I don't have a scale to put the watch on, but to my mind, it weighs about the same as my IWC Minute Repeater in platinum, 
but feels definitely lighter than an earlier Portugieser Platinum model I was interested in a few years ago.
I really liked that one back then, but ultimately it was far too heavy and too bulky for me.
The Eternal is significantly lighter and more wearable. I think that's also because it uses a lot more glass and the case seems thinner.
Best
Thomas