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What is Class Speed Rating?

You may have heard or seen speed ratings starting to appear on SD, Micro SD and Mini SD memory cards, these ratings are split into three categories as defined by the Secure Digital Association (SDA). These categories were introduced to make it easier for the consumer to identify the minimum speed capabilities of their SD cards. The three categories are defined in the below table:

Class Rating 2 Class Rating 2 Transfer speed is a minimum of 2MB/sec.
Class Rating 4 Class Rating 4 Transfer speed is a minimum of 4MB/sec.
Class Rating 6 Class Rating 6 Transfer speed is a minimum of 6MB/sec.
Class Rating 10 Class Rating 10 Transfer speed is a minimum of 10MB/sec.

Previously different manufacturers have indicated data transfer speeds in various ways and there has been no standard. The existing speed ratings in use state the memory cards maximum Data Transfer Rate (DTS) which may vary, the new SDA version 2.0 speed rating states a guarantee minimum sustained DTS.

The new SDA class ratings will be adopted by all memory card manufacturers which will make life a lot easier when comparing different brands. Speed classes logos will appear near the SD cards logo as shown on the below Sandisk SDHC 32GB memory card which is a class 4:

32GB SD Memory Card

As technology moves forward devices such as Digital Cameras and Digital Video Recorders are requiring higher specification memory cards. Manufacturers of these host devices are now starting to stipulate the appropriate speed rating class to achieve the best performance. Having this information readily available and in an easy to understand format is definitely a great step forward and will save us all a lot of time researching what is the best memory card for my Camera!

Written by msowa at 14/08/2008 17:58:00

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Comments made about this article

Written By John on 27th August 2008 at 16:27
Very helpful notes. If you have the expertise, it would also be good to have the class ratings linked to likely needs. I have a 10MP camera that uses Sd cards and higher rated cards seem to make no difference at all. I am going to try a HD camcorder that records to SD cards, but apart from the manufacturer wanting me to buy their own brand, I don't know what spec of card is best. If you know the data transfer rate such a camcorder needs, that would be a help.

There is a second concern that may just be marketing spin. Sandisk do an SD card called 'Video', which is class 4. Is it any different from any other class 4, including Sandisk's own Ultra?

Thanks John Reed
Written By pderbyshire on 28th August 2008 at 14:10
Not all devices can take advantage of higher speed rated cards. High performance cards such as the Sandisk extreme IV and extreme III range of memory cards are designed for the professional or high end amateur DSLR camera. These digital cameras specifically benefit from high performance read/write speeds when shooting in RAW format or high res JPEG format. As these formats produce larger files a high speed card allows these files to be saved at a faster rate which in turn will increase your continuous shooting capabilities.

In regards to Digital Camcorders a high speed card will definitely be of advantage as data is continually being written to the memory card. Once again it would be advisable to contact the manufacturer as to what is the best suited speed class rating card for your device. As a general rule we would always recommend a class 4 speed rating as a minimum for digital Camcorders.

When transferring data from your memory card to your PC the benefits of a high speed memory card will be in the time it takes to transfer data. Obviously large amounts of data will take less time but, it is important to note that this will also depend on the Card Reader and the connection that you are using. For example to take advantage of the full 40MB/sec transfer speed of Sandisks CF IV cards you will need to use a Sandisk Extreme Firewire reader.
Written By mothandrust1 on 21st December 2009 at 10:23
Thanks! U answered my questions!
Written By Jaundalynn on 27th August 2011 at 01:01
AFAIC that's the best asewnr so far!
Written By Alan on 30th August 2011 at 23:51
Very interesting and informative. Glad I read this prior to buying an sd micro card.
Written By Greg on 15th September 2011 at 04:30
I'm a little confused, why does the chart above show that a Class 4 is 4MB/Sec yet on the SC card, it shows that it's 15MB/Sec?
Written By Floyd on 8th November 2011 at 04:30
Like Jeff, I want to know why the card shown, Class 4 transfer rate 4MB/sec, says 15 MB/sec in another location on the face of the card.
What does this mean, why two different rates?
I am interested in getting my camera to quickly record the picture, so I can take another picture. Which of these transfer rates applies?
The narrative refers to the transfer of pictures from the memory card to a computer. Which transfer rate applies here??
I'm confused too.
Written By Rohan on 10th November 2011 at 12:38
@Greg: Class refers to minimum speed for the transfer of data. But for vendors, they need to show the maximum speed of transfer as the greater the digits, the more it maybe that's y they have written.
Written By rak on 5th December 2011 at 15:15
very good clear information
Written By NedR on 27th December 2011 at 08:10
It might have been too obvious to mention but, the transfer speed refers to the minimum write speed guaranteed by the manufacturer.
Written By Che Bramley on 6th January 2012 at 10:52
Greg the chart states minimum speed the card supports a maximum of 15mb/s. Hope that has cleared any confussion.
Written By Fred on 13th January 2012 at 17:10
Greg, that is most likely the READ speed, whereas the ratings above are the WRITE speeds.
Written By ty on 17th January 2012 at 23:12
Greg. a Class 4 SDXC card will also brandish 15MB/s on its exterior - a claim that can only be made as a possible speed rather than a full-time sustainable one.
Written By pritish on 6th February 2012 at 12:18
what about mobile phone? my phone is sony xperia neo v..which speed class should i prefer???
Written By Donald on 4th March 2012 at 22:48
Thanks for the info. I now have to check my cameras handbook to find out what the transfer speed is and if it is not written there I will have to contact the manufactere to find out. Will cameras have these ratings printed on them in the future?
Written By Donald Jacobs on 5th March 2012 at 15:42
Greg it seem to me that it was tested at 15MB/s as you may have noted on the "CLASS" classification it is the minimum of 4MB/s that it will transfer. The 15MB/s is not guaranteed
This is only my opinion ,I might be wrong
Written By geezer on 10th March 2012 at 00:38
its the read speed that is of 15mBs
the write speed is of at least 4mBs (3.9mBs on your pc)
Written By Lynn on 13th March 2012 at 11:39
My camera maual says i need a class 6, i wondered if a clas 10 would work as well or if it would be too fast as such
Written By Someone on 18th March 2012 at 19:33

For sure it says UP TO 15MB/s.
Written By Kenny on 15th April 2012 at 18:21
Hi the images are not displayed properly and can you also add a table for the class with different speeds and the price for the same.
Written By Andy on 8th June 2012 at 07:11
Very useful info thanks for that. It has helped me before buying a new micro sd card.
Written By ken on 5th July 2012 at 18:48
thanks i have now got an idea of the right one to buy for my camera. thanks
Written By saqib on 19th July 2012 at 14:22
Class 4 is 4MB/Sec defines minimum read/write speed where as 15MB/Sec on card shows maximum read/write speed

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