What do the numbers and symbols on an SD card mean?

SD cards are a popular type of removable flash memory card used for storing digital information in devices such as cameras, phones, drones, handheld consoles, and more. They are small, lightweight, and portable storage devices that allow you to capture hi-resolution photos, HD video, music, documents, and other files.

SD cards come in different capacities and speeds to support various needs. The SD format was introduced in 1999 by the SD Association and has gone through several evolutions like SDHC and SDXC to support ever-growing storage demands. SD cards use non-volatile memory, meaning data is retained even when power is removed.

This article will provide an in-depth look at the different numbers, symbols, sizes, speeds, and formats of SD cards to help decipher their technical specifications.

SD Card Sizes

SD cards come in a variety of storage capacities, ranging from smaller cards that hold a few gigabytes to larger capacity cards that can store hundreds of gigabytes. Some of the most common SD card sizes include:

  • 32GB – This mid-size SD card can hold thousands of photos and songs. It’s a good all-around choice for many uses.
  • 64GB – With double the space of a 32GB card, 64GB cards allow you to store even more photos, videos, and files. Many flagship phones now come with 64GB cards.
  • 128GB – High-capacity SD cards like 128GB have become very affordable in recent years. They offer ample space for media storage.
  • 256GB – 256GB SD cards are ideal for devices that capture high resolution photos and 4K video. This capacity allows tons of media to be stored.
  • 512GB – Top-tier SD cards now reach 512GB for massive storage needs. This space can hold hours of 4K footage.

Larger SD cards up to 1TB and 2TB are also available. The right size card depends on your storage needs. Higher capacity cards allow you to capture more photos/videos before needing to offload media.

Speed Class Ratings

SD cards have speed class ratings that indicate their minimum guaranteed write speeds. The numbers refer to the minimum sustained write speed in megabytes per second (MB/s).

Some common speed class ratings are:

  • Class 2 – Minimum write speed of 2 MB/s
  • Class 4 – Minimum write speed of 4 MB/s
  • Class 6 – Minimum write speed of 6 MB/s
  • Class 10 – Minimum write speed of 10 MB/s (SD Association)

There are also UHS speed classes for faster cards:

  • U1 – Minimum 10 MB/s write speed
  • U3 – Minimum 30 MB/s write speed (Kingston)

For video recording, the Video Speed Class ratings V6, V10, V30, V60, and V90 indicate the minimum write speeds. For example, V30 supports at least 30 MB/s write speed.

Bus Interface Symbols

SD cards use symbols to denote the bus interface or data transfer speed between the card and device. The most common bus interface symbols are:

  • UHS-I – Denoted by the Roman numeral I. UHS-I cards support a bus speed up to 104 MB/s.1
  • UHS-II – Denoted by the Roman numeral II. UHS-II cards support a bus speed up to 312 MB/s.2
  • UHS-III – Denoted by the Roman numeral III. UHS-III is a new standard that supports a bus speed up to 624 MB/s. Very few devices support UHS-III currently.

The UHS bus interface determines the maximum possible data transfer speed between the SD card and host device. Newer devices and card readers will support faster interfaces like UHS-II and UHS-III for better performance.

Video Speed Class Symbols

SD cards can have video speed class symbols to indicate their minimum write speed for recording video. These are signified by a ‘V’ followed by a number.

The main video speed classes are:

  • V6 – Minimum 6 MB/s write speed
  • V10 – Minimum 10 MB/s write speed
  • V30 – Minimum 30 MB/s write speed
  • V60 – Minimum 60 MB/s write speed
  • V90 – Minimum 90 MB/s write speed

Higher video speed classes, with a higher minimum write speed, are better suited for recording high resolution or high frame rate video. V30 is common on many modern SD cards and provides sufficient performance for 4K video recording. V60 or V90 cards are recommended for 8K video or very high frame rate 4K recording.[1]

The video speed class indicates sustained minimum write performance to ensure smooth video recording without dropping frames. Having a card with a higher video speed class than required by your camera can provide extra headroom.


[1] https://www.sdcard.org/developers/sd-standard-overview/speed-class/


The main difference between SD, SDHC, and SDXC cards is the storage capacity they offer. SD cards have a maximum capacity of 2GB. SDHC cards can store between 4GB and 32GB. SDXC cards have a much higher maximum capacity starting at 64GB going up to 2TB.

