While many rumors have suggested that the iPad 3 wil be slightly thicker than its predecessor, and a leaked iPad 3 casing did, in fact, appear slightly thicker, Apple.pro measured the casing with a caliper, and reports (via MacRumors) that it is indeed nearly 1mm thicker than the iPad 2.
According to the photo, the iPad 2 rear shell measures 8.69 mm, whereas the iPad 3 part measures 9.50mm. thick while the iPad 3 part comes in at 9.50 mm, a difference of 0.81 mm. Apple’s official iPad 2 specs peg the device at 8.8 mm thick (complete with front glass), which is just a touch thicker than the rear shell alone.
MacRumors points out that, using the same front glass thickness as an assumption, the iPad 3 would be around 9.6mm – far thinner than the original iPad, and about 9% thicker than the iPad 2. Translation: The thickness will barely be noticeable when holding the device.