SD Card Performance Standards

Speed Classifications and Markings

Removable memory cards adhering to the Secure Digital (SD) standard employ various speed classifications to indicate their data transfer capabilities. These classifications assist users in selecting cards appropriate for specific applications, such as capturing high-resolution photos or recording video.

Original Speed Class

The initial classification, represented by a number inside a "C" symbol (e.g., Class 2, Class 4, Class 6, Class 10). These classes define the minimum sequential write speed in megabytes per second (MB/s). For instance, Class 4 guarantees a minimum write speed of 4 MB/s.

UHS Speed Class

Introduced with the UHS (Ultra High Speed) bus interface, indicated by a number inside a "U" symbol (e.g., U1, U3). UHS Speed Class denotes the minimum sequential write speed and is typically used for demanding applications. U1 guarantees at least 10 MB/s, while U3 guarantees at least 30 MB/s.

Video Speed Class

Defined for high-resolution video recording, denoted by a "V" symbol followed by a number (e.g., V6, V10, V30, V60, V90). Video Speed Class specifies the minimum sequential write speed in MB/s, ensuring smooth video capture without frame drops. The higher the number, the higher the sustained write performance. For example, V30 guarantees at least 30 MB/s.

Application Performance Class

Focuses on both sequential and random read/write speeds, crucial for applications running directly from the card (e.g., in smartphones). Indicated by an "A" symbol followed by a number (A1 or A2). These classes specify minimum random read IOPS (Input/Output Operations Per Second) and write IOPS, alongside minimum sustained sequential write speed.

Visual Identification

The speed class symbol (C, U, V, or A) and associated number are generally printed on the face of the removable memory card. Locating this marking is the most direct method of determining the card's performance rating.

Device Compatibility

While a device may support a particular speed class, the actual performance achieved is also dependent on the device's hardware and software capabilities. Inserting a higher-rated memory card than supported by the host device will not result in performance exceeding the device's maximum capacity.

Capacity and Bus Interface

Beyond speed class, the card's capacity (e.g., SD, SDHC, SDXC, SDUC) and bus interface (e.g., UHS-I, UHS-II, UHS-III) also influence overall performance. Higher capacity standards generally support faster transfer speeds. Faster bus interfaces like UHS-II and UHS-III can enable significantly higher maximum read and write speeds, provided both the card and the host device support them.