Percent change = (new − old) ÷ |old| × 100. A positive result is a percentage increase, a negative one is a percentage decrease. The starting value is always the base, so going from 80 to 100 is +25%, but going back from 100 to 80 is only −20%.
Example: a price rises from $80 to $100. The difference is 20, and 20 ÷ 80 = 0.25, so the price went up 25%.
Subtract the old value from the new value, divide by the old value, and multiply by 100. From 50 to 60: (60 − 50) ÷ 50 × 100 = 20% increase.
Use the same formula. If the new value is lower, the result is negative. From 200 to 150: (150 − 200) ÷ 200 × 100 = −25%, a 25% decrease.
Because the base changes. A 50% drop needs a 100% rise to get back to where you started.
Percent change is undefined when the starting value is 0, because you cannot divide by zero.
No, everything is calculated in your browser.
See also: Percentage Calculator · Discount Calculator · Average Calculator