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Three calculators in one, matching how percentage questions are actually phrased: "what is 15% of 200", "30 is what percent of 200", and "what's the change from 120 to 150". Each row computes live as you type — no Calculate button, no remembering which number goes where.
Frequently asked questions
How do I work out a percentage increase between two numbers?
Use the change row: (new − old) ÷ old × 100. Going from 120 to 150 is +25%; going from 150 to 120 is −20%. They differ because the base changes — the most common percentage mistake.
How do I calculate X% of a number by hand?
Multiply by the percentage over 100: 15% of 200 = 200 × 0.15 = 30. For mental math, 10% is one decimal shift, and 5% is half of that.
Why is a 50% gain not cancelled by a 50% loss?
Because each percentage applies to the current value: 100 + 50% = 150, then 150 − 50% = 75. Sequential percentages multiply, they don't add.