The SD card is a proprietary, non-volatile, flash memory card format developed by the SD Association (SDA). They come in three physical forms: the full-size SD, the smaller miniSD (now obsolete), and the smallest, microSD. Owing to their compact form factor, SD cards have been widely adopted in a variety of portable consumer electronics, including digital cameras, camcorders, video game consoles, mobile phones, action cameras, and camera drones.
The format was introduced in August 1999 as Secure Digital by SanDisk, Panasonic (then known as Matsushita), and Kioxia (then part of Toshiba). It was designed as a successor to the MultiMediaCard (MMC) format, introducing several enhancements including a digital rights management (DRM) feature, a more durable physical casing, and a mechanical write-protect switch. These improvements, combined with strong industry support, contributed to its widespread adoption.
To manage licensing and intellectual property rights, the founding companies established SD-3C, LLC. In January 2000, they also formed the SD Association, a non-profit organization responsible for developing the SD specifications and promoting the format. As of 2023, the SDA includes approximately 1,000 member companies. The association uses trademarked logos owned by SD-3C to enforce compliance with official standards and to indicate product compatibility.
History
Origins and standardization In 1994, SanDisk introduced the CompactFlash (CF) format, one of the first successful flash memory card types. CF outpaced several competing early formats, including the Miniature Card and SmartMedia. However, the late 1990s saw a proliferation of proprietary formats such as Sony's Memory Stick and the xD-Picture Card from Olympus and Fujifilm, resulting in a fragmented memory card market.
To address these challenges, SanDisk partnered with Siemens and Nokia in 1996 to develop a new postage stamp-sized memory card called the MultiMediaCard (MMC). While technically innovative, MMC adoption was slow, and even Nokia was slow to integrate support for it into its mobile devices.
In 1999, SanDisk was approached by Panasonic (then known as Matsushita) and Kioxia (then part of Toshiba) to develop a new format as a second-generation successor to MMC. The goal was to create a portable, high-performance memory card with integrated security features and broader interoperability. Concerned about losing market share to Sony's proprietary Memory Stick, Toshiba and Panasonic saw the collaboration as an opportunity to establish an open, industry-backed standard.
Panasonic and Toshiba, who had previously collaborated on the Super Density Disc (a DVD precursor), reused its stylized "SD" logo for the Secure Digital (SD) card format. Anticipating the growth of MP3 players, they also advocated for digital rights management (DRM) support seeking to reassure content publishers wary of piracy. The DRM system adopted—Content Protection for Recordable Media (CPRM)—had been developed earlier in partnership with IBM and Intel and complied with the Secure Digital Music Initiative standard. Although often cited as a factor in the format's broad industry support, CPRM was rarely implemented in practice. SD cards also featured a mechanical write-protect switch, and early SD slots maintained backward compatibility with MMC cards. In early 2000, the first commercial SD cards offering of storage were released, with larger capacity versions following shortly after. By August 2000, cards were being sold for approximately . According to SanDisk, consumer adoption was accelerated by Toshiba and Panasonic's commitment to launching compatible devices in parallel with the cards.
To support standardization and interoperability, SanDisk, Toshiba, and Panasonic announced the creation of the SD Association (SDA) at the January 2000 Consumer Electronics Show (CES). Headquartered in San Ramon, California, the SDA initially included 30 member companies and has since grown to encompass around 800 organizations worldwide.
Smaller formats
At the March 2003 CeBIT trade show, SanDisk introduced and demonstrated the miniSD card format. The SD Association (SDA) adopted miniSD later that year as a small-form-factor extension to the SD card standard, intended primarily for use in mobile phones. However, the format was largely phased out by 2008 following the introduction of the even smaller microSD card.
The microSD format was introduced by SanDisk at CeBIT in 2004, initially under the name T-Flash, later rebranded as TransFlash or TF. In 2005, the SDA adopted the format under the official name microSD, though the TransFlash name remains in common use as a generic term for microSD cards. A passive adapter allows microSD cards to be used in standard SD card slots, maintaining backward compatibility across devices.
Increasing storage density
The storage capacity of SD cards increased steadily throughout the 2010s, driven by advances in NAND flash manufacturing and interface speeds. In January 2009, the SDA introduced the Secure Digital eXtended Capacity (SDXC) format, supporting up to of storage and transfer speeds up to . SDXC cards are formatted with the exFAT file system by default.
The first SDXC cards appeared in 2010, with early models offering capacities of and read/write speeds of several hundred megabits per second. Consumer adoption accelerated as digital cameras, smartphones, and card readers gained SDXC compatibility.
By 2011, manufacturers offered SDXC cards in capacities, with some models supporting UHS Speed Class 10 and faster. In the following years, capacity milestones were reached: in 2013, in 2014, in 2019, and in 2022.
The Secure Digital Ultra Capacity (SDUC) specification, announced in 2018, expanded maximum capacity to and increased theoretical transfer speeds to . In 2024, Western Digital announced the first SDUC card, scheduled for commercial release in 2025.
Capacity standards There are four defined SD capacity standards: Standard Capacity (SDSC), High Capacity (SDHC), Extended Capacity (SDXC), and Ultra Capacity (SDUC). In addition to specifying maximum storage limits, these standards also define preferred file systems for formatting cards.
