A Guide To Speed Classes For SD And MicroSD Cards

Speed Class and Ultra-High Speed (UHS) Class

Speed Class was one of the original standards and was created to define minimum write speeds for what are now older memory technologies, classified by four performance tiers.

• C2 - minimum sustained write speed of 2MB/s
• C4 - minimum sustained write speed of 4MB/s
• C6 - minimum sustained write speed of 6MB/s
• C10 - minimum sustained write speed of 10MB/s

Minimum sequential write speed	Speed Class			Corresponding video format
Speed Class	UHS SpeedClass	Video SpeedClass (new)	8K video	4K video	Full HD /HD video	StandardVideo
Card image				The necessary speed varies by each recording/playback device condition, even in the same format.
90MB/sec	II
60MB/sec	II
30MB/sec	/ I
10MB/sec	/ I
6MB/sec
4MB/sec
2MB/sec

As technology improved and higher write speeds were reached more consistently, the SDA created the Ultra-High Speed (UHS) Class to categorize newer builds into 10MB/s and 30MB/s minimum write performance tiers and add UHS bus interface classification.

• U1 – minimum sustained write speed of 10MB/s
• U3 – minimum sustained write speed of 30MB/s

Take a look at your microSD or SD card. Does it say C10 and U1? In terms of speed class, don’t those both mean minimum write speed of 10MB/s? Yes, they do! So why do we still include both?? Because not everyone has read this article so spread the word! But seriously, many card manufacturers continue to use both logos on their product markings and in their specification material purely because not everyone is aware they are the same and may be looking for one and not the other. If you do see a card that has one logo and not the other, rest assured, you are still getting a 10MB/s minimum write speed card.

U3 on the other hand, can actually achieve 30MB/s minimum write speed. This is desirable for high-end cameras and those taking photos and videos in 4K resolution. U3 is also shared by the two most popular UHS bus interfaces, whereas U1 is limited to UHS-I:

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

(Of course we won’t forget to mention UHS-III, the latest classification with theoretical maximum transfer speeds up to 624MB/s but its applications are more enterprise and industrial than consumer.)

What does UHS bus interface mean and why is it important? I’m glad you asked! Simply put, the UHS bus interface dictates the method in which the card communicates with the host device, which could be a camera or a reader. If you have a UHS-II camera and use a UHS-I card, the best theoretical transfer performance your card can do is 104MB/s, unless it is built with performance-enhancing firmware. And if you have a UHS-II card with U3 markings and nothing else, it means you have a card with a theoretical transfer performance of up to 312MB/s (with UHS-II supporting products) and a minimum write speed of 30MB/s.

But if you have a professional grade UHS-II camera to shoot 4K, 8K or 3D videos, don’t you need higher than 30MB/s min. write speed?