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Memory speeds and compatability

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 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)
DDR3-800 PC3-6400 6400 MB/s 800
DDR3-1066 PC3-8500 8533 MB/s 1066
DDR3-1333 PC3-10600 10667 MB/s 1333
DDR3-1600 PC3-12800 12800 MB/s 1600

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)
DDR2-400 PC2-3200 3200 MB/s 400
DDR2-533 PC2-4200 4266 MB/s 533
DDR2-667 PC2-5300 5333 MB/s 667
DDR2-800 PC2-6400 6400 MB/s 800
DDR2-1066 PC2-8500 8533 MB/s 1066

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.

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-200 PC-1600 1600 MB/s 200
DDR-266 PC-2100 2100 MB/s 266
DDR-300 PC-2400 2400 MB/s 300
DDR-333 PC-2700 2700 MB/s 333
DDR-400 PC-3200 3200 MB/s 400

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.

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.

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).

About adding faster memory ...

It may seem confusing, but faster memory will not necessarily make your system faster. You can't speed up your computer by adding faster memory if any of the other components in your computer (your processor or other memory modules) operate at a slower speed.

Keep in mind, that the right memory for your computer is the kind of memory it was designed to take. Check your system manual or look up your system in the Crucial Memory Advisor or System Scanner tools to find the memory guaranteed to be 100 percent compatible or your money back!

Two ways to find your upgrades

Crucial Advisor tool
If you know your system's specs, use this tool.

Crucial System Scanner
If you don't know your system's specs, use this tool.