Choose the formula from the wording of the question
A percentage means “per hundred,” but the same two numbers can answer different questions. First decide whether you need a percentage of an amount, one value’s share of a reference value, or the change from an earlier value to a later value. The calculator changes the field labels after that choice so the order remains visible.
What is a percentage of a whole value?
Use this path for questions such as “What is 20% of 150?” Divide the percentage by 100 and multiply by the whole value: 20 ÷ 100 × 150 = 30. The percentage may exceed 100 or be negative when the situation genuinely calls for it. For a store reduction followed by tax, use the discount calculator, because the order of operations matters.
What percentage is one value of another?
Use this path when a part or measured value must be expressed relative to a whole or reference value. Divide the part by the whole and multiply by 100. If 30 people out of 120 chose an option, 30 ÷ 120 × 100 = 25%. The whole cannot be zero because division by zero does not define a percentage.
What is the percentage change?
Use the starting value as the reference: (new value − starting value) ÷ starting value × 100. Moving from 80 to 100 is a 25% increase because the 20-unit difference is one quarter of 80. Moving from 100 back to 80 is a 20% decrease, not 25%, because the reference value is now 100. The starting value cannot be zero; report the absolute change or use a domain-specific convention instead. With a negative starting value, the signed formula can still be shown, but ordinary “increase” and “decrease” wording may be misleading, so the tool flags that case.
Percentage change is not percentage-point change
If a rate moves from 20% to 25%, the difference is 5 percentage points. Relative to the starting rate, it is a 25% increase: (25 − 20) ÷ 20 × 100. State which measure you mean. Percentage points are usually clearer when comparing tax rates, survey shares, interest rates, margins, or probabilities that are already written as percentages.
Do not round the inputs too early
Enter the best available source values and round only the final answer to the precision the decision needs. A display with several decimals does not make an estimate more certain. When the values come from prices, use consistent before-tax or after-tax amounts. For business pricing decisions, the margin and markup calculator keeps revenue, cost, margin, and markup definitions separate; for compensation changes, use the pay raise calculator.
The tool performs arithmetic on the values entered. It does not determine which population, price base, time period, tax base, or comparison is appropriate for a particular decision.