SD stands for Secure Digital and was the original format that was introduced in 1999. As technology advanced and higher capacity cards were needed, SDHC (Secure Digital High Capacity) and SDXC (Secure Digital eXtended Capacity) were introduced in 2006 and 2009 respectively.

While the cards may look similar physically, they use different filesystems internally to support the higher capacities. SD cards use FAT12 or FAT16. SDHC uses FAT32 filesystem. SDXC uses the exFAT filesystem.[1]

SD, SDHC, and SDXC cards are backwards compatible so you can use a high capacity card in a device that only supports lower capacities. The device will simply be unable to access data beyond its max capacity. However, you can’t use a lower capacity card like SD in a device expecting SDXC.

In summary:

  • SD – up to 2GB capacity
  • SDHC – 4GB to 32GB capacity
  • SDXC – 64GB to 2TB capacity

Higher capacity cards use different filesystems and are backwards compatible in devices supporting lower capacities.

microSD Cards

microSD cards are a smaller physical size version of standard SD cards. Despite the size difference, microSD cards utilize the same terminology for types, speed classes, and symbols as standard SD cards. For example, they come in microSD (SDSC), microSDHC, and microSDXC types to indicate capacity ranges. Speed classes like Class 10 and UHS Speed Class 1 are also used. And bus interface symbols like UHS-I and Video Speed Class symbols are present as well. This allows microSD cards to be fully compatible with devices designed for standard SD cards when used with a simple physical adapter.

The main advantages of microSD cards are their tiny size, making them well-suited for small devices like phones and tablets. However, their small physical size can also make them easier to lose and more difficult to handle. Performance is generally comparable to standard SD when used in an adapter. So for most purposes besides fitting into the smallest devices, standard SD cards are likely the better choice.


The CPRM symbol stands for Content Protection for Recordable Media, which is a type of DRM (digital rights management) technology that prevents unauthorized copying and sharing of copyrighted content stored on SD cards. The SD Association developed CPRM to enable secure distribution of music, video, and other media while still allowing consumers to make copies for personal use.

When you see a CPRM logo on an SD card, it means the card is equipped with CPRM technology to protect files from unauthorized duplication. CPRM uses encryption and other security protocols to control access to copyrighted data. It allows content providers to specify how files can be used, such as setting limits on how many copies can be made.

In practice, CPRM is mainly used to protect commercial media like audio and video files. Consumers typically won’t notice any impact from CPRM on SD cards used for everyday photo and document storage. But CPRM does enable more secure distribution of copyrighted materials on SD cards while still granting end users reasonable rights.

MB/s vs Mb/s

There is an important distinction between MB/s (megabytes per second) and Mb/s (megabits per second). While they may look similar, they measure different things.

MB/s stands for megabytes per second, and it measures data transfer speed or storage capacity. 1 megabyte (MB) = 8 megabits (Mb). Megabytes are used to measure the speed of storage devices like hard drives and SSDs, as well as data transfer speeds over networks.

Mb/s stands for megabits per second, and it measures the speed of data transfer over networks. 1 megabit (Mb) = 1/8 megabyte (MB). Mb/s is used to measure network connection speeds like broadband, 4G, WiFi, and Ethernet.

To convert between the two:

  • 1 MB/s = 8 Mb/s
  • 1 Mb/s = 0.125 MB/s

So a 100 Mb/s internet connection speed is equivalent to 12.5 MB/s data transfer speed. This distinction is important when comparing network connection speeds with storage drive speeds.


In summary, the numbers and symbols on SD cards indicate important information about the card’s storage capacity, speed, and compatibility. The main factors to consider when selecting an SD card are:

  • Storage capacity – Higher capacity cards offer more storage but are more expensive. Consider how much storage you need.
  • Speed class and rating – Faster cards have higher speed classes and ratings. Choose a card with sufficient speed for your device.
  • Bus interface – Cards are either SD, SDHC, or SDXC. Check which interface your device supports.
  • Form factor – Standard SD cards are larger than microSD. Ensure you get the right physical size.

When buying an SD card, check the manufacturer ratings and your device requirements. A higher speed or capacity card may provide better performance and future-proofing but is not always necessary. Consider when you’ll likely upgrade devices. With storage needs increasing over time, it can help to buy more capacity than you need today if the price difference is small.

To recap, focus on matching the SD card’s specifications to your device’s capabilities and storage requirements. Checking the markings on the card and reading reviews can help ensure you select a reliable, high-quality card that provides good value for money.