What’s the difference between micro SD card classes?

Micro SD cards, also known as microSD or μSD, are a type of removable flash memory card used for storing data in portable devices like smartphones, tablets, and cameras. Micro SD cards come in different speed classes that indicate their minimum guaranteed read and write speeds.

The speed class rating printed on micro SD cards represents the minimum read and write speeds they are guaranteed to reach. Higher speed classes allow you to transfer data like photos, videos, apps, and other files faster. When shopping for a micro SD card, it’s important to match its speed class rating with your device and usage needs.

There are several micro SD card speed classes defined by the SD Association. The main classes are Speed Class 2, 4, 6, and 10, UHS Speed Class U1 and U3, and SD Express. Within these speed ratings there are alsoperformance tiers, with more advanced cards able to reach faster minimum speeds. Understanding the differences helps ensure you buy a micro SD card with suitable capabilities.

SD and microSD Speed Classes

The SD Association has defined several speed classes and categories for full-size SD cards and micro SD cards to indicate their performance capabilities:

  • Speed Class 2 – Minimum write speed of 2MB/s
  • Speed Class 4 – Minimum write speed of 4MB/s
  • Speed Class 6 – Minimum write speed of 6MB/s
  • Speed Class 10 – Minimum write speed of 10MB/s
  • UHS Speed Class 1 (U1) – Minimum write speed of 10MB/s
  • UHS Speed Class 3 (U3) – Minimum write speed of 30MB/s
  • SD Express – Leverages PCI Express and NVMe interfaces for 985MB/s+ speeds

Cards with higher speed class ratings offer improved performance and transfer speeds. Class 2 cards have a minimum write speed of 2MB per second while Class 10 cards have a guaranteed minimum write speed of 10MB/s. The fastest SD cards available today use the SD Express standard capable of 985MB/s.

Speed Class 2

Speed Class 2 micro SD cards have a minimum write speed of 2MB/s and are labeled with a 2 in a circle. They are ideal for standard definition video recording and basic phone applications. Class 2 cards provide adequate performance for simple tasks like viewing documents, listening to music, and taking photos.

While Class 2 is the slowest SD card speed class, it is sufficient for entry-level uses like in budget smartphones and basic digital cameras. The cards are not suitable for HD video recording or transferring large files. For advanced functionality, higher micro SD speed classes are recommended.

Speed Class 4

Speed Class 4 cards have a minimum sustained write speed of 4MB/s and are marked with a 4 in a circle. Class 4 micro SD cards are recommended for recording standard definition video and basic phones. They can capture up to 15 frames per second video. Some older midrange digital cameras may also use Class 4 cards.

While faster than Class 2, Class 4 micro SD cards have limited capabilities and are outdated for many devices today. They do not support HD video recording or efficiently running apps and games on mobile devices. Class 4 cards are better suited for simple, non-demanding tasks.

Speed Class 6

Micro SD cards with a Class 6 rating offer a minimum sustained write speed of 6MB/s and are identified by a 6 in a circle. These cards support recording HD 720p video at 30fps along with quick file transfers. Many point and shoot cameras use Class 6 cards for improved functionality over lower classes.

In addition to 720p video recording, Class 6 micro SD cards work well for mid-range smartphones and basic digital cameras. They provide good performance for reading photo files, loading apps, and transmitting data quickly. Class 6 strikes a balance between price and capabilities for general use SD cards.

Speed Class 10

Speed Class 10 represents cards with a minimum 10MB/s sustained write speed and UHS-I interface. Class 10 micro SD cards are denoted by a 10 symbol. Performance-wise they are a large jump up from Class 6, capable of recording full HD 1080p video at 30fps.

Class 10 is the minimum SD card class recommended for HD camcorders and advanced photography. The faster speeds allow you to store high resolution photos quickly without the camera buffer filling up. Class 10 micro SD cards have the necessary performance for many action cams, drones, Android devices, and DSLR cameras.

UHS Speed Class 1 (U1)

UHS Speed Class 1 (U1) is designed for Ultra High Speed bus interfaces and sustain a minimum 10MB/s write speed. While also Class 10, U1 micro SD cards support full HD video with high resolution picture recording. Cards meeting the U1 standard are rated for real-world minimum speeds of 10MB/s.

U1 SD cards are ideal for compact to mid-range point and shoot cameras, mirrorless cameras, drones, camcorders, and 1080p video recording. They provide good performance for both continuous shoots and HD video capture. U1 represents a well-balanced choice suitable for advanced functions.

