The formula
Subtract the old value from the new value, divide by the old value, then multiply by 100. From 80 to 100: (100 − 80) ÷ 80 × 100 = 25% increase.
A decrease is the same formula
From 100 to 80: (80 − 100) ÷ 100 × 100 = −20%. The minus sign means it fell.
Why up 25% then down 25% is not back to the start
Each change is measured from a different starting number. 100 rising 50% gives 150, and falling 50% from there gives 75, which is 25 below where you began.
Percent versus percentage points
If an interest rate moves from 4% to 6%, that is a rise of 2 percentage points, but a 50% increase in the rate itself.