What to Look For in SD and microSD Cards

microSDA lot of people are going to be receiving gadgets that require SD or microSD cards this holiday season. When you go to the store, it’s easy to just grab a random one from the shelf based entirely on capacity and price. Unfortunately, it’s easy to miss some important information that could make the difference between the card working properly and causing your digital camcorder or DSLR to hang up during operation.

While capacity is indeed very important, it isn’t exactly the best indicator to the speed or capability of the card. Whether you’re dealing with SD or microSD devices, consistently high transfer speeds can make a world of difference to their operation.


Not all devices are compatible with all SD and microSD cards. SD, SDHC, and SDXC are different standards which require support from the hosting device. SDHC and SD cards operate at different voltages, which requires support to work properly.

Consult the owner’s manual for your device to make sure which types of SD cards work. Devices that support SDHC typically support SDXC cards, though that isn’t an absolute universal certainty. If a device doesn’t specifically list SDHC or SDXC in its documentation, there’s a good chance it doesn’t support the higher capacity cards.

For more on compatibility, check out my previous article comparing the different SD standards.

Class and Transfer Speed

The class rating on an SD card is usually represented as a number within a circle printed somewhere on the front. This number indicates the minimum data transfer speed of the card. For example, a class 10 card has a minimum sustained transfer rate of 10 MB/s. Likewise, a class 6 card has a lower speed of 6 MB/s. There are spikes in transfer rates and some class 10 cards are technically a little faster than others, but there are a number of variables that come into play there which aren’t apparent to the buyer.

Some cards have packaging that indicates speed as a number followed by the letter x. This is a throwback to the old speed rating based on CD-ROM speeds during the earlier days of computing. The higher the number, the faster the card. 56x speed SD cards are typically considered 56 times as fast as a base CD-ROM.

Speed becomes a factor in data transfer from the device to storage. For example, a HD camcorder requires a bit more data transfer speed than a point-and-shoot camera. If you’re capturing 6 MB of data every second, a class 4 card will not be able to keep up. This can cause your device to stop, stutter, or buffer.

This is another specification pointed out in user manuals. Find out what type of class the manufacturer recommends and buy a card that is a step above it. A class 6 card requirement can be filled by a class 6 card, but it’s always safer to go with a class 10 card if you can afford the difference.

UHS Speed Class

Some cards don’t have a number class. Instead, they have a big U with the number 1 or 2 (alternatively I or II) inside. UHS stands for ultra high-speed and is indicative of some of the fastest SD cards you can find today. Some products specifically point out UHS as a requirement, and this is where it comes into play.

When I shop for SD cards for HD video, I tend to stick to UHS-1 SDXC cards.

What do you look for when buying an SD or microSD card?