SD and CompactFlash memory cards often carry speed ratings but it isn’t clear what the numbers mean exactly. To make matters worse the two card formats use different methods of designating the speeds. Although many devices only take one card type what do you use if you have a camera or media player that will take either?

SD card speed ratings are quite simple – the cards are rated into three different speed classes – class 2, class 4 and class 6. The numbers indicate the minimum transfer speed of each class in Megabytes per second:

Class 2 – minimum transfer speed 2 MBytes/sec

Class 4 – minimum transfer speed 4 MBytes/sec

Class 6 – minimum transfer speed 6 MBytes/sec

CompactFlash speed ratings are completely different. A few manufacturers have applied their own simplified labels, such as SanDisk’s Ultra 2, Extreme 3 and Extreme 4, but the majority of manufacturers use an ‘x’ rating. You will find CompactFlash cards with speed ratings from 12x to 300x. It seems clear that 300x is faster than 12x (especially when you look at the prices) but what is the actual transfer speed?

It may seem odd but CompactFlash speed ratings are the number of times the card is faster than an old 1x speed CD-ROM. This is because CF cards use the same IDE interface as CD-ROMs and can be thought of as an alternative type of removable storage.

The speed of those original CD drives was 150 kBytes/sec or 0.15 Mbytes/sec, so a 12x card will support a transfer speed of 12 x0.15 = 1.8 Mbytes/sec. As you can see this appears to be comparable with a class 2 SD cards. Unfortunately some card manufacturers use maximum write speeds not minimum speeds, which may be significantly lower. Major brands are more honest with Lexar for example making it clear that their card ratings are minimum sustained write speeds.

Here is a table showing some common card ratings. You can clearly see why high-end digital cameras have continued using CompactFlash cards, as all but the slowest are faster than the fastest SD cards:

12x = 1.8 Mbytes/sec

40x = 6 Mbytes/sec

80x = 12 Mbytes/sec

100x = 15 Mbytes/sec

200x = 30 Mbytes/sec

266x = 40 Mbytes/sec

300x = 45 Mbytes/sec

So there you have it – speed ratings for SD and CF cards explained.