SD Card Specifications

The SD card is a flash memory card format developed by the SD Association (SDA), which was introduced in August 1999 as Secure Digital by SanDisk, Panasonic (then known as Matsushita), and Kioxia (then part of Toshiba).

Formats

Format	When introduced	Note
SD	August 1999	SD meaning Secure Digital
miniSD	March 2003	phased out by 2008 following the introduction of microSD
microSD	March 2004	initially under the name T-Flash, later rebranded as TransFlash or TF

Capacity Standards

	SDSC	SDHC	SDXC	SDUC
Full name	SD Standard Capacity	SD High Capacity	SD eXtended Capacity	SD Ultra Capacity
Mark
Max capacity	2 GB	32 GB	2 TB	128 TB
File system	FAT12, FAT16	FAT32	exFAT
When introduced	August 1999	January 2006	January 2009	June 2018

Bus Marks

Bus marks indicate the maximum interface speed and data transfer protocol.

Interface	Mark	Bus speed (MB/s)
Default		12.5
High Speed		25
UHS-I		50
		104
UHS-II		156
		312
UHS-III		312
		624
SD Express		985
		1969
		3938

Speed Classes

Speed classes guarantee the minimum sustained write speed required for steady tasks like video recording.

The SDA created four types of speed classifications:

• Original Speed Class – represented by a number encircled with a "C"
• Ultra-High Speed (UHS) Class – represented by a number inside a "U"; designed for high-bandwidth tasks such as 4K video recording
• Video Speed Class – represented by a stylized "V" followed by the number; introduced to support high-resolution formats such as 4K and 8K
• SD Express Speed Class – represented by a stylized "E" followed by the number, enclosed in a rounded rectangle; designed for data-intensive applications such as large-scale video processing, real-time analytics, and software execution

Min speed (MB/s)	Speed class				Video format
	Original	UHS	Video	SD Express	SD	HD	4K	8K
2					Yes	No	No	No
4						Yes
6							Yes
10
30								Yes
60
90
150
300
450
600

Application Performance Classes

Earlier SD card speed ratings focused on sequential read and write performance, which is important when transferring large files. However, running apps and operating systems involves frequent access to many small files—a pattern known as random access—which places different demands on storage, in which case a new performance metric became necessary. To address this, the SD Association introduced Application Performance Classes.

Class	Minimum random IOPS		Minimum sustained sequential writing	When introduced
	Read	Write
	1,500	500	10 MB/s	November 2016
	4,000	2,000		February 2017

References

• SD card - Wikipedia
• A guide to speed classes for SD and microSD Cards - Kingston Technology