SD cards have speed class ratings defined by the SD Association.
The SD class ratings specify the minimum write speed for the device based on "the best fragmented state where no memory unit is occupied":
Minimum Serial Data
|SD Bus Mode||Application|
|UHS Speed Class||UHS Class 3||30MB/s||
|4K2K Video Recording|
|UHS Speed Class||UHS Class 1||10MB/s||
|Full HD Video Recording HD Still Image Continuous Shooting|
|Speed Class||10MB/s||High Speed||Full HD Video Recording HD Still Image Continuous Shooting|
|Speed Class||6MB/s||Normal Speed||HD and Full HD Video Recording|
|Speed Class||4MB/s||Normal Speed||HD and Full HD Video Recording|
|Speed Class||2MB/s||Normal Speed||Standard Video Recording|
UHS Speed Class is designed for UHS** devices only and designates minimum writing performance to record video on UHS products. The UHS Speed Classes defined by the SD Association are UHS Speed Class 1 (U1) and UHS Speed Class 3 (U3).
* (Speed Class and the UHS Speed Class are two different speed classes.)
**UHS (Ultra High Speed), the fastest performance category available today, defines bus-interface speeds up to 312 Megabytes per second for greater device performance. It is available on SDXC and SDHC memory cards and devices.