+ Comparison of capacity standards SDSC SDHC SDXC SDUC Mark Max capacity File system FAT12, FAT16 FAT32 exFAT
SD (SDSC) The original Secure Digital (SD) card was introduced in 1999 as a successor to the MMC format. The name SD Standard Capacity (SDSC) was applied later to distinguish it from newer variants. Although based on the same electrical interface as MMC, the SD format introduced several enhancements aimed at improving usability, durability, and performance: A notched, asymmetrical shape to prevent incorrect insertion. Recessed electrical contacts to protect against damage and contamination. A four-line data bus for faster transfers, compared to MMC's single data line. A mechanical write-protect switch. These features came at the expense of increased card thickness: for standard SD cards, compared to for MMC. A Thin SD variant was also defined, but saw little use.
SDSC cards support capacities up to and use the FAT12 or FAT16 file system. They remain compatible with most SD-capable devices but have been largely superseded by higher-capacity formats.
Because of physical differences, full-size SD cards do not fit in slim MMC-only slots.
SDHC
SD High Capacity (SDHC) was introduced in SD specification version 2.0, released in January 2006. It expands the maximum capacity to , compared to the limit of SDSC.
SDHC cards are physically identical to earlier standard-capacity SD (SDSC) cards, but differ in how they store and address data. This includes a redefinition of the Card-Specific Data (CSD) register (for details, see § Storage capacity calculations). Additionally, SDHC cards are typically preformatted with the FAT32 file system.
SDHC-compatible devices are required to support older SDSC cards. However, older SDSC devices may not recognize SDHC cards without a firmware update. Older operating systems like Windows XP require patches or service packs to access SDHC cards.
SDXC SD eXtended Capacity (SDXC) was introduced in SD specification version 3.01, released in January 2009. It expands the maximum capacity to compared to the limit of SDHC. SDXC cards are formatted with the exFAT file system, which is required by the SDXC standard. While Windows Vista SP1 and later and Mac OS X 10.6.5 and later support exFAT natively, support in BSD and Linux distributions was limited until Microsoft released the exFAT specification and Linux kernel 5.4 included an open-source driver.
SDXC cards can be reformatted to other file systems (e.g., ext4, UFS, VFAT or NTFS), which may improve compatibility with older devices or systems lacking exFAT support. Many SDHC-compatible hosts can use SDXC cards if reformatted to FAT32, but full compatibility is not guaranteed.
SDUC SD Ultra Capacity (SDUC) was introduced in SD specification version 7.0, released in June 2018. It expands the maximum capacity to , compared to the limit of SDXC. Like SDXC cards, SDUC cards use the exFAT file system by default.
Bus marks Bus marks indicate both the bus interface and the minimum data transfer performance of a device (as opposed to speed class ratings which indicate card performance) in terms of sustained sequential read and write speeds. These are most relevant for handling large files—such as photos and videos—where data is accessed in contiguous blocks. The SD specification has improved bus speed performance over time by increasing the clock frequency used to transfer data between the card and the host device. Regardless of the bus speed, a card may signal that it is "busy" while completing a read or write operation. Compliance with higher-speed bus standards typically reduces reliance on this "busy" signal, allowing for more efficient and continuous data transfers.
+ Comparison of bus speeds Interface Mark Bus Capacity standard Speed Duplex PCIe SD SDHC SDXC SDUC Default rowspan="2" Half rowspan="8" rowspan="2" rowspan="11" rowspan="11" rowspan="11" 1.01 Half 1.10 UHS-I Half rowspan="9" 3.01 UHS-II Full 4.004.10 Half UHS-III Full 6.00 SD Express rowspan="3" 3.1x1 7.007.10 3.1x24.0x1 8.0 4.0x2 +Bus speed of host and card combinations (in MB/s) colspan="2" rowspan="2" UHS-I UHS-II UHS-III Express UHS50 UHS104 UHS-I UHS50505050505050 UHS10450104104104104104UHS-II 50104156156156156 50104156312312312UHS-III50104156312624624Express501041041041043,938
Default Speed The original SD bus interface, introduced with version 1.00 of the SD specification, supported a maximum transfer rate of . This mode is referred to as Default Speed.
High Speed With version 1.10 of the specification, the SD Association introduced High-Speed mode, which increased the maximum transfer rate to . This enhancement was designed to meet the growing performance requirements of devices such as digital cameras.
UHS (Ultra High Speed)
The Ultra High Speed (UHS) bus interface enables faster data transfer on SDHC, SDXC and SDUC cards.
UHS-compatible cards are marked with Roman numerals next to the SD logo, indicating the version of the UHS standard, and therefore the bus speeds, they support. These cards offer significantly faster read and write speeds than earlier SD card types, making them well suited for high-resolution video, burst photography, and other data-intensive applications.
To achieve higher transfer speeds, UHS cards and devices use specialized electrical signaling and hardware interfaces. UHS-I cards operate at instead of the standard and use a four-bit transfer mode. UHS-II and UHS-III introduce a second row of interface pins to add a second lane of data transfer and use low-voltage differential signaling (LVDS) at to increase speed and reduce power consumption and electromagnetic interference (EMI).
The following UHS speed classes are defined:
UHS-I