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Percentage Calculator🔒 100% local
Calculate percentages: X% of Y, X is what % of Y, or percentage change. Privacy-first, no server uploads.
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Answers three percentage questions from one form. Pick the mode, enter two numbers, and the page computes with parseFloat and prints a single line: X percent of Y, X as a percentage of Y, or the change from X to Y. The answer is formatted to two decimal places and can be copied as text.
How to use it
- Choose one of the three radio options. The two field labels change to match the mode.
- Fill both fields - they start at 20 and 150.
- Press Calculate for the result line, or Reset to return to those starting values and the first mode.
Worked example
What is X% of Y? with X = 20 and Y = 150becomes20% of 150 = 30.00X is what % of Y? with X = 30 and Y = 150becomes30 is 20.00% of 150Percentage change X -> Y with X = 150 and Y = 100becomes150 → 100 = 33.33% decreaseChange mode divides by the absolute value of X, so the number is always positive and the direction word carries the sign.
When to use it
- Working out a tip, discount or sales tax without opening a spreadsheet.
- Checking what share of a total a single line item represents.
- Running the same three questions in a row while resetting between them - the Reset button restores the mode as well as the numbers.
What it does not do
- A zero denominator is refused instead of reported as infinite: 'X is what % of Y?' with Y = 0 shows 'Total cannot be zero', and change mode with X = 0 shows 'Original value cannot be zero'.
- Change mode divides by |X|, so a move from -100 to -50 comes back as a 50.00% increase. Mathematically defensible, and surprising if you were thinking about a shrinking negative balance.
- Both fields are numeric inputs, so anything the browser cannot read as a number reaches the component as an empty string and produces 'Please enter valid numbers' rather than a partial parse.
- Exactly one answer is shown at a time and the output box holds only the last one, so there is no history to scroll back through.
- For a fuller report on a single change - ratio, reverse check and the substituted formula - use the Percentage Change Calculator, which takes 'old, new' on one line.