Full Details from Amazon.com : http://www.amazon.com/gp/product/B00JFVMPF8?tag=onigi-20

4GB Upgrade for a Lenovo ThinkPad W500 Series System (DDR3 PC3-12800 NON-ECC )

204-pin SODIMM
DDR3-1600
DDR3 PC3-12800 Unbuffered NON-ECC DDR3-1600 135V
The only brand of memory upgrade available directly from a DRAM manufacturer tested to meet the most stringent industry specifications
Install it yourself in about 15 minutes The same top-quality memory that the worlds largest computer manufacturers install in the systems they sell