There are a lot of ways that SD cards are classified, which can make comparing two cards difficult. If you’ve ever heard an SD card called a “Class 4” card or “Class 10” card, maybe you’ve wondered what that actually means.

While all of this is about SD cards, the information also applies to microSD cards, which follow the same standards.

Class 4 vs Class 10 SD Card

Class 4, also referred to as C4 and noted on SD cards with the number 4 inside the letter C, has a minimum write speed of 4MB/s. Class 10, or C10 and noted by the number 10 inside the letter C, has a minimum write speed of 10MB/s.

Knowing the minimum write speed for a card is important because that is a different measure of performance than how fast data can be read off a card. 

If you’re using the card for video recording, particularly, the write speed can mean the difference between being able to record more than a few seconds at a time.

What Is Speed Class For SD Cards?

Buying the right kind of SD card is important, and the speed of the card is definitely one of the main considerations. One of the multiple ways that SD card makers categorize their offerings is by Speed Class.

Speed Class was the first way SD cards were categorized by write speed.

There are four Speed Classes: Class 2, Class 4, Class 6 and Class 10. In each case, the number refers to the card’s minimum sequential write speed in megabytes per second.

Speed Class Chart

What Are UHS And Video Speed Classes For SD Cards?

Speed Class is sometimes called Original Speed Class because two other classes have also been added. Those classes, UHS Speed Class and Video Speed Class, measure the same thing as Original Speed Class, but offer different classes and different ranges.

UHS Speed Class

UHS Speed Class was introduced as 4K video was starting to come to consumer cameras and denoted cards able to handle the more demanding task.

There are two UHS Speed Classes: Class 1, also called U1 and identified on cards by the number 1 inside the letter U, and Class 3, also called U3 and marked with the number 3 inside the letter U.

Class U1 cards have minimum sequential write speeds of 10MB/s while U3 cards have speeds of at least 30MB/s.

SD Card UHS Speed Chart

Video Speed Class

Video Speed Class is the most recent addition and came about as 8K video became available. The classes are V6, V10, V30, V60 and V90, with the second number indicating minimum write speed in MB/s.

The classes are identified on SD cards with the letter V followed by the class number.

Video Speed Class Chart

Speed Class vs Bus Type

In addition to various speed classes, there are multiple bus types for SD cards. 

This shouldn’t be confused with the speed class, though they use the same basic terminology and similar names. 

The speed classes tell you how fast the card will write data. The bus type, on the other hand, tells you the maximum data transfer speed for the card. 

Because writing is a slower process, the top speed is usually only achieved when reading from the card.

Originally, SD cards had a maximum transfer speed of 12.5MB/s, with the High Speed bus type doubling that. The Ultra High Speed I or UHS I bus type can transfer at 50 or 104MB/s, while the Ultra High Speed II or UHS II bus type can hit 156MB/s in one configuration and 312MB/s in another.

Which Is Better, a Class 4 SD Card or Class 10 SD Card?

Class 10 cards will be faster than Class 4 cards, making them a better choice if you’re planning to record video or high resolution photos. Generally, slower cards will be less expensive, making them a better value if you don’t need the speed.

Read More:

What’s the best SD card for 4K video?

How many pictures can 256GB hold?