All SD cards work in fundamentally the same way, providing removable solid-state storage for portable devices such as cameras or smartphones. However, not all cards are compatible with all devices, as the three SD card form factors -- regular, mini and micro -- vary in physical size. A card's form factor can also indirectly affect its performance, as not all SD card specifications are available in all form factors.
Perhaps the most obvious difference between the three SD card form factors is their physical size. Regular SD cards measure approximately 1.25 by 0.95 inches, miniSD cards measure 0.87 by 0.79 inches and microSD cards measure 0.43 by 0.59 inches. You should buy the correct size SD card for your device, as you cannot fit a larger SD card into a smaller slot. However, you can buy adapters that let you put a smaller card into a larger slot -- fitting a microSD card into a miniSD interface, for example.
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As well as having different form factors, SD cards adhere to different standards. The SD card's standard affects its maximum storage capacity. The original SD card standard allows for capacities of up to 2GB, while cards using the SD High Capacity standard can have capacities of up to 32GB. The SD Extended Capacity standard allows for capacities of between 32GB and 2TB. SD and SDHC cards come in all three sizes. However, SDXC cards are only available as full-size or microSD cards.
SD cards can also be categorized by speed class. There are four SD speed classes: Class 10, Class 6, Class 4 and Class 2. Of these, Class 10 is the fastest, producing speeds high enough to record full HD video, while Class 2 is the slowest. Some cards also support the Ultra High Speed bus standard, allowing for further speed gains. All classes are available in all form factors, meaning there is little direct correlation between SD card size and speed.
Typically, smaller SD cards tend to cost a little more than larger ones. This is largely due to higher manufacturing costs, as smaller components require increased precision to build. However, card specification generally has a greater impact on cost than size alone. For example, a full-size Class 10 SDXC card would likely cost more than a Class 2 Micro SDHC card, even though the Class 2 card is physically smaller.