The calculator above handles the three ways a "percent drop" question shows up: the decrease between two numbers, what is left after cutting a value by some percent, and the original value before a known decrease. Each mode prints the arithmetic under the result.

Formula

$\text{percentage decrease}=\frac{\text{initial}-\text{final}}{|\text{initial}|}\times 100\%$

$\text{final}=\text{initial}\times\left(1-\tfrac{p}{100}\right) \qquad \text{initial}=\frac{\text{final}}{1-\frac{p}{100}}$

The change is always measured against the initial value: 80 to 60 is a 25% decrease, but 60 to 80 is a 33% increase - same gap, different base. Percent change is undefined when the initial value is zero.

Two worked examples:

$\frac{50-38}{50}\times 100=24\% \qquad \frac{51}{1-0.40}=\frac{51}{0.60}=85$

The second is the one people get wrong: a jacket at 51 after a 40% markdown cost 85, not 51 plus 40% (71.40) - the discount was taken from the bigger number.

Scientific calculator - for the division and rounding by hand.
Weighted grade calculator - percentages weighted across categories.

Sources and further reading

Wikipedia: Relative change - the formal definition of percent change and its edge cases.
Wikipedia: Percentage point - percent change vs. point change when the values are themselves percentages.

Percentage Decrease Calculator