A Guide to SD Card Speed & Other Specs
Courtesy of SanDisk

We recently got our hands on the new 64GB SanDisk Extreme PRO SDXC UHS-II card, which offers read and write speeds of 300MB/s and 260MB/s respectively, and belongs to the V90 video speed class. If you’re not 100% clear on the importance of these specifications, don’t fret. In this post, we breakdown everything you need to know about SD card speed, including why high-speed models are ideal for professional photographers and videographers. 

The specifications and ratings listed on every SD card’s label and packaging are regulated by the SD Association, an international not-for-profit organization.  

Who is the SD Association? 

SD Association logo
the logo of the SD Association

In 1999, a group of memory card manufacturers including SanDisk came together to develop Secure Digital: a non-proprietary, flash memory card format, abbreviated as SD. Despite being competitors, the manufacturers formed the SD Association to continue establishing industry-wide standards for the design, development, and manufacturing of SD cards.

What Does SDXC Mean? 

64gb SanDisk Extreme PRO SDXC and MicroSD cards
The 64GB SanDisk Extreme PRO SDXC UHS-II Card and a 64GB SanDisk Extreme PRO MicroSDXC UHS-I Card

Today, SD cards come in three form factors and four capacity types. The form factors include the standard SD card size, MiniSD, and MicroSD. The capacity types are listed below:

  • SD: Up to 2GB of memory, using the FAT12 and FAT16 file systems 
  • SDHC: 2GB –32GB of memory, using the FAT32 file system 
  • SDXC: 32GB – 2TB of memory, using the exFAT file system 
  • SDUC: 2TB – 128TB of memory, using the exFAT file system

The new SanDisk Extreme PRO card comes in four sizes: 32GB (SDHC), 64GB (SDXC), 128GB (SDXC), and 256GB (SDXC). The capacity types are designed to have backward compatibility; however, you should confirm which sizes are supported by your camera/recording device. For example, the maximum card size for a Zoom H5 audio recorder is 32GB, so it can only use SD or SDHC cards. 

SD Card Speed 

SD card speed, or the speed at which the card can read and write data, has significant implications for your project workflow. Most cards list their read and write speeds on their label. For example, the Extreme PRO UHS-II SDXC cards can reach a read speed up to 300MB/s, and a write speed up to 260MB/s.

However, the SD Association grades cards based on their minimum write speed. They use three separate ratings: Speed Class, UHS Speed Class, and Video Speed Class.

Speed Class

Speed class is indicated on the card label by a number enclosed within a C-shaped symbol. There are four classes: 

  • Class 2: a minimum write speed of 2MB/s 
  • Class 4: a minimum write speed of 4MB/s 
  • Class 6: a minimum write speed of 6MB/s 
  • Class 10: a minimum write speed of 10MB/s

Innovation over the past 20 years means that most modern SD cards have a class 10 rating. In fact, most modern cards have much higher minimum write speeds than 10MB/s, which has led the SD Association to introduce additional standards for grading SD card speed. 

UHS Speed Class 

The UHS speed class rating is indicated on the card label by a number enclosed within a U-shaped symbol. There are two ratings:

  • U1: a minimum write speed of 10MB/s 
  • U3: a minimum write speed of 30MB/s

Video Speed Class 

Video speed class, the third and final write speed class, is indicated on the label by a number next to a V-shaped symbol.

This standard was developed as videographers and filmmakers began shooting higher definition video and required faster read/write speeds. Additionally, the video speed class standard supports multi-file recording. That is, when a camera also wants to record additional data such as geographic location, height from the ground, date and time, and others. This makes it ideal for photographers and videographers that use drones, action cameras, and 360° cameras.

There are five ratings:

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

While most SD cards available today have a minimum write speed of 6MB/s, negating the need for the previous two speed classes, all three rating standards are still used. That’s why the SanDisk Extreme PRO UHS-II SDXC cards have a Class 10, U3, and V90 speed class rating, indicating they have a minimum write speed of 90MB/s.  Click here for a detailed SD card speed class chart from the SD Association.

What Does UHS-II Mean? 

The Extreme PRO SDXC cards also have a UHS-II rating. The UHS rating is distinct from the UHS speed class above; it indicates the bus speed of the card, instead of its write speed. However, there is a correlation between bus speed and write speed. Let’s break it down.

The bus is an important, internal component of an SD card; think of it as a road that data must travel along inside the card so it can be read and written to the card’s memory. A wider road means more data can travel simultaneously, thus enabling the card to read and write data at a higher speed.   

Early SD cards had a default bus speed of 12.5 MB/s. However, as the demand for faster cards increased, the SD Association introduced new bus interfaces to enable faster bus speeds, and as a result, increase the theoretical read and write speeds. Today, most cards use one of the following, ultra-high-speed (UHS) bus interfaces: 

  • UHS-I: Bus speed up to 50MB/s 
  • UHS-II: Bus speed up to 156MB/s 
  • UHS-III: Bus speed up to 312MB/s

While a higher bus speed can help an SD card achieve a faster minimum write speed, it’s not the only contributing factor. That’s why you can get UHS-I cards that still have decent write speeds and are also considerably more affordable. For example, the 64GB Extreme PRO UHS-I SDXC card has a V30 speed class, and costs just $24.99

Additionally, some cameras may only support UHS-II cards in one or some of their card slots. For example, the Sony a7 III has two card slots. The camera can utilize SDXC UHS-II cards in slot 1, but only SDXC UHS-I cards in slot 2. UHS-II cards are also backwards compatible though, so you can still use it in Slot 2. The card will simply be limited to a UHS-I level bus speed (up to 50MB/s).

Why Does Minimum Write Speed Matter?

While you can save money by investing in a slower SD card, certain photographers, videographers, and filmmakers should invest in a card with a high minimum write speed.

For example, wildlife photographers, sports photographers, or any photographer that shoots continuously at a high speed should use a card with a minimum write speed of 90MB/s (V90), the fastest SD card speed currently available. When shooting at a high frame rate (e.g. 10fps or higher), a high minimum write speed will ensure your photos are correctly read and written to your SD card. A slower write speed means the card may struggle to process every image, increasing the risk of data loss. 

Similarly, when recording high definition video, a high minimum write speed is crucial. While V90 is the ideal SD card speed for 4K video, you may make do with a speed class of V10 or higher. V60 or V90 are effectively required for stable 8K recording and smooth playback.

Why Choose the SanDisk Extreme PRO SDXC UHS-II Card?

SanDisk Extreme PRO SDXC UHS-II Card
Courtesy of SanDisk

SanDisk has cultivated a reputation for producing high quality, durable, and reliable memory cards. Their latest release, the Extreme PRO SDXC UHS-II cards, are ideal for professional photographers, videographers, and filmmakers that require a high-speed, high-performing memory card for their camera. They’re also affordably priced, with the 64GB card available for just $99.99.


Read and write speeds are also important to consider when shopping for an external drive to use for editing photos and/or video. Check out our post discussing the SSD vs HDD external drive debate to learn more. Otherwise, let us know in the comments below if you have any further questions about SD card speed or any other technical specs.


  1. Hello my loved one! I wish to say that this article is awesome,
    great written and come with approximately all significant infos.
    I would like to look more posts like this .


Please enter your comment!
Please enter your name here