Understanding Memory Card Ratings

Understanding SD Card Speeds

While the first SD cards were very similar between brands, each brand now offers cards in several different speeds for each capacity. To make things more confusing, SD card speeds are measured in two different ways: in kilobytes or megabytes per second (KB/s or MB/s) and in an “x” rating similar to that of CD-ROMS. The “x” is the minimum CD-ROM speed of 150KB/s, so here’s a table of equivalent speeds:

• 4x: 600 KB/s
  
• 16x: 2.4 MB/s
  
• 40x: 6.0 MB/s
  
• 66x: 10MB/s
  
• 133x: 20MB/s
  

Understanding available options

While this varies between manufacturers, most offer two or three categories of speed. At this writing, here’s a summary of what each category means:

• Basic SD cards (Sandisk, Lexar, etc.) are the cheapest models. They generally have a speed of 1x to 16x, but often don’t guarantee a particular speed, so you won’t find anything about speed listed on the package.
  
• “Pro” SD cards (Sandisk Ultra, Lexar High Speed, etc.) list a speed of 32X or higher. Some have speeds up to 66x (10Mbps).
  
• High-end SD cards (Sandisk Extreme, Lexar Professional, etc.) have a speed of at least 66x. The latest models (i.e. Sandisk Extreme III) have a speed of 133x.
  

Since these categories are vague, and not always meaningful—for example, Sandisk’s “Ultra II” and “Extreme” cards are the same speed, but “Extreme III” is a higher speed—you should always compare based on the actual speed listed on the package. If it isn’t listed, assume you’re looking at one of the slowest cards.