The speed classes of SD cards
There are several different ways to measure and notate an SD card’s speed, so it can be confusing to compare SD cards. Today, the most commonly used measurements are Speed Class Ratings, which guarantee a minimum level of sustained read and write speeds.
You’ll find speed class labels on most SD cards, but some manufacturers have started going further and printing the actual read and write speeds on SD cards now as well. If those aren’t available, look to the following speed classes and their meanings:
Speed Class: Denoted by a “C” symbol on the SD card with a number inside it. That number represents the minimum sequential write speed of the card. It can go faster at times, but will never go slower. The four levels include C2 (2MB/s), C4 (4MB/s), C6 (6MB/s), and C10 (10MB/s).
UHS Speed Class: UHS stands for Ultra High Speed. Denoted by a “U” symbol on the SD card with a number inside it. That number is either 1 or 3, indicating either U1 or U3 ratings. These ratings correspond to minimum sequential write speeds of either 10MB/s (U1) or 30MB/s (U3).
Within the UHS speed rating, you’ll also see that some SD cards are marked as UHS-I, UHS-II, or UHS-II, which correspond to maximum write speeds of 104MB/s (UHS-I), 312MB/s (UHS-II), and 624MB/s (UHS-III).
Video Speed Class: Denoted by a “V” symbol on the SD card with a number next to it. That number is either 6, 10, 30, 60, or 90, referring to the minimum MB/s speed of the card. For example, an SD card rated V60 guarantees at least 60MB/s when writing. This standard was created for videographers who need guaranteed speeds to ensure all video frames are saved while recording at high resolutions or frame rates.
SD Express Speed Class: This is the newest SD card standard that builds on the previous Video Speed Class, increasing speed and bandwidth with multi-stream support. It’s denoted by an “E” symbol with a number next to it, with that number being either 150, 300, 450, or 600. Like the Video Speed Class, the number refers to the guaranteed minimum speed of the SD card, so E600 guarantees at least 600MB/s all the time.
