Percentage change is the universal way to compare two numbers: a stock price last month versus this month, your weight last year versus today, last quarter’s revenue versus this quarter. The trick is always to divide by the original value, not the new one — otherwise an increase and a decrease will not match up.
How percentage change is calculated
The formula is % change = (new − old) ÷ old × 100. A positive result is an increase; a negative result is a decrease. Going from 80 to 100 is a 25% increase, but going from 100 to 80 is only a 20% decrease — the base is different.
Worked example
A company’s revenue grew from $1.2M to $1.5M: percentage change = (1.5 − 1.2) ÷ 1.2 × 100 = 25%.
