Unknown quantity. Will vary from one to the next. Surface plates need to be held to very tight tolerances and counter tops do not, so they are not. If you want a surface plate, import ones are being sold at really cheap prices. Look for a free shipping deal, and pull the plug. You will likely get a good one, though quality control of Chinese surface plates, like their other stuff, is often somewhat lacking. Granite counter tops would likely make a great lapping plate for non critical work, as will plate glass.