Percentage Calculator

Three Percentage Problems, One Tool

This calculator handles the three percentage questions that come up most often. Choose the mode that matches your question — the tool does the arithmetic instantly.

Percentages show up everywhere: shopping discounts, exam scores, salary increases, tax amounts, growth rates, and loan interest. If you find yourself reaching for a calculator whenever a percentage is involved, the modes below cover the most common situations.

The Three Calculation Modes

1. What is X% of Y? — Use this to find a portion of a value. For example, "What is 20% of 500?" → 100. Formula: (X / 100) × Y
2. X is what % of Y? — Use this to express a part as a percentage of a whole. For example, "100 is what % of 500?" → 20%. Formula: (X / Y) × 100
3. % change from X to Y? — Use this to measure growth or decline. For example, "% change from 400 to 500?" → +25%. Formula: ((Y − X) / X) × 100

Example Calculations

Everyday Situations Where This Helps

• Shopping discounts — A jacket was $120 and is now 30% off. Mode 1: what is 30% of $120? → $36 saved, so you pay $84.
• Exam scores — You scored 78 out of 120. Mode 2: 78 is what % of 120? → 65%.
• Salary increase — Your pay went from $45,000 to $49,500. Mode 3: % change → 10% rise.
• Tax and VAT — An invoice is $500 plus 18% tax. Mode 1: what is 18% of $500? → $90 tax, total $590.
• Tipping — Your restaurant bill is $68 and you want to leave 15%. Mode 1: 15% of $68 → $10.20 tip.

Common Percentage Mistakes Worth Avoiding

Even simple percentage questions can trip people up. Here are three errors that come up regularly:

• Direction matters for % change — A price rising from $100 to $150 is a 50% increase. But going back from $150 to $100 is only a 33.3% decrease, not 50%, because the base changes. Mode 3 accounts for this automatically when you enter the numbers in the right order.
• Stacking percentages is not additive — A 20% discount followed by a further 10% discount is not a 30% discount. The second 10% applies to the already-reduced price, giving a combined discount of 28%.
• Round at the end, not in the middle — When chaining multiple percentage calculations, keep full decimal precision in intermediate steps. Rounding too early causes small errors to accumulate.

Common Mistakes to Avoid

• Picking the wrong mode for the question — "What is 20% of 500?" and "20 is what % of 500?" look similar but need different modes and give very different answers (100 vs. 4%). Read the question carefully before choosing.
• Entering values in the wrong order for % change — Mode 3 expects the starting value first (X) and ending value second (Y). Swapping them flips the sign and changes the magnitude of the result.
• Trying to calculate % change from zero — Percentage change is undefined when the starting value is zero, because you can't divide by zero. This calculator blocks that input rather than showing a meaningless number.
• Adding percentages instead of compounding them — A 20% pay rise followed by a 20% pay cut does not return you to your original salary; the cut applies to the already-raised amount, so you end up lower than where you started. Run each step through the calculator separately rather than adding or subtracting percentages directly.
• Misreading percentage points vs. percent — Going from a 5% interest rate to a 6% rate is a 1 percentage point increase, but a 20% relative increase. Be clear about which one your context requires.

Frequently Asked Questions