UHS Speed Class 3 (U3)

UHS Speed Class 3 (U3) cards represent the best consumer performance available in the UHS speed class designations. They guarantee a minimum sustained write speed of 30MB/s and interface using UHS-I technology.

U3 micro SD cards support 4K Ultra HD video recording at 30fps along with continuous burst mode photography. Devices like high resolution sports cams, mirrorless and DSLR cameras, and Android smartphones and tablets benefit from U3’s fast speeds.

For professional use and recording high quality 4K video, U3 micro SD cards deliver the performance needed. They offer a cost-effective storage solution versus CF or XQD memory cards.

SD Express

SD Express combines the SD interface with PCI Express and NVMe technology using two lanes for transfer speeds up to 985MB/s. The SD 7.0 specification introduces SD Express cards and microSD Express cards with blazing performance capabilities.

At nearly 1GB/s read and write speeds, SD Express cards are designed for professional use cases like cinema quality 8K video. They offer speeds rivaling more expensive CFast 2.0 and XQD cards. SD Express brings dramatic speed improvements to the SD card standard.

SD Express micro SD cards require compatible host devices and consumers will not experience their full speeds. But SD Express represents the future direction delivering accelerated performance using mature, proven technologies.

Speed Class Comparison Table

Class Minimum Write Speed Capabilities
Class 2 2 MB/s Basic phone usage, SD video
Class 4 4 MB/s Entry-level digital cameras, SD video
Class 6 6 MB/s 720p HD video recording
Class 10 10 MB/s Full HD 1080p video, mid-range cameras
UHS Class 1 (U1) 10 MB/s Full HD video, mid-range & mirrorless cameras
UHS Class 3 (U3) 30 MB/s 4K Ultra HD video, advanced cameras
SD Express 985+ MB/s Professional 8K video, high resolution cameras

Choosing the Right Speed Class

When shopping for a new micro SD card, the speed class provides an indication of its performance capabilities. Consider the following factors when deciding on a suitable speed class rating:

  • Device Support – Check your device’s manual for the recommended micro SD card class. Using a lower class may result in poor performance.
  • Usage Needs – Assess how you plan to use the card. Recording 4K video requires U3 class, while standard photos and media are fine with Class 10.
  • Future Proofing – Getting a faster UHS class card than currently needed allows room to upgrade devices later.
  • Price – Faster cards cost more. Balance speed class with your budget constraints.

While higher speed micro SD cards are backwards compatible with devices, you won’t get their full speeds unless your device supports the faster standard. Avoid using a lower class card than recommended or you may experience issues.

SD Card Labeling

Micro SD card speed class ratings, capacity, bus interface, and other details are labeled on the card itself for easy identification. A Class 10 UHS-1 card displays its class symbol, UHS-I designation, and rated speeds.

When inserting the micro SD card, align the label side facing up according to device directions. SD cards may have a label covering their metal contacts – leave this in place to avoid damaging the card from static discharge.

Other Performance Considerations

In addition to read and write speeds, micro SD cards are further segmented by their performance tier or UHS speed bus rating. These indicate capabilities beyond just the minimum speed class.

UHS Bus Ratings

UHS-I cards support up to 104 MB/s speeds. UHS-II cards reach up to 312 MB/s for double the bandwidth over UHS-I. UHS-III standard delivers up to 624MB/s. Cards will be labeled as UHS-I, UHS-II, or UHS-III for the bus interface used. Matching the device’s UHS rating ensures maximum performance.

Speed Class U1 and U3

As faster versions in the Class 10 standard, U1 and U3 SD cards have higher sustained write speeds for improved real-world performance. Their speeds are tested under full card conditions versus empty cards.

U1 represents a 10% enhancement over Class 10 while U3 delivers 4x higher performance. Choosing U1 or U3 provides a further speed differentiation at Class 10 for better complex workload handling.

Video Speed Class

The Video Speed Class defines a minimum write speed for video recording, reflected in MB/s. V6, V10, V30, V60, and V90 correspond to Classes 6, 10, U1, U3, and above. A V30 card is suitable for 4K video capture.

Video Speed Class uses MB/s for minimum sequential writes to simplify matching a card to video recording needs. It complements the speed class symbols for general performance.


Micro SD cards with faster speed classes and ratings provide improved performance and capabilities. They allow you to utilize advanced functionality like HD video recording, gaming, live video streaming, and burst mode photography based on their level. When purchasing a new micro SD card, match its speed class and rating to your device specs and intended use for optimized reliability and transfer speeds.