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  • SD Cards Explained
Technology Articles > Computers > Storages > SD Cards Explained

SD cards come in many shapes, capacities and classes. But what does it mean? And will one SD card work in all of your devices? Find out what you need to know by reading on.

Physical Size

There are three types of SD card sizes available: regular SD cards, mini SD cards and micro SD cards. The normal SD cards (referred to simply as an SD card) measures up at 32 x 24 mm. The mini SD card, or miniSD card, is the next smallest at 21.5 x 20.0 mm. Lastly, there is the microSD card or micro SD card, which is 11.00 x 15.00 mm.

Obviously, the size of the slot dictates which cards the device will accept. For example, cell phones are designed to accept microSD cards, while laptops and printers might accept larger SD cards or multiple SD cards. Either way, you can purchase an adapter that lets you use a miniSD card in a regular sized SD card slot, a micro SD card in a regular SD card slot, etc. The adapter is shaped like the target SD card size and the smaller card is simply inserted into it before sliding it into the computer or device.

Capacity and Compatibility

The next attribute to pay attention to for SD cards is the capacity. The first flash-based memory cards were measured in MBs, such as 512 MB, 256 MB, etc. Today, SD cards are large enough to be expressed in GB or TB. It’s important to pay attention to SD card capacity, because not all devices can read SD cards of all capacities.

To help you keep track of compatibility, SD cards are given different classifications based on their maximum capacity. Standard SD cards have capacities between 1 MB and 4 GB. Secure Digital High Capacity (SDHC) cards have capacities between 4 GB and 32 GB. Secure Digital eXtended Capacity (SDXC) cards range from 32 GB to 1 TB. All are the same physical size and shape.
When devices are marketed, the max SD card capacity it can accept is typically included in the technical specifications. This is important to note when buying flash-based media for your camera, cell phone or computer. For example, a cell phone that only accepts flash cards up to 8 GB won’t be able to read SDXC cards—so don’t waste your money.


The other specification you’ll notice in SD cards is the Class, which refers to the speed of SD card.

Similar to CDs and DVDs, SD cards have certain rated write speeds. Class 0 cards are those that were designed before the Speed Class Rating was defined by the SD Association. Class 2 SD cards have write speeds of 2 MB per second. Class 4 is 4 Mbps. Class 6 is 6 Mbps, and Class 10 is 10 Mbps and so on.

You may also notice cards with x speed ratings. Each x is equal to 1.2 Mbps. To get the overall speed, multiply the number that comes before x by 1.2 Mbps.

For example, 13x = 16.0 Mbps, 40x = 48.0 Mbps and 66x = 80 Mbps.

One thing to watch out for: the x-rating can refer to the read speed or the write speed. Almost always, the maximum read speed is faster than the maximum write speed. So, a card rated at 13x may have a read speed of 16.0 Mbps but only a write speed of 2 Mbps.

For this reason, paying attention to the Class speed rating is a more accurate way to judge SD cards. Speed Class Ratings are overseen by the SD Association, and manufacturers can only reference the Speed Class
Rating when talking about the minimum speed, whereas x speed ratings can refer to the maximum read or write speed.

Getting a fast card is important for camcorders and digital audio recorders, where audio or video is written in real time. It’s less important for things like cell phones and computers, where you’ll mostly be transferring data.


There’s much more to learn about SD cards, but these three key areas will help you when purchasing media. Even this brief overview is fairly complex, so don’t feel bad if you can’t remember it all. Just be sure to keep your receipt!