Percentage Calculator
Solve three everyday percent problems live: what is X% of Y, X is what % of Y, and percentage change from A to B.
What is X% of Y?
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X is what % of Y?
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Percentage change A → B
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How do I calculate percentages quickly?
What this tool does
Three live calculators cover the percent questions people ask most often: what is X% of Y, X is what percent of Y, and percentage change from A to B. Type two numbers in any mode and the answer updates immediately. Copy a single result or everything you have filled with one action.
Why you need it
Percent math is simple on paper but slow under pressure — and easy to invert. Is a €30 discount on €200 “15% off” or “15% of something else”? Did revenue grow 25% or did you accidentally divide new by old the wrong way? Spreadsheets help, but opening one for a two-field check is overhead. This page gives correct formulas with guardrails (like refusing divide-by-zero) without leaving the browser.
How to use it
- Pick the mode that matches your question — each section is independent.
- Enter the two labeled values; results refresh as you type.
- Click Copy on a mode to grab just that answer, or Copy all for every filled section.
- Use Clear to reset all fields before a new problem.
Practical examples
- Retail discount: “What is 20% of €149?” → first mode, X = 20, Y = 149.
- Exam score: “42 out of 50 — what percent?” → second mode, X = 42, Y = 50.
- Metrics dashboard: “Users went from 8,400 to 10,500” → change mode, A = 8400, B = 10500.
- Tip estimate: 18% of a €86 bill → first mode without mental multiplication.
How it works
What is X% of Y? converts the percent to a fraction: (X ÷ 100) × Y. Example: 15% of 200 = 0.15 × 200 = 30.
X is what % of Y? treats X as a part of whole Y: (X ÷ Y) × 100. Example: 30 of 200 → (30 ÷ 200) × 100 = 15%.
Percentage change A → B measures relative growth or shrink vs the starting value: ((B − A) ÷ A) × 100. Going from 80 to 100 is +25%; from 100 to 80 is −20% because the denominator is always the original A.
All three modes accept decimals and negative inputs where meaningful. If Y or A is zero where division is required, the tool reports an error instead of infinity or NaN.
Common mistakes
- Swapping part and whole: “30% of 200” is not the same question as “30 is what % of 200” — pick the matching mode.
- Change vs ratio: Percentage change uses the old value as baseline; comparing B/A without subtracting one gives a different number.
- Adding percents incorrectly: 20% off then 10% off is not 30% off — successive discounts multiply, not add.
- Forgetting the % sign in head math: 15% means 0.15, not 15, when you multiply directly.
FAQ
How do I calculate X percent of Y?
Use the first mode: enter the percent and the base value. Result = (X / 100) × Y.
How is percentage change calculated?
Percentage change = ((new − old) / old) × 100. Positive means increase; negative means decrease.
What if the base value is zero?
Division by zero is undefined. The tool shows an error instead of a fake result.
Can I use decimals and negative numbers?
Yes. All three modes accept decimal inputs. Negative values follow standard signed arithmetic.
Which mode answers "30 is what percent of 200"?
Use the second mode (X is what % of Y). Enter 30 as X and 200 as Y to get 15%.
Is my data stored?
No. Calculations run locally in your browser and are not uploaded.