Apple didn’t announce a lot of specs of the Apple Watch besides the 2 different face sizes. There’s a 42 mm one and a 38 mm one. Thickness or screen size are nowhere to be found in Apple’s marketing material.
Based on the 42 mm Watch and several images on Apple’s website I did some measurements to determine some of the specs we don’t know yet.
According to Apple this watch should be 42 mm high.
I checked the 42mm with this image of a Watch next to an iPhone 6 of which we know the height (138.1 mm). It’s not a 6 Plus, I checked that too. The Watch is indeed 42mm high (metal to metal).
Based on this information it’s trivial to get all the other measurements. Now the size of the sapphire is known and we have this handy image of the side of the Watch.
I checked to make sure this is the 42 mm watch as well by overlapping the first image in this post. The screen is also a mystery, but I tried to get a sense it by finding an image on which the content is bordered by the screen on all sides. I assume that is the case in this image:
Now we’re able to get all the dimensions of the 42 mm Apple Watch.
42 mm Apple Watch dimensions
The body including sensor: 36.2 mm x 42 mm (12.46 mm thick)
The body without sensor: 36.2 mm x 42 mm (10.6 mm thick)
The sapphire part: 33.0 mm x 39.1 mm
The screen: 24.3 mm x 30.5 mm (about 39 mm / 1.54 inch diagonally), that makes the aspect ratio about 4:5.
38 mm Apple Watch dimensions
The body including sensor: 32.9 mm x 38 mm (12.3 mm thick)
There was some discussion in the comments on the thickness of the 38 mm unit. Thanks to the commenters I found images of the 38 mm Apple Watch from the side. Based on those images the 38 mm Watch is a little thinner than the 42 mm Watch.
The Digital Crown and button on the side of the Watch are the same size as well:
About the screens
I’ve seen the number 1.5 and 1.7 inch been thrown around as screen sizes, but that just seems be a direct translation of the 38 mm and 42 mm numbers Apple gave us, which are the heights of the devices.
I’ve got no idea of pixels dimensions or ppi, maybe that can be calculated based on screenshots of the demo during the keynote. Based on some images on Apple’s website I would take an uneducated guess at 280 x 350 which would put it at around 290 ppi.
Dave Chap shared an image showing the relative thickness to the iPhone5/5S and iPhone 6:
— DaveChap (@DaveChap) September 11, 2014
I updated the thickness of the 42 mm Apple Watch from 12.6 to 12.46 mm and added dimensions for the 38 mm one as well. I also added a comparison image looking at both watches from the side, including a very high resolution image.