What do the numbers and symbols on an SD card mean?

Secure Digital (SD) cards have become the most widely used memory cards for consumer electronics like digital cameras, smartphones, and tablets. With several types and speed ratings available, the numbering and symbols on SD cards can be confusing to understand.

This article will explain what all those numbers and symbols on your SD card mean and how to pick the right SD card for your device.

SD Card Types

There are several types of SD cards available today, with different capacities and speeds:


The original SD card design offered capacities up to 2GB. They have mostly been replaced by higher capacity formats today.


SD High Capacity cards offer capacities between 4GB to 32GB. The format was introduced to support higher storage needs. All SDHC cards are backwards compatible with SD devices.


SD Extended Capacity cards go from 64GB up to 2TB storage. The SDXC format uses exFAT file system instead of FAT32 for SD and SDHC cards. So SDXC cards will not work in devices that don’t support exFAT.


microSD is a smaller form factor designed for smartphones and tablets. They are available in microSDHC and microSDXC types with different capacities and speeds just like regular SD cards.


SDIO cards are designed to add capabilities beyond memory storage to devices like WiFi, Bluetooth, GPS etc. They are mostly used in older PDAs and some digital cameras.

SD Card Capacities

The capacity of the SD card is identified by a number that denotes the amount of data that can be stored on the card. It’s usually prominently printed on the front of the card.

Common capacity numbers you will see are 4GB, 8GB, 16GB, 32GB, 64GB, 128GB, 200GB, 256GB, 400GB, 512GB, 1TB and 2TB. The higher the GB number, the more data the SD card can store.

1 GB = 1,000 MB = 1,000,000 KB = 1,000,000,000 bytes

The actual formatted capacity of a memory card is always a bit lower than the advertised capacity. That’s because things like file tables also take up space.

SD Card Speed Classes

SD card speed is identified by both a speed class symbol and a separate speed rating printed on the card. The combination identifies the minimum read/write speed capability of the card for capturing HD video or sequential burst mode photography.

Speed Class Symbols

Symbol Speed Class Minimum Speed
No symbol Class 2 2 MB/s
C2 Class 2 2 MB/s
C4 Class 4 4 MB/s
C6 Class 6 6 MB/s
C10 Class 10 10 MB/s
U1 UHS Class 1 10 MB/s
U3 UHS Class 3 30 MB/s
V6 Class 6 6 MB/s
V10 Class 10 10 MB/s
V30 Class 10 30 MB/s
V60 Class 10 60 MB/s
V90 Class 10 90 MB/s

Cards with no Speed Class symbols should be assumed to be Class 2 (2MB/s).

UHS Bus Ratings

UHS stands for Ultra High Speed which is available on advanced SDHC and SDXC cards for professional use. UHS interfaces are rated by numbers denoting the minimum bus speed:

  • UHS-I (U1/U3) – 104 MB/s
  • UHS-II (U3) – 156 MB/s
  • UHS-III (U3) – 312 MB/s

Cards with higher UHS ratings allow for faster data transfer rates when used with compatible devices.

What Do the Class Ratings Mean?

The class ratings define the minimum write speed for sequential operations on the card measured in MB/s. Different classes allow cards to support various use cases:

Class 2

Class 2 cards have a minimum write speed of 2MB/s. They allow basic recording of 720p HD video.

Class 4 & Class 6

Class 4 and 6 SD cards with 4MB/s and 6MB/s write speed support recording of 1080p full HD video at low bitrates.

Class 10

Class 10 SD cards with 10MB/s write speed can record full HD 1080p video at higher quality settings and bitrates up to 10Mbps.

UHS-I U1 & U3

UHS-I Class 1 (U1) guarantees 10MB/s minimum write speed for recording 1080p video at high quality. UHS-I Class 3 (U3) boosts it up to 30MB/s for 4K video recording.

V30, V60, V90

V30, V60 and V90 guarantee minimum write speeds of 30MB/s, 60MB/s and 90MB/s respectively for recording high bitrate 4K and 8K video.

How to Pick the Right SD Card Speed

When buying an SD card, match its speed rating with your usage – slower cards compromise performance. Here are some tips:

  • Class 10, U1, V10 cards are good for full HD video.
  • U3, V30 cards allow 4K video recording and burst photography.
  • V60 and above minimum for stable 8K video recording.
  • Higher speeds provide transfer buffer for demanding photography.
  • Always check your device’s recommended SD card rating.

Avoid extremely cheap cards that skip speed ratings. Slow cards affect the performance of your devices.

SD Card Interface & Bus Ratings

SD cards use various interfaces to transfer data between devices. The interface used is printed on the SD card label:


Uses SD 1-bit interface with a bus speed of 12.5MB/s for cards up to 2GB.


Uses SD 4-bit interface with a bus speed of 25MB/s for cards between 4GB-32GB capacity.


Supports SD 4-bit interface with higher bus speeds. SDXC cards above 32GB use various interfaces:

  • UHS-I – Uses SD 4-bit interface with a bus speed of 104MB/s.
  • UHS-II – Uses SD 8-bit interface with a bus speed of 156MB/s.
  • UHS-III – Uses SD 8-bit interface with a bus speed of 312MB/s.

The interface logo like UHS-I/UHS-II is printed on compatible SDXC cards.


Uses SD 4-bit interface and supports card bus speeds of 25MB/s or higher depending on card class rating.


SD cards have evolved a lot over the years in terms of capacities, speeds, and interfaces. When purchasing an SD card, take note of the capacity, speed class rating, UHS rating, and interface support printed on the label.

Match the SD card’s specifications like Class 10 or UHS-I with your device requirements for seamless performance. For demanding needs like 4K video or burst mode photography, choose higher rated UHS-III or V90 cards. Avoid cards without any ratings marked on them.

Understanding what the numbers and symbols denote on the SD card helps pick the optimal one for your use case. The standardization in ratings by the SD Association makes it easy to decode the capabilities of memory cards from different manufacturers.