SD and microSD cards of various speed classes

A Guide to Speed Classes for SD and microSD Cards

The SD Association came up with a way to standardize the speed ratings for memory cards. They are known as Speed Classes and refers to the absolute minimum sustained write speed for both SD and microSD cards. There are three types of speed class:

  • Speed Class
  • UHS Speed Class
  • Video Speed Class
Speed ClassUHS Speed ClassVideo Speed Class
"Speed Class" Mark "UHS Speed Class" Mark "Video Speed Class" Mark

Numbers with a circular "C" symbol, "U" symbol or a "V" symbol next to or around it indicates the speed classes of the memory card. Manufacturers use these speed class symbols to indicate the type of speed class and rating on the memory card. They are intended to help consumers choose the right memory card for their hardware devices in terms of speed.

Speed Class

The very first speed class is simply known as the original speed class but since they’re denoted with a "C" symbol, let’s refer to it as C-Class for the rest of this guide. There are four ratings within the C-Class:

  • C2 (Class 2): minimum write speed of 2MB/s
  • C4 (Class 4): minimum write speed of 4MB/s
  • C6 (Class 6): minimum write speed of 6MB/s
  • C10 (Class 10): minimum write speed of 10MB/s

Minimum sequential write speedSpeed ClassCorresponding video format
Speed Class UHS Speed
Video Speed
Class (new)
8K video 4K video Full HD /
HD video
Card image The necessary speed varies by each recording/playback device condition, even in the same format.

C2 is the slowest speed class while C10 is the industry standard. Because of newer hardware devices requiring newer speed classes with higher speeds and capabilities, hardly anyone uses Class 2–6 nowadays. C10 is usually the minimum speed class requirement for most hardware devices made today.

UHS Speed Class

The next speed class up is the UHS (Ultra-High Speed) Speed Class and it’s denoted with the “U” symbol. There are two ratings within the UHS Speed Class:

  • U1 (UHS Speed Class 1): minimum write speed of 10MB/s
  • U3 (UHS Speed Class 3): minimum write speed of 30MB/s

The UHS Speed Class is more commonly used nowadays than the Speed Class and many of the high-end cameras require at least a U3-rated memory card for many of its functions like recording high-resolution videos. The UHS Speed Class mainly refers to the minimum sustained writing performance for recording videos and came about due to 4K-capable video recording devices needing faster write speeds. As a rule of thumb, 4K-capable recording cameras will usually require at least a U3-rated SD card.

What makes the U1 and U3 memory cards more advanced than those in the Speed Class are that they use one of two UHS bus interfaces:

  • UHS-I: theoretical maximum transfer speeds up to 104MB/s
  • UHS-II: theoretical maximum transfer speeds up to 312MB/s

Both U1 and U3 memory cards can utilize the UHS-I bus interface but are not compatible with the UHS-II bus interface.

These UHS bus interfaces indicate the theoretical maximum read and write speeds unlike the sustained write speeds of speed classes. The UHS bus interfaces are denoted with a Roman numeral “I” or “II” symbol on the front of the card. The bus speeds refer to the theoretical data transfer rate of the interface itself while a U3-rated SD card has its own sustained write speed of 30MB/s. For example, a UHS-I U3-rated card guarantees a write speed of 30MB/s but has the potential for a read and write speed of up to 104MB/s if used with a device that supports a UHS-I bus interface.

A UHS-II compatible card has a potential read and write speed of up to 312MB/s. The UHS bus interfaces are backwards-compatible so you can use a UHS-II card in a device that supports UHS-I, but you won’t see the speed benefits of UHS-II as the card will default back to the lower specs of UHS-I. Both the card and bus interface must be fully compatible to experience the speed benefits.

Video Speed Class

The latest Video Speed Class was created to enable higher video resolution and recording features such as multiple video streams, 360-degree capture, VR content, and 4K or 8K resolution videos. They’re usually denoted with the “V” symbol. There are five ratings for the Video Speed Class:

  • V6 (Video Speed Class 6): minimum write speed of 6MB/s
  • V10 (Video Speed Class 10): minimum write speed of 10MB/s
  • V30 (Video Speed Class 30): minimum write speed of 30MB/s
  • V60 (Video Speed Class 60): minimum write speed of 60MB/s
  • V90 (Video Speed Class 90): minimum write speed of 90MB/s

The Video Speed Class is unique because it’s capable of utilizing both the UHS-I and UHS-II bus interfaces. V6 to V90 speed class memory cards can use the UHS-II bus interface, but the UHS-I bus interface can only support V6 to V30 speed class memory cards.

The Video Speed Class offers the fastest speeds available and is ideal for ultra-high-resolution videos, high-quality videos and multi-file recording in drones and 360-degree cameras. It supports HD formats up to 8K video in drones, 360-degree cameras, action cams and VR cameras.

You can find the best card for your hardware device by choosing the same speed class or higher that’s required for your device. For example, if your device requires a Class 4 memory card, you can use Speed Class 4, 6, or 10. If your device requires a UHS Speed Class 1 card, you can use UHS Speed Class 1 or 3. The same functionality applies to the Video Speed Class as well. Note, using a higher-rated card that goes beyond the speed class requirement for a device will still work, but you won’t be experiencing the full benefits of the higher speed class since the device only supports the lower speed class.


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