If I'm not mistaken, the iPhone 7 and iPhone 8 are the same exact size, so you can test an Iphone 7 case on an iPhone 8 and see if it fits. It seems like the buttons/holes configuration is the same as well? On the other hand, I believe the Plus' are different sizes.
Haven't gotten a hold of an iPhone 8 or I would try my iPhone 7 cases on it.
Haven't gotten a hold of an iPhone 8 or I would try my iPhone 7 cases on it.