Percent change formula
Percent change = (new − old) ÷ |old| × 100
A positive result is an increase and a negative result is a decrease. For example, a price going from $250 to $310 is (310 − 250) ÷ 250 × 100 = +24%. Going from $310 back to $250 is (250 − 310) ÷ 310 × 100 = −19.35%.
Percent change vs. percentage difference
Use percent change when there's a clear "before" and "after" — prices, salaries, populations, stock values. Use percentage difference when comparing two values with no natural order, such as two quotes or two measurements. It divides the gap by the average of both values, so the answer is the same whichever way round you enter them.
Percent vs. percentage points
If an interest rate rises from 4% to 5%, it went up by 1 percentage point, which is a 25% increase in the rate. Mixing these up is a classic error in news reports.