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SD Card’s Different Types and Speed Classes Explained
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SD Card’s Different Types and Speed Classes Explained

by Syed Hassan AlgadrieJuly 10, 2020
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Have you ever wondered what the numbers and symbols on certain SD cards signify? Or what the terms SD, SDHC and SDXC mean despite all the SD cards look pretty much the same? With so much information shown on these SD cards, it can become pretty confusing which of the one is the most suitable for your needs. Well, here we will attempt to explain what those symbols and numbers mean and what the differences are between one another.

First, a quick history recap. SD cards have been around since the year 1999 and was created as an improvement security-wise over the previous MMC (MultiMediaCard) format released a couple of years prior as well as a direct competitor to Sony’s Memory Stick. The SD Card was a joint development between SanDisk, Panasonic and Toshiba and is now the industry standard when it comes to memory card formats.

SD Card Differences

SD Card's Different Types and Speed Classes Explained 22

Different SD Card Logos

Now that we’ve got the history lesson out of the way, let’s get on to the main differences of the SD cards. The difference between SD, SDHC, SDXC  and SDUC cards are purely based on the memory capacity range. Here’s a little breakdown on the different types of SD cards:

  • SD (Secure Digital) – up to 2GB storage capacity only
  • SDHC (Secure Digital High Capacity) – storage capacity from 2GB up to 32GB
  • SDXC (Secure Digital eXtended Capacity) – storage capacity from 32GB up to 2TB
  • SDUC (Secure Digital Ultra Capacity) – storage capacity from 2TB up to 128TB

As mentioned before, SD, SDHC, SDXC and SDUC cards all look the same and can fit in pretty much any device that has an SD card slot. Despite that, this isn’t really a “one size fits all” kind of situation as most older devices can’t actually support newer SD card types. For example, a device that supports SD card formats prior to SDXC cards introduction will only be compatible with the SD and SDHC card formats. They are backwards compatible however, so if a device is made with SDXC compatibility then it will be able to support both the SDHC and SD card formats as well.

SD Card Type Storage Capacity
SD (Secure Digital) Up to 2GB
SDHC (Secure Digital High Capacity) Up to 32GB
SDXC (Secure Digital eXtended Capacity) Up to 2TB
SDUC (Secure Digital Ultra Capacity) Up to 128TB

SD Card Speed Classes

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Symbols for Speed Class, UHS Speed Class and Video Speed Class

There are three different types of speed classes when it comes to SD cards. These speed classes are usually rated from the card’s sequential read and write speed. The three types of speed classes are Speed Class, UHS Speed Class, and Video Speed Class.

Speed Class

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Speed Class Symbols

Under this Speed Class, the SD cards will speed ratings of either 2, 4, 6 or 10. These ratings correspond with the number of megabytes per second of the cards. This means that Class 2 supports a minimum write speed of 2MB/s, Class 4 supports 4MB/s, Class 6 supports 6MB/s and Class 10 supports 10MB/s. It’s important to note that these are all the cards’ minimum speeds and it can very well go faster.

The symbol for the Speed Class has a number within the letter ‘C’.

Speed Class Minimum Write Speed
Class 2 2MB/s
Class 4 4MB/s
Class 6 6MB/s
Class 10 10MB/s

UHS Speed Class

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UHS Speed Class Symbols

UHS stands for Ultra High Speed and was developed due to the fact that 4K cameras needed faster writing speed to shoot. The UHS Speed class has two different speed ratings with Class 1 which supports a minimum write speed of 10MB/s and Class 3 which supports 30MB/s minimum write speed. Curiously, UHS Speed Class 1 cards and Speed Class 10 cards have the same minimum write speed of 10MB/s, which is why sometimes you can see these two speeds labelled together in certain SD cards.

UHS Speed Class cards can be broken down to three different types which are UHS-I, UHS-II and UHS-III. All these cards have different maximum potential write speeds with UHS-I supprting to up to 104MB/s, UHS-II supporting up to 312MB/s and UHS-III supporting up to 624MB/s.

The symbol for the UHS Speed Class has a number within the letter ‘U’.

