Percentage Calculator – Increase, Decrease & Of
Three percentage questions, one tool: what is X% of Y, what percent X is of Y, and how much a value changed in percent.
For “what is X% of Y” enter the percentage here. For “X is what percent of Y” enter the part. For percent change enter the starting value.
The base or ending value, depending on the calculation type.
15% of 200
30
- Formula
- 15 ÷ 100 × 200
- Half of it
- 15
Useful for tips, commissions, deposits and VAT-style quick maths.
For informational purposes only — not financial or tax advice.
The three percentage formulas
Almost every percentage question is one of three types, and each has its own formula:
- What is X% of Y? → X ÷ 100 × Y. Example: 15% of 200 = 30. Used for tips, sales tax, commissions and deposits.
- X is what percent of Y? → X ÷ Y × 100. Example: 42 of 60 = 70%. Used for test scores, budget shares and conversion rates.
- Percent change from X to Y → (Y − X) ÷ X × 100. Example: 80 → 100 is a 25% increase. Used for price moves, KPIs and growth rates.
Pick the question type in the form, enter your two values, and the result updates instantly with the formula shown — so you can check the logic, not just the answer.
The classic traps
- Direction matters: a rise from 50 to 100 is a 100% increase, but falling back from 100 to 50 is only a 50% decrease. Percent change is always relative to the starting value.
- Successive changes multiply: two 50% discounts leave 25% of the original price, not 0%. A +10% year followed by −10% year does not return to the start — it lands 1% below.
- Percentage points ≠ percent: an interest rate moving from 2% to 3% is up one percentage point, but a 50% relative increase. Both are correct — they answer different questions.
Worked examples
- Tip: 18% of a $64.50 restaurant bill → $11.61.
- Exam score: 42 of 60 → 70%.
- Price move: $240 → $180 → 25% decrease.
- Successive discounts: 20% off, then another 20% → multiply 0.8 × 0.8 = 0.64, i.e. 36% off in total.
Shopping maths with a tax step? The Discount Calculator adds sales tax after a percent-off. For pay maths see the Salary to Hourly converter.
Frequently asked questions
How do I work out a percentage of a number?
Multiply the number by the percentage and divide by 100. For example, 15% of 200 is 15 ÷ 100 × 200 = 30. The tool does this instantly and shows the formula.
How do I calculate percentage increase or decrease?
Subtract the old value from the new one, divide by the old value, and multiply by 