Find a percentage of a total
Part = total × percentage ÷ 100. Example: 15% of 250 = 250 × 15 ÷ 100 = 37.5.
Find what percentage one value is of another
Percentage = part ÷ whole × 100. The whole must not be zero. Example: 34 out of 40 = 34 ÷ 40 × 100 = 85%.
Find the whole from a part and percentage
Whole = part ÷ (percentage ÷ 100). Example: if 30 is 15% of a whole, the whole is 30 ÷ 0.15 = 200. A percentage of zero cannot be used as a divisor.
Percentage change
Change = (ending − starting) ÷ starting × 100. Use this for a positive, non-zero starting value. Explain zero and negative starting values separately rather than presenting a result as universally meaningful.
Sequential discounts
Final price = original × (1 − first discount) × (1 − second discount). For 20% followed by 10%, multiply by 0.80 and then 0.90; the combined discount is 28%, not 30%.
Tax added after a price
Final price = price before tax × (1 + tax rate). Tax law and retailer pricing conventions differ. This is arithmetic, not tax advice.
Use the interactive calculators to see the formula update with your own inputs.