Secure Digital cards – more commonly known as SD cards – come in all sorts of capacities, physical sizes and speeds, all of which you need to consider when buying one. Here’s what you need to look out for.

SD cards are flash-based storage devices that are used to store data from modern digital devices like cameras, mobile phones, MP3 players and drones. Flash memory doesn’t require power to retain data, and because NAND flash chips don’t wear out easily, can also be written to many times, making them the ideal storage solution for digital cameras, where they’re most commonly found. In addition, because they contain no moving parts, they are more resilient against knocks, bumps or falls that can cause serious damage to traditional spinning media; and because SD cards are typically used in mobile devices, this is especially important. Some devices will contain an amount of internal storage, which can then be topped up using an SD card; others will require an SD card to store any data. If you need to transfer your data from an SD card to your computer or laptop, you can use an external SD card reader, or a built-in reader on your machine, which are fairly common now. These typically only take standard SD cards, and not others like microSD or miniSD.  

SD cards vary in speed, and whether this is an issue or not depends what you’re using it for. For example, a professional photographer who shoots weddings will need to take photos in rapid succession, as well as saving them in RAW format so they are high resolution enough. For a task like this, an SD card with a high-speed class is important. There are four classes – 10, 6, 4 and 2, in order of how fast they are. Class 10 SD cards are the fastest, and are suitable for HD video recording and high resolution still photos, while Class 2 SD cards are the slowest, and might struggle with anything more than SD video recording. In addition to these four standard SD card speed classes, there are also two more Ultra High Speed (UHS) classes, designed with professionals in mind – Class 1 and 3, with the latter being the fastest. However, only certain cameras support the use of UHS SD cards. We’d advise against going for a Class 2 SD card, as the cost difference isn’t much between those and higher classes, but you’ll notice the difference in speed.

SD cards come in a wide range of storage sizes, from a gigabyte or so right up to a whopping 2TB, made possible by the newer SD Extended Capacity (SDXC) standard. As well as storage size, SD cards also come in three physical sizes – standard SD cards, miniSD cards, and microSD cards. The largest, standard SD cards, are still incredibly small, measuring just 32x24mm and just 2.1mm thick. MiniSD cards are 21.5x20mm, and 1.4mm thick, while microSD cards are the smallest, 15x11mm, and only 1mm thick. MiniSD cards have been phased out in recent years, and replaced with smaller microSD cards. Choosing a physical size depends on your device, which will only take one. You can, however, buy an adapter that’s the size of a standard SD card, allowing the use of microSD cards in more devices.