Memory speeds and compatibility

Memory is designed to be backward-compatible, so generally speaking, you can safely add faster memory to a computer that was designed to run slower memory. However, your system will operate at the speed of the slowest memory module.
One thing to keep in mind is that the memory does need to be the same type - for example, SDRAM cannot be mixed with DDR, and DDR cannot be mixed with DDR2 and DDR2 cannot work in a DDR3 system.
We recommend that you use the Crucial Memory Advisor™ or System Scanner tools to find the right memory for your computer. Rather than give memory modules catchy names, the industry refers to modules by their specifications. But if you don't know a lot about memory, the numbers can be confusing. Here's a short summary of the most popular types of memory and what the numbers refer to.

DDR3

DDR3 memory is the latest generation of memory for computing applications and offers many benefits over DDR2 including lower latencies, higher speeds, and more. DDR3 is not backward-compatible with DDR2. For this type of memory, a higher number represents faster memory, or more bandwidth. Occasionally DDR3 is referred to by a "friendly name" like "DDR3-1066" or "DDR3-1333". When written this way, the numbers after "DDR3" refer to the data transfer rate per second (/s) of the components.

Friendly Name Industry Name Peak Transfer Rate Data Transfers/Second (in Millions)
DDR3-800PC3-64006400 MB/s800
DDR3-1066PC3-85008533 MB/s1066
DDR3-1333PC3-1060010667 MB/s1333
DDR3-1600PC3-1280012800 MB/s1600

DDR2 PC2-4200, DDR2 PC2-5300, DDR2 PC2-6400, and DDR2 PC2-8000

In DDR2 modules, the numbers that come after the "PC2" refer to the total bandwidth of the module. For this type of memory, a higher number represents faster memory, or more bandwidth. Occasionally DDR2 is referred to as a "Friendly name" like "DDR2-533" or "DDR2-667". When written this way, the numbers after "DDR2" refer to the data transfer rate per second (/s) of the components. DDR2 is not backward-compatible with DDR.
Friendly Name Industry Name Peak Transfer Rate Data Transfers/Second (in Millions)
DDR2-400PC2-32003200 MB/s400
DDR2-533PC2-42004266 MB/s533
DDR2-667PC2-53005333 MB/s667
DDR2-800PC2-64006400 MB/s800
DDR2-1066PC2-85008533 MB/s1066
DDR2 PC2-4200 (commonly referred to as DDR2-533) memory is DDR2 designed for use in systems with a 266MHz front-side bus (providing a 533MT/s data transfer rate). The "4200" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 4200MB/s, or 4.2GB/s.
DDR2 PC2-5300 (commonly referred to as DDR2-667) memory is DDR2 designed for use in systems with a 333MHz front-side bus (providing a 667MT/s data transfer rate). The "5300" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 5300MB/s, or 5.3GB/s. PC2-5300 is backward-compatible for PC2-4200.
DDR2 PC2-6400 (commonly referred to as DDR2-800) memory is DDR2 designed for use in systems with a 400MHz front-side bus (providing an 800MT/s data transfer rate). The "6400" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 6400MB/s, or 6.4GB/s. PC2-6400 is backward-compatible for PC2-4200 and PC2-5300.
DDR2 PC2-8000 (commonly referred to as DDR2-1000) memory is DDR2 providing a 1,000MT/s data transfer rate). The "8000" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 8000MB/s, or 8GB/s. PC2-8000 is backward-compatible for PC2-4200, PC2-5300, and PC2-6400.

DDR PC1600, PC2100, PC2700, and PC3200

Like DDR2 modules, in DDR modules the numbers that come after the "PC" refer to the total bandwidth of the module. For this type of memory, a higher number represents faster memory, or more bandwidth. Occasionally DDR is referred to as "DDR400" or "DDR333," for example. When written this way, the numbers after "DDR" refer to the data transfer rate per second (/s) of the components.
Friendly Name Industry Name Peak Transfer Rate Data Transfers/Second (in Millions)
DDR-200PC-16001600 MB/s200
DDR-266PC-21002100 MB/s266
DDR-300PC-24002400 MB/s300
DDR-333PC-27002700 MB/s333
DDR-400PC-32003200 MB/s400
PC1600 memory — which Crucial no longer carries - is DDR designed for use in systems with a 100MHz front-side bus, (providing a 200 mega transfers per second [MT/s] data transfer rate). The "1600" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 1600MB/s, or 1.6GB/s. PC1600 has been replaced by PC2700, which is backward-compatible.
PC2100 memory — which Crucial no longer carries - is DDR designed for use in systems with a 133MHz front-side bus (providing a 266 MT/s data transfer rate). The "2100" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 2100MB/s, or 2.1GB/s. PC2100 is used primarily in AMD® Athlon® systems, Pentium® III systems, and Pentium IV systems. PC2100 has been replaced by PC2700, which is backward-compatible.
PC2700 memory — the slowest DDR memory speed that Crucial now carries - is DDR designed for use in systems with a 166MHz front-side bus (providing a 333 MT/s data transfer rate). The "2700" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 2700MB/s, or 2.7GB/s. PC2700 is backward-compatible for PC1600 and PC2100.
PC3200 (commonly referred to as DDR400) memory is DDR designed for use in systems with a 200MHz front-side bus (providing a 400 MT/s data transfer rate). The "3200" refers to the module's bandwidth (the maximum amount of data it can transfer each second), which is 3200MB/s, or 3.2GB/s. PC3200 is backward-compatible for PC1600, PC2100, and PC2700.

SDRAM PC100, 125MHz, and PC133

In SDRAM modules, the numbers that come after the "PC" refer to the speed of the system's front-side bus. PC100 memory — which Crucial no longer carries - is SDRAM designed for use in systems with a 100MHz front-side bus. It is used in many Pentium II, Pentium III, AMD K6-III, AMD Athlon, AMD Duron, and Power Mac G4 systems. PC100 has been replaced by PC133, which is backward-compatible.
125MHz memory is SDRAM designed for use in systems with a 125MHz front-side bus. 125MHz has been replaced by PC133, which is backward-compatible.
PC133 memory is SDRAM designed for use in systems with a 133MHz front-side bus. It is used in many Pentium III B, AMD Athlon, and Power Mac G4 systems. PC133 is backward-compatible for PC100 and 125MHz.

Older memory technology — PC66 SDRAM, FPM, and EDO

PC66 memory is SDRAM designed for use in systems with a 66MHz front-side bus. It is used in the Pentium 133MHz systems and Power Macintosh G3 systems. FPM and EDO speeds are written in nanoseconds (ns), which indicates their access time; the lower the number, the faster the memory (it takes fewer nanoseconds to process data).
 
 
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