At first glance, an SD card is a small, flat, rectangular object with a notch along one edge and copper leads, called pins, embedded on one side along another edge. But there are a few different form factors. Here are the physical sizes you'll see for SD cards:
- 32 x 24 millimeters, 2.1 millimeters thick - This is the most common size you'll see. This size has nine pins and features a write-protection switch on one side to toggle between read-only and read-write states.
- 20 x 21.5 millimeters, 1.4 millimeters thick - This is the "mini" version with 11 pins, and it's less common than the other two sizes.
- 15 x 11 millimeters, 1 millimeter thick - This is the "micro" version with eight pins. Because it's so small, this size has become popular in mobile devices like smartphones.
It's not just the physical makeup of the card that's standardized. SD cards also come in different capacities. These are the capacity formats for SD cards listed in order from oldest to newest:
- SD - Standard format with up to 2 GB per card, available in all three sizes, under $10
- SDHC - High-capacity format with 4 to 32 GB per card, available in all three sizes, $10 to $100
- SDXC - eXtended capacity format with 32 GB to 2 TB per card, available in the larger and micro sizes only, starting around $80
That's three capacity formats, each in two or three sizes. To read and write data to the card, you'll need card-reader hardware that can handle both the size factor and the capacity class indicated. Also, if you're using a mini- or micro-size card but have the large-size reader, you can insert the smaller card into a special adapter in the shape of the larger card. Some SD card manufacturers even sell microSDHC cards together with SDHC-sized adapters to make it easier to move the cards between devices.
This is where you have to be cautious. For example, if you have an older digital camera that takes SD format cards, an SDHC card might not work, even if it fits into the camera. Check your hardware specification to find out what capacity formats your device supports. Then look for cards featuring the SD logos that correspond to the capacity-size combination you need. You can see all those logos at the SD Association Web site here.
In addition to these SD card types, there is another classification you might want to know when purchasing an SD card: its speed. Next, let's look at what activities require higher SD card speeds and how the SD Association classifies cards by speed.