Four percentage questions, one tool — of, is-what-percent, change, and reverse.
Multiply the base value by the percentage, then divide by 100.
What is 15% of $200?
Percentage arithmetic is exact within double-precision floating point. Percentage change uses the absolute value of the original as the base — the standard convention in finance and statistics.
What is X% of Y, X is what percent of Y, percentage increase/decrease, and X is Y% of what — four percentage calculators in one, with formulas and worked examples. Free.
One tool for all four percentage questions people actually ask: What is X% of Y? (discounts, tips, tax), X is what percent of Y? (scores, market share, proportions), percentage increase or decrease from X to Y (price changes, growth rates, KPI movement), and X is Y% of what? (reverse percentage — finding the original before a markup or discount). Each mode displays its formula and a step-by-step worked example you can replicate by hand, so the tool doubles as a refresher on percentage arithmetic.
Shoppers verify discount math and sale prices. Students check homework and learn the underlying formulas from the worked examples. Analysts and managers compute period-over-period changes with the standard (old-value-base) convention. Freelancers and small-business owners reverse out pre-discount prices, compute margins, and check markup math. Anyone comparing two numbers as a share of each other gets an instant, precise answer.
(1) Pick the question type from the dropdown — the input labels change to match. (2) Enter your two numbers. (3) Read the answer, stated in plain language beneath the figure. (4) Check the formula block and by-hand worked example to see exactly how the arithmetic proceeds, step by step. Percentage change uses the absolute value of the original as the base, so decreases compute correctly even when values go negative.
A common source of error: moving from 10% to 15% is a 5 percentage-point change, but a 50% relative change (5 ÷ 10). This calculator computes relative percentage change — the standard for growth rates and financial returns. When a headline says "rates rose 2%", check whether the source means percentage points or percent: the difference between 2 points on a 10% rate (12%) and a 2% relative rise (10.2%) is material. Stating which convention you used, as the formula block does here, prevents that ambiguity.
Multiply the number by the percentage and divide by 100. For example, 15% of 200 = 200 × 15 ÷ 100 = 30. The "What is X% of Y?" mode does this instantly and shows the formula.
Subtract the old value from the new one, divide by the absolute old value, and multiply by 100: (New − Old) ÷ |Old| × 100%. Positive results are increases, negative are decreases. Example: 200 → 230 is +15%.
Divide the known value by the percentage expressed as a decimal. If 30 is 15% of a number, the number is 30 ÷ 0.15 = 200. Use the "X is Y% of what?" mode.
Percentage change measures growth relative to where you started, so the original value is the base. Dividing by the old value answers "what fraction of the original was gained or lost" — the convention used in finance, statistics, and retail discounting.
This tool is also known by these tasks — each link opens the same tool with a focused guide:
What do you call a crab that plays baseball?
No paywalls, no signups, no data sold. Built by a solo developer who believes useful tools should be accessible to everyone.
☕Support me on Ko-fi— keep tools free100% of proceeds go towards hosting & building more free tools.