Calculation details
Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
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What does this calculator estimate?
A percentage calculator handles the three everyday percent problems: what is X% of Y, X is what percent of Y, and percent change between two values.
- X% of Y = X ÷ 100 × Y
- X as % of Y = X ÷ Y × 100
- Change % = (new − old) ÷ old × 100
The three percentage problems
Percentages appear in discounts, tips, taxes, growth, and statistics. The three core operations — finding a percentage of a value, finding what percentage one value is of another, and measuring change — cover nearly every everyday use.
Limitations to watch for
Percent change is relative to the starting value: a 50% rise then a 50% fall does not return to the original (100 → 150 → 75). Percentage points differ from percents when comparing rates (e.g., 5% to 7% is +2 percentage points, +40% relative).
How to use it in practice
Pick the mode that matches the question: 'of' for discounts/tips/tax, 'is what percent' for proportions, 'change' for growth or decline. Use the tool on the numbers, then sanity-check with round numbers.
['Choose the calculation mode.', 'Enter the two values.', 'Read the result.']
How this calculator works
Formula
Three modes: (1) X% of Y = X ÷ 100 × Y; (2) X as a percent of Y = X ÷ Y × 100; (3) percent change = (new − old) ÷ old × 100.
Worked example
15% of 200 = 30. 25 is 31.25% of 80. A price rising from $50 to $75 is a 50% increase.
Assumptions to verify
- The selected mode matches the question.
- Values are non-negative except where change allows negatives.
- Relative change is measured from the old value.
Frequently asked questions
How do I calculate X% of Y?
Multiply: X ÷ 100 × Y. 15% of 200 = 0.15 × 200 = 30.
How do I find what percent X is of Y?
Divide: X ÷ Y × 100. 25 ÷ 80 × 100 = 31.25%.
How do I calculate percent change?
(New − old) ÷ old × 100. $50 to $75 = (25 ÷ 50) × 100 = 50% increase.
What's the difference between percent and percentage points?
A rate going from 5% to 7% is a 2-percentage-point rise but a 40% relative increase. They measure different things.
Why doesn't a 50% rise then fall return to the original?
Because the fall is relative to the larger value: 100 → 150 → 75. Percentage changes don't cancel symmetrically.
How do I calculate a tip?
Use 'of': 18% of the bill. On a $40 bill that's $7.20.
How do I calculate a discount?
Subtract X% of the price from the price — or multiply by (1 − X/100).
Cite this tool
BoringToolsKit. “Percentage Calculator.” boringtoolskit.com/percentage-calc/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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