What is a class SD card?

SD cards, which stands for Secure Digital cards, are a type of removable flash memory card used for storing digital information. There are different classes of SD cards that denote the speed and performance capabilities of the card.

SD Card Classes Explained

SD cards are given speed class ratings that indicate the minimum guaranteed speeds that the card can achieve. Higher speed classes typically have faster performance and are better suited for certain applications like high resolution video recording.

The main classes for SD cards are:

  • Class 2 – Minimum sequential write speed of 2 MB/s
  • Class 4 – Minimum sequential write speed of 4 MB/s
  • Class 6 – Minimum sequential write speed of 6 MB/s
  • Class 10 – Minimum sequential write speed of 10 MB/s
  • UHS Speed Class 1 – Minimum sequential write speed of 10 MB/s
  • UHS Speed Class 3 – Minimum sequential write speed of 30 MB/s

In addition to the speed class ratings, SD cards can also have a UHS speed class rating:

  • UHS Speed Class 1 – Minimum sequential write speed of 10 MB/s
  • UHS Speed Class 3 – Minimum sequential write speed of 30 MB/s

The UHS speed classes denote ultra high speed capabilities beyond the standard speed classes. UHS-I cards support up to 104 MB/s while UHS-II cards can handle up to 312 MB/s.

Class 2

Class 2 SD cards have a minimum write speed of 2 MB/s. They are an older and lower class of SD card that is not commonly used anymore. Class 2 cards are not ideal for recording video or other write-intensive operations.

Some uses for Class 2 SD cards include:

  • Basic point-and-shoot digital cameras
  • Low resolution digital video cameras
  • Audio recording devices

Class 2 cards lack the speeds necessary for full HD or 4K video recording or burst mode photography. Their slower speeds also result in longer buffer clearing times after bursts of images are taken.

Class 4

Class 4 SD cards can sustain a minimum sequential write speed of 4 MB/s. This makes them faster than a Class 2 card. They are considered acceptable for standard definition video recording.

Typical uses for Class 4 SD cards include:

  • Standard definition camcorders
  • Mid-range point-and-shoot cameras
  • Some entry-level DSLR cameras

While Class 4 cards are generally outdated, they can still be usable in devices that do not require high speed memory cards. However, for full HD video recording and burst mode photography, a Class 4 card is still rather slow.

Class 6

Class 6 SD cards offer a minimum sequential write speed of 6 MB/s. This makes them even faster than Class 2 and Class 4 cards. They are considered fast enough for some full HD video recording at 1920×1080 resolution.

Typical devices that can leverage Class 6 SD cards are:

  • Higher end point-and-shoot cameras
  • Entry-level to mid-range DSLR cameras
  • Mirrorless cameras
  • Full HD camcorders

While Class 6 cards are adequate for some full HD video, they may not be fast enough for extended recording sessions, especially with high bitrate data. Memory buffer clearing also can be slower than higher speed classes.

Class 10

Class 10 SD cards are a major jump up in speed from the previous classes. They guarantee a minimum write speed of 10 MB/s. This makes them fast enough for full HD video and moderate 4K video recording.

Class 10 cards are commonly used in:

  • Advanced point-and-shoot cameras
  • DSLR cameras
  • Mirrorless cameras
  • Full HD and 4K camcorders
  • High resolution sports photography

The increased speeds of Class 10 cards allow them to handle large image files from burst mode shots without slowing down. They also enable extended lengths of full HD and 4K video to be recorded without dropping frames or having the recording stop prematurely if a slower card is used.

UHS Speed Class 1

UHS Speed Class 1 SD cards also maintain a minimum sequential write speed of 10 MB/s, similar to a Class 10 card. The main difference is the UHS classification guarantees they can achieve higher read/write speeds when used in UHS-I bus devices.

