The iPhone 8 is
Dimensions: 138.4mm x 67.3mm x 7.3mm
and
iPhone 6 is
Dimensions: 138.3mm x 67.1mm x 7.1 mm
So virtually the same size, if the bumper case still fits would be almost as useful as it was with the 6. But to be on the safe side best to get a case designed for the iPhone 8.