Memory cards: A guide to speed classes for SD and microSD Cards

A guide to speed classes for SD and microSD Cards


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Dec 2020

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The SD Association came up with a way to standardise the speed ratings for memory cards. They are known as Speed Classes and refer 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 Class UHS Speed Class Video 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 them indicate 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 and is denoted with a “C” symbol. There are four ratings within the Speed 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 speed Speed Class Corresponding 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. Since newer hardware devices require 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 and is the most shipped memory card on the market.

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 high-end cameras require at least a U3-rated memory card for many of its functions, such as recording high-resolution videos. The UHS Speed Class mainly refers to the minimum sustained write 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 utilise 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 by 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 utilising 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 than the one 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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Memory cards. Which one to choose?

Memory cards. Which one to choose?

Why you shouldn’t take a slow card

  • Video recording creates corrupted files or downgrades video recording to match card speed.
  • When you press the shutter button, the camera does not immediately allow you to take a second photo (recording to the memory card).
  • The camera freezes, the video suddenly breaks when the maximum recording speed of the memory card is reached, creating videos that are short in time.
  • SD cards (Secure Digital)0003

    This is the most common type of memory card. We will dwell on it in more detail.

    available 2 variations :

    SDHC – cards up to 32 GB ;

    SDXC – cards up to 2TB .

    Speed ​​class (“C”) indicates the maximum possible write speed. It happens 2, 4, 6, 10 class . The class number points to is the minimum allowed write speed for the memory card in megabytes per second.

    Also introduced for SD cards standard UHS (Ultra High Speed) – high-speed communication protocol.

    Now there are three versions of the UHS standard:

    UHS-I – transmits data at speeds up to 100 Mb / s;

    UHS-II – Transfers data at up to 312 Mbps;

    UHS-III – Transfers data at speeds up to 624 Mbps.

    If the second version allows you to record 4K video, then with the third version you can already record 8K and 360 !

    Inside UHS there are also speed classes :

    Class 1 (U1) – from 10 MB / s;

    Class 3 (U3) – from 30 MB/s.

    Last speed class Video Speed ​​Class was created to support higher resolution video. Such memory cards are usually marked with a “V”.

    There are 5 categories in the Video Speed ​​Class:

    V6 (Class 6) : minimum recording speed 6MB/s – allows you to record HD video;

    V10 (Class 10) : 10MB/s minimum write speed – allows you to record Full HD video;

    V30 (Class 30) : 30MB/s minimum write speed – allows 4K recording at 60/120 fps;

    V60 (Class 60) : minimum write speed 60MB/s – allows you to record 8K at 60/120 fps;

    V90 (Class 90) : 90MB/s minimum write speed – allows you to record 8K at 60/120 fps.

    SD cards can also use the card speed designation as a multiplier of , for example 133x. In this case, 1x corresponds to the speed of 150 Kb / s.

    Unfortunately, this standard does not specify what speed the memory card means: writing or reading. Therefore, manufacturers often indicate a read speed that is greater than the write speed.

    to exFAT which does not have this limitation.

    Another important point: SDHC memory cards use the FAT32 file system, the maximum file size on which is 4 GB. Therefore, when recording, the maximum length of one clip will be limited.

    SDXC, in turn, uses the exFAT file system, which does not have this limitation.

    Micro SD

    MicroSD cards are used in certain cases (for example, when the device is small).

    The cards also have 2 variations:

    microSDHC – drives up to 32 GB, work on devices with SDHC and SDXC support;

    microSDXC – drives up to 2 TB, work only on devices that support SDXC.

    These cards have all designations and standards that correspond to SD cards. In addition, with the help of a Micro SD adapter, it is easy to turn it into an SD card.

    There are also faster memory cards – CFexpress Type-A and Type-B , СFast , XQD . These cards are used in photo and video cameras that are more demanding on recording speed. In such as, for example: Canon R5 , Sony A7S III , Nikon Z7 II .

    We will talk about them in a separate article.

    CFexpress – XQD – CFast – CF


    Divided into three types: A, B and C .

    Type A.

    The smallest card in the CFexpress family.

    Its dimensions are:

    20mm x 28mm x 2.8mm.

    The maximum theoretical write speed can reach 1,000MB/s.

    Currently used in new Sony cameras, for example: A7SIII and A1

    Type B.

    The most common card in the 9 family0005 CFexpress .

    Its dimensions are:

    38.5mm x 29.6mm x 3.8mm. They are identical to the size of XQD cards, and therefore, with firmware, manufacturers added the ability to use CFexpress cards in XQD slots, for example: Nikon Z6 and Z7.

    The maximum theoretical write speed can reach 2,000MB/s.

    Used in most cameras with CFexpress slot:

    Canon EOS R5, Nikon Z7II, Panasonic S1R, etc.

    Type C.

    Largest card in the CFexpress family.

    Its dimensions are:

    54mm x 74mm x 4.8mm. Designed to work with computers and SSDs.

    The maximum theoretical write speed can reach 4,000MB/s.

    In order not to get confused on the cards, the type of the card is always indicated.


    A memory card format developed by SanDisk, Sony, and Nikon and approved by the CompactFlash Assotiation in 2010-2011. As mentioned earlier, this format is identical to CFexpress Tybe B, both in size and contact group.

    The interface bandwidth is 2.5Gbps , the current write speed is up to 125MB/s

    The format is licensed by Sony, cameras with XQD connector are manufactured by Nikon.

    CompactFlash (CF)

    Brand name for one of the first flash memory card formats. The format was developed by SanDisk Corporation in 1994.

    Despite their age, cards of this format are still popular in photographic equipment thanks to record speed and capacity. In 2014, the maximum capacity of CompactFlash drives reached 512 GB .

    There are two types of cards: CompactFlash Type I and Type II . The Type II card variant was created exclusively for miniature Microdrive hard drives and may be considered obsolete today. The dimensions of CompactFlash Type I cards are 42 mm by 36 mm, the thickness is 3. 3 mm, CompactFlash Type II is 5 mm.

    Cards CompactFlash Type I can be inserted into slots of both sizes , CompactFlash Type II – only in slot for CompactFlash Type II .


    CFast is an evolution of the CompactFlash standard on the SATA bus.

    CFast cards are not compatible with CF, have a standard 7-pin SATA connector and a 17-pin power connector.

    CFast 2.0 – the second generation of cards. Recording speed 3400× (510 MB/s) . In 2016, Canon built an additional memory card slot of this standard into its Canon EOS-1D X Mark II camera, replacing the duplicating CompactFlash slot with it.

    Memory card size

    How many photo and video files will fit on the device depends on the choice of volume.

    For example, with a single photo size of 5 MB and a memory card with 1 GB of memory, about 200 photos will fit.

    Also, if you shoot 4K video, then 1 second in this quality can take at least 10 MB, that is, approximately 1 hour of shooting will fit on 32 GB.

    If you know that you have heavy files, we recommend choosing a memory card from 128 GB so that there are no unpleasant “surprises” with unexpectedly running out of space during the shooting.

    Speed ​​load

    Memory card load is the maximum data rate

    (bitrate), which the device outputs. It should not exceed its maximum write speed.

    To understand how fast your memory card can withstand when taking photos, check the camera’s specifications. When recording a video, the data stream can be viewed in the properties of the video file.

    Remember : the higher the class of the camera, the greater the load on the memory card. However, when choosing memory cards, it should be borne in mind that most of the more or less serious models have the so-called burst buffer . Technically, this is a built-in volatile memory that takes on photos, and only then they are written to a memory card.

    To avoid high speed load, we recommend taking a card no lower than 10 class .


    Delkin Devices Power SDXC 256GB 2000X UHS-II Class 10 Memory Card [DDSDG2000256] In stock
    40560 Y


    6990 Y

    Transcend TS128GUSD500S microSDXC 128Gb memory card In stock
    6990 Y



    SONY Memory Card SONY CFexpress Type A Memory Card 80 GB CEA-G CEA-G80T In stock
    15990 Y

    27990 Y

    Memory card Sony CEB-G128 CFexpress 128GB Type B out of stock
    27990 Y

    5990 Y

    SanDisk Extreme Pro SDXC UHS-I Class 3 V30 Memory Card 170/90 MB/s 128GB SDSDXXY-128G-GN4IN out of stock
    5990 Y


    27380 Y

    Memory Card Delkin Devices Premium Cinema CFast 2. 0 128GB 560X VPG 13 0 [DCFSTV128] In stock

    Source: Pixel24


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