UHS-I cards support extra data lanes that increase the maximum possible speeds over the standard interface. This allows UHS Class 1 cards to often achieve speeds over 20 MB/s in UHS-I compatible devices. Uses can include:

  • DSLR cameras
  • High resolution video recording
  • Burst mode photography
  • High bitrate 4K video

The UHS interface allows the card to better handle professional level cameras and camcorders that need fast read/write speeds when recording large image and video files.

UHS Speed Class 3

UHS Speed Class 3 SD cards are the fastest main speed class available today. They guarantee minimum sequential write speeds of 30 MB/s. They fully leverage UHS-I interfaces to reach speeds between 30-90 MB/s.

Speed Class 3 cards are ideal for:

  • Professional DSLR and mirrorless cameras
  • High bitrate and high resolution 4K video
  • Burst photography in sports and action scenarios
  • Recording 8K video

The blazing fast speeds of UHS Class 3 cards enable professional cameras and camcorders to capture rapid fire bursts of large images without any performance lag or bottlenecking from the memory card. These cards provide the best performance for demanding photographers and videographers.

SD Card Class Comparisons

To help summarize and compare the different SD card classes, here is an overview of the minimum write speeds and typical use cases for each:

Class Minimum Write Speed Typical Uses
Class 2 2 MB/s Basic cameras, audio recorders
Class 4 4 MB/s Standard definition video
Class 6 6 MB/s Full HD video
Class 10 10 MB/s Full HD/4K video, action photography
UHS Class 1 10 MB/s 4K video, burst mode photography
UHS Class 3 30 MB/s Professional cameras, 8K video

As you move up in speed class, the cards can handle more intensive workloads like video recording at higher resolutions and bitrates. The fastest UHS Class 3 cards are optimal for professionals needing to capture high resolution images and video.

Physical Size Differences

In addition to speed class differences, SD cards also come in different physical sizes. The SD card form factor was originally the size of a postage stamp at 24mm x 32mm x 2.1mm.

MiniSD and MicroSD cards were later introduced to provide even more compact options. Adapter cards are available to convert the mini and micro cards to full size SD for use in SD compatible devices.

The different physical SD card sizes include:

  • Standard SD – 24mm x 32mm x 2.1mm
  • MiniSD – 21.5mm x 20mm x 1.4mm
  • MicroSD – 15mm x 11mm x 1mm

In most cases, the mini and micro versions of the cards have the same speeds and classifications as full size SD cards, just in a smaller physical footprint. Pay attention to the speed class markings to determine the performance capabilities.

SD, SDHC, SDXC: What’s the Difference?

In addition to size and speed differences, SD cards are available in three different storage standards and capacities:

  • SD – Up to 2GB capacity
  • SDHC – Between 4GB and 32GB capacity
  • SDXC – Between 32GB and 2TB capacity

The SDHC (Secure Digital High Capacity) and SDXC (Secure Digital eXtended Capacity) standards were introduced to support higher storage capacities beyond the original SD specification. They utilize different file systems to properly handle the increased capacities.

When it comes to speed, SD, SDHC, and SDXC cards can come in all the same speed classes. The speed class relates to performance, while the specification relates to storage capacity and file system used.

Choosing the Right Class for Your Needs

When shopping for an SD card, the most important factors are the speed class and storage capacity. Higher speed cards ensure better performance when recording video or taking burst photos.

For basic point-and-shoot cameras, Class 10 cards offer a good balance of speed and affordability. For professional use, UHS-3 Class 10 cards provide the fastest speeds to handle high resolution images and video.

In terms of capacity, 32GB to 64GB is a safe range for most users. Higher capacities up to 256GB may be required for 8K video recording and other professional needs. With the SD card classes and capacities sorted out, you can find the right SD card for your camera or device.


SD cards have evolved a long way since the original standard defition cards. Today, blazing fast UHS-3 SD cards are capable of recording professional level 8K video thanks to ultra high speeds over 90 MB/s.

By understanding the different speed classes and capacities available, you can pick the right SD card to provide the performance your camera or device needs. Pay attention to the speed class in particular to ensure the card can handle your desired recording quality and video resolutions.