-50% is the change from 256 to 128
Calculation steps:
[( 128 - 256 ) ÷ 256] x 100 = -50%
Formula
Percentage change = [(128 - 256) / 256] x 100
Subtract the new number by the original number
128 - 256 = -128
-128 ÷ 256 = -0.5
-0.5 x 100 = -50%
Example