See below for an explanation of various terms used to describe the speed of memory modules, USB drives, and Flash Cards:
DDR3
DDR3 memory is the latest generation of memory and came in to widespread use in late 2007. It offers many benefits over DDR2 including lower latencies, higher speeds, and lower power requirements. DDR3 is not backward-compatible with DDR2, nor will DDR3 modules fit in a system designed for DDR2.For all types of memory, a higher number represents a faster speed module, or more bandwidth. DDR3 is often referred to by its common name using terms like "DDR3-1066" or "DDR3-1333". When written this way, the numbers after "DDR3" refer to the data output rate in Megahertz of the components.
DDR3 is also referred to by terms like "PC3-6400" or "PC3-8500". When written this way, the numbers after "PC3" refer to the peak bandwidth of the memory module.
| Common Name | Industry Name | Peak Bandwidth | Data Output Rate |
| DDR3-800 | PC3-6400 | 6400 MB/s | 800Mhz |
| DDR3-1066 | PC3-8500 | 8533 MB/s | 1066Mhz |
| DDR3-1333 | PC3-10600 | 10667 MB/s | 1333Mhz |
| DDR3-1600 | PC3-12800 | 12800 MB/s | 1600Mhz |
DDR2 PC2-4200, DDR2 PC2-5300, DDR2 PC2-6400, and DDR2 PC2-8000
For DDR2 modules, the numbers that come after the "PC2" refer to the peak bandwidth capability of the module.For all types of memory, a higher number represents a faster speed module, or more bandwidth. DDR2 is often referred to by its common name using terms like "DDR2-667" or "DDR2-800". When written this way, the numbers after "DDR2" refer to the data output rate in Megahertz of the components. Note: DDR2 is not backward-compatible with DDR.
| Common Name | Industry Name | Peak Bandwidth | Data Output Rate |
| DDR2-400 | PC2-3200 | 3200 MB/s | 400Mhz |
| DDR2-533 | PC2-4200 | 4266 MB/s | 533Mhz |
| DDR2-667 | PC2-5300 | 5333 MB/s | 667Mhz |
| DDR2-800 | PC2-6400 | 6400 MB/s | 800Mhz |
| DDR2-1066 | PC2-8500 | 8533 MB/s | 1066Mhz |
As an example, PC2-4200 (commonly referred to as DDR2-533) memory is DDR2 designed for use in systems with a 266MHz FSB (front-side bus), providing a 533Mhz data output rate). The "4200" refers to the module's peak bandwidth (the maximum amount of data it can transfer each second), which is 4200 MegaBytes per second (also written as 4200MB/s, or 4.2GB/s).
Note: A Megabyte is equivalent to 8 Megabits, and the terms can cause some confusion with consumers. A Megabyte is written in shorthand using a capital M followed by a capital B - MB. A Megabit is written in shorthand using a capital M, followed by a lower case b - Mb. Put differently, a single 2GB module (4 Gigabytes) may contain sixteen 2Gb (2 Gigabit) chips.
DDR PC1600, PC2100, PC2700, and PC3200
Like DDR2 modules, in DDR modules the numbers that come after the "PC" refer to the peak bandwidth of the module. As with other types of memory, a higher number represents faster memory, or more bandwidth. Occasionally DDR is referred to as "DDR400" or "DDR-333," for example. When written this way, the numbers after "DDR" refer to the data output rate (in Megahertz or Mhz for short) of the components.
| Common Name | Industry Name | Peak Bandwidth | Data Output Rate |
| DDR-200 | PC-1600 | 1600 MB/s | 200Mhz |
| DDR-266 | PC-2100 | 2100 MB/s | 266Mhz |
| DDR-300 | PC-2400 | 2400 MB/s | 300Mhz |
| DDR-333 | PC-2700 | 2700 MB/s | 333Mhz |
| DDR-400 | PC-3200 | 3200 MB/s | 400Mhz |
As an example, PC3200 (commonly referred to as DDR400) memory is DDR designed for use in systems with a 200MHz FSB (front-side bus), providing a 400Mhz data output rate). The "3200" refers to the module's peak bandwidth (the maximum amount of data it can transfer each second), which is 3200 MegaBytes per second (also written as 3200MB/s, or 3.2GB/s).
SDRAM PC100 and PC133
With SDRAM modules, the numbers that come after the "PC" are different in meaning to those used for DDR modules, now referring to the speed of the system's front-side bus.As an example, PC100 memory is SDRAM designed for use in systems with a 100MHz front-side bus. It is used in some older HP, Toshiba, and Apple Power Mac G4 systems. Some resellers have stopped selling PC100 in favour of PC133, which is backward-compatible. RamCity are still selling Kingston modules in PC100 speeds for systems that require it..
RDRAM PC800 and PC1066
RDRAM or RAMBUS Ram up until a few years ago didn't have the same dual-data-rate features as DDR so was named according to the speed it ran at. So PC800 RDRAM ran at 800Mhz, and PC1066 at 1066Mhz. Because each standard RDRAM module has only a single 16-bit channel, the modules have to be installed in pairs to achieve the same bandwidth transfer rates as DDR or DDR-2 ram.
| Common Name | Industry Name | Peak Bandwidth | Data Output Rate |
| PC800 | PC-800 | 3200 MB/s (per module) | 800Mhz |
| PC1066 | PC-1066 | 4200 MB/s | 1066Mhz |
Flash Memory
Flash Memory, (including USB Drives, CF Cards, SD Cards, and other types) is often labelled with a speed of 10x, 45x, 133x, etc. Typically this figure is worked out by dividing the maximum write speed in bits per second by 150. The latter figure represents the old standard 1x CD-ROM drive write speed - being 150 bits/sec or 1x. Therefore if a flash card is listed with a speed of 50x, then the maximum write speed in ideal conditions is going to be about 7500 bits per second, or around 7.5 Mbits per second.
To complicate matters further, a new type of flash memory card was introduced, termed "High Capacity". Presently these cards have speed classified as "Class 2" (2 MB/sec minimum data transfer rate), "Class 4" (4 MB/sec minimum data transfer rate), or "Class 6" (6 MB/sec minimum data transfer rate). Already some manufacturers have started introducing their own write speed specification (such as 133x, or 20MB/s) to indicate that their High Capacity cards perform much better than the minimum standard.
