini and microSD cards don’t have this. There’s also a further 5 categories within the size classes, that indicate the connection system and data capacity of the card:

  • SD or SDSC (Secure Digital Standard Capacity): maximum storage of 2 GB
  • SDHC (Secure Digital High Capacity): More than 2 to 32 GB of storage
  • SDXC (Secure Digital Extended Capacity): More than 32 GB to 2 TB of storage
  • SDUC (Secure Digital Ultra Capacity): More than 2 to 128 TB of storage

The fifth category, Secure Digital Input Output (SDIO), is special in that they contain more than just storage. These cards sport an extra device that provides additional functions, such as Bluetooth or a GPS receiver. Because there is a big difference in the storage sizes, each category also has restrictions of the file format used.

SDSC is restricted to FAT12, FAT16, and FAT16B. SDHC is nearly always FAT32 and the XC/HC versions use exFAT. The exFAT file format was specifically designed for NAND flash devices and is likely to remain the standard for many more years.

SDSC, SDHC, and SDXC cards are supported in a wide range of devices, such as laptops, smartphones, drones, and digital cameras. The need for increased storage continually grows, thanks to bigger games, more complex apps, and cameras sporting ever higher resolutions — but there will always be an SD card for everyone’s needs and budget.

SDUC is still very new, so it will be some time before we see producers routinely supporting it; 128 TB of storage should be enough for the majority of users for years to come!

SD Card Performance: Speed Classes

All SD cards use the little brass contacts at the end of the package to receive and send information, in the form of instructions and data. The interface between the card and the reading device has evolved with each specification revision — in some cases, the updated system just runs faster (i.e. the bus clock is higher) but in some cases, the SD card has extra contacts to provide more channels for the data.

However, not all NAND flash chips are the same, so the speed classes also indicate the minimum sequential write rate — the slowest speed at which data can be put onto the memory chip in a structured, rather than a random, way.

With so many speed classes to get your head around, it can be tricky to figure out what rating you really need. In the table below, we can see how they roughly compare.

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