Explaining percentage change in one sentence

For a change from an old value to a new one, divide the difference by the old value and multiply by 100. Report the direction, both values, and sensible rounding: “from 80 to 100, up 25%.”

Percentage Calculator can check the arithmetic, but the denominator still needs to be stated. If the starting value is zero, ordinary percentage change is undefined; describe the absolute change instead.

What are the starting and ending values?