What do the classes of SD cards mean?

SD cards, short for Secure Digital cards, are a popular type of memory card used in digital cameras, smartphones, tablets, and other devices. SD cards come in different classes that designate their speed capabilities. The class rating is an important factor to consider when choosing an SD card for your device.

SD Card Classes Explained

The SD Association, the organization that sets standards for SD cards, defines several speed classes for SD cards based on their minimum guaranteed speeds:

Class Minimum Speed
Class 2 2 MB/s
Class 4 4 MB/s
Class 6 6 MB/s
Class 10 10 MB/s
UHS Class 1 (U1) 10 MB/s
UHS Class 3 (U3) 30 MB/s

The class number represents the minimum write speed that card guarantees, measured in megabytes per second (MB/s). So a Class 10 card has a minimum write speed of 10 MB/s, while a UHS Class 3 card has a minimum write speed of 30 MB/s.

The speed class gives you an idea of the card’s transfer rate performance. However, the actual speeds can often exceed the class rating significantly. Two cards with the same rating may have very different actual speeds.

Class 2

Class 2 is the lowest class designed for standard definition video recording. Class 2 cards have a minimum write speed of 2 MB/s. This is too slow for most uses today, so Class 2 cards are now obsolete.

Class 4

Class 4 SD cards have a minimum speed of 4 MB/s. They can support standard definition video recording but are still too slow for high definition video. Class 4 cards are outdated and not commonly used anymore.

Class 6

Class 6 cards have a guaranteed minimum write speed of 6 MB/s. They can record standard definition and some high definition video. However, for full HD video recording and advanced photography, Class 6 cards are inadequate.

Class 10

Class 10 is currently the most popular class of SD card for general use. Class 10 cards have a guaranteed minimum write speed of 10 MB/s, though they often have faster speeds. This makes them fast enough for full HD video recording as well as burst mode photography.

UHS Class 1 and 3

UHS (Ultra High Speed) cards are designed for intense workloads like burst mode photography and 4K video recording. UHS Class 1 cards have a minimum speed of 10 MB/s, while UHS Class 3 cards have a minimum of 30 MB/s.

Only certain cameras and devices are compatible with UHS cards. Many devices can use UHS cards as standard cards, but will not reach UHS speeds unless the device supports it.

Speed Classes Explained

In addition to the SD card classes, there are also video speed classes:

Speed Class Minimum Speed
V6 6 MB/s
V10 10 MB/s
V30 30 MB/s
V60 60 MB/s
V90 90 MB/s

The video speed class indicates the minimum write speed when recording video. A V30 card, for example, has a minimum write speed of 30 MB/s when recording video.

There are also UHS speed classes:

UHS Speed Class Minimum Speed
UHS Speed Class 1 (U1) 10 MB/s
UHS Speed Class 3 (U3) 30 MB/s

The UHS speed rating is designed for 4K video recording and burst photography. UHS Speed Class 3 (U3) cards have a minimum 30 MB/s write speed in these scenarios.

Reading the Labels

SD card labels can list multiple ratings that indicate the card’s capabilities:

  • Speed class (2, 4, 6, 10)
  • UHS speed class (U1, U3)
  • Video speed class (V6, V10, V30)

For example, a card might be labeled:

UHS-I / V30 / U3 / Class 10

This means it meets the requirements for:

  • UHS-I interface
  • V30 video recording (30MB/s minimum)
  • U3 UHS speed class (30MB/s minimum)
  • Class 10 standard (10MB/s minimum)

Recommendations by Use Case

Here are the SD card class recommendations for different types of devices and uses:

Digital cameras

  • Basic point-and-shoot: Class 10 or UHS-I U1
  • Enthusiast compact & mirrorless: UHS-I U3
  • High-end DSLR: UHS-II U3

Action cameras

  • 4K video: UHS-I U3
  • Slow motion video: UHS-II V30/V60/V90


  • HD video: Class 10 or UHS-I U1
  • 4K video: UHS-I U3


  • Most phones: UHS-I U1
  • High-end phones: UHS-I U3


  • 1080p video recording: UHS-I U1
  • 4K video recording: UHS-I U3


  • Nintendo Switch: UHS-I U1
  • Steam Deck: UHS-I U3 or A2

SD Cards and Devices

When choosing an SD card, it’s important to consider your device’s capabilities. The device needs to support the card’s speed class to take full advantage of it.

Many devices specify which SD card classes are recommended:

  • Entry-level DSLR cameras often recommend UHS-I U1 cards.
  • Advanced mirrorless cameras work best with UHS-II U3 cards.
  • Mid-range action cameras support up to UHS-I U3.
  • High-end action cameras can utilize V90 cards.

Consult your device’s manual or specifications to see which SD card speed classes it can utilize. Buying a card that exceeds your device’s capabilities is wasteful.

SD Cards and Capacities

In addition to speed class, SD cards come in different storage capacities. Capacity indicates how much data the card can hold, typically ranging from 4GB up to 1TB for high capacity cards.

A higher capacity card allows you to store more photos, videos, music, and other files. However, high capacity cards are more expensive. Choose the smallest capacity that can adequately meet your storage needs.

For example, a 32GB card may be sufficient for a point-and-shoot camera but too small for a DSLR used for 4K video. Think about your usage – more capacity gives you more flexibility before needing to offload files.


SD card speed classes indicate the guaranteed minimum performance. Higher classes mean better transfer speeds and ability to handle advanced workloads.

Make sure to choose a class rating that matches your device specs and usage needs. The class alone doesn’t determine real-world speeds, but it’s a good baseline indicator of what the card can handle.

Beyond speed class, also consider overall card performance, reputation of the brand, warranty policies, and capacity for determining the right SD card for your needs.

Using the appropriate SD card class allows your device to operate at its full potential so you can smoothly capture high quality photos, video, and other content.

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