UHS Speed Class MInimum Write Speed
Class 1 10MB/s
Class 3 30MB/s

Video Speed Class

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Video Speed Class Symbols

This speed class is the most recent addition and is introduced as a response to the demand for high resolution and high quality 4K and 8K video recording. Video Speed Class cards also has an important feature to support next generation flash memory such as 3D NAND. This class of SD cards also different classes that corresponds to their different write speeds which are:

  • V6 – supports minimum write speed of 6MB/s
  • V10 – supports minimum write speed of 10MB/s
  • V30 – supports minimum write speed of 30MB/s
  • V60 – supports minimum write speed of 60MB/s
  • V90 – supports minimum write speed of 90MB/s

Due to the similarities in minimum write speeds, the Video Speed Class SD cards can be lumped together with the corresponding UHS Speed Class cards. V6 and V10 can be applied to High Speed and UHS-I cards, while the V30 can be applied to UHS-I cards and the V60 and V90 can be applied to UHS-II and UHS-III cards.

The symbol for the Video Speed Class has the letter ‘V’ followed by the number that corresponds to the card’s write speed.

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

Below is a chart that summarises the SD cards and their speed classes.

Minimum Sequential Write Speeds Speed Class Video Format Support
Speed Class UHS Speed Class Video Speed Class SD HD & FHD 4K 8K
2MB/s Class 2 YES NO NO NO
4MB/s Class 4 YES
6MB/s Class 6 V6 YES
10MB/s Class 10 Class 1 V10 YES
30MB/s Class 3 V30 YES
60MB/s V60
90MB/s V90

Application Performance Class

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Application Performance Class Symbols

Application Performance Class is a recently added standard for SD cards which doesn’t just define sequential reading speeds but also calls for a minimum IOPS for reading and writing. IOPS or input/output operations per second; is a benchmark to measure the performance of input/output of storage devices such as SD cards, computer hard disk drives and SSDs. The Application Performance Class has two different classes; Class A1 and Class A2. 

Class A1 requires a minimum of 1500 reading and 500 writing operations per second, whereas Class A2 requires 4000 and 2000 IOPS. Furthermore, Class A2 cards needs a suitable host driver support to take advantage of their higher speeds, if not their speeds might end up even slower of that of Class A1.

Class Minimum IOPS
Read Write
Class A1 1500 500
Class A2 4000 2000

One thing to note, if you’re planning on shooting using devices that are 4K-capable like a GoPro, you are better off using a UHS Speed Class 3 card or higher as slower cards can cause problems such as jittering in the recorded footage.

SD Card Kingston

We hope that this helps you in clearing up the confusion surrounding the technical terms and symbols of SD cards. At the end of the day however, before choosing which SD card to go with we recommend that you check your device manual or user guide first. Once you’ve got an inkling on which card your device can support then you can proceed on purchasing your favoured card.

Generally, cards with higher speeds would benefit most if you’re planning on getting it for your camera, especially if you’re shooting in 4K. Other gadgets such as smartphones or consoles like the Nintendo Switch aren’t too dependent on higher speed cards, although that doesn’t mean that they wouldn’t benefit from them. Happy choosing!

To recap, here’s a summary of all the SD card types and speed classes:

  • SD cards have four different types; SD, SDHC, SDXC and SDUC.
  • SD can store up to 2GB, SDHC up to 32GB, SDXC up to 2TB and SDUC up to 128TB.
  • SD cards have three different speed classes; Speed Class, UHS Speed Class and Video Speed Class.
  • Speed Class has write speed of up to 10MB/s, UHS Speed Class up to 30MB/s and Video Speed Class up to 90MB/s.
  • Application Performance Class is a standard that measures the input/output operations of SD cards and has two different classes; Class A1 and Class A2.
  • Class A1 requires 1500 reading and 500 writing while Class A2 requires 4000 and 2000 IOPS.
About The Author
Syed Hassan Algadrie
Just a guy who loves Star Wars, Transformers, tech, 90s nostalgia and all things geek. Is in a constant battle with procrastination.