Four percentage calculators in one page: find what X% of Y is, work out what percent one number is of another, calculate percentage increase or decrease between two values, and apply a percent change to a number.
Nearly every everyday percentage question falls into one of four categories, and this tool provides a dedicated calculator for each. 'What is X% of Y?' answers questions where you know a rate and a total and want an amount (a tip, a discount, a tax). 'X is what % of Y?' answers questions where you know a part and a whole and want a rate (a test score, a completion percentage, a market share). 'Percentage Increase/Decrease' answers questions where you know two values and want to know how much one changed relative to the other (a price change, a population change, a performance change). 'Increase/Decrease X by Y%' answers questions where you know a starting value and a rate of change and want the resulting new value (applying a raise, a markup, a discount). Recognizing which of these four shapes your specific question fits is often the hardest part of a percentage problem — once you know which one applies, the arithmetic itself is simple.
This is the most fundamental percentage operation: converting a percentage into an actual amount relative to some total. The formula — (X ÷ 100) × Y — works because a percentage is, by definition, a fraction out of 100; dividing by 100 converts it into an equivalent decimal fraction, and multiplying by the total scales that fraction up to a real amount. This calculation underlies tip calculations, sales tax, commission, interest on a principal amount, and any scenario where a rate is applied to a base amount to produce a concrete result.
This is the inverse operation: instead of starting with a rate and a total to find an amount, you start with a part and a whole and want to express that relationship as a rate. The formula — (part ÷ whole) × 100 — first expresses the part as a plain decimal fraction of the whole, then converts that fraction into a percentage by multiplying by 100. This calculation is everywhere in reporting and analysis: test scores, project completion percentages, budget category shares, survey response rates, and win/loss ratios are all, mathematically, exactly this same calculation applied to different kinds of numbers.
Percentage change measures how much a value has grown or shrunk relative to where it started, expressed as a percentage of that starting point: ((new − old) ÷ old) × 100. The key detail that trips people up is the denominator — it's always the original (old) value, never the new value, since percentage change is inherently about the size of the change relative to where you began, not relative to where you ended up. A positive result means an increase; a negative result means a decrease. This calculation is standard in finance (price changes, investment returns), business (revenue growth, cost changes), and statistics (year-over-year comparisons) whenever two values from different points in time or different categories need to be compared proportionally rather than just by their raw difference.
This calculator runs the percentage-change relationship in the opposite direction from the previous one: instead of starting with two values and solving for the percentage between them, you start with one value and a percentage and solve for the resulting new value. Increasing multiplies by (1 + rate); decreasing multiplies by (1 − rate). This is the calculation behind applying a raise or markup (increase), applying a discount (decrease), or projecting a value forward by a known growth or shrinkage rate. It's worth remembering that increasing and then decreasing by the same percentage (or vice versa) does not return you to the original value, since each step's percentage is calculated relative to a different base — this is one of the most common sources of error when reasoning about sequential percentage changes like stacked discounts or repeated rate adjustments.
Percentage Calculator handles general-purpose percentage math. These related calculators apply percentage concepts to more specific everyday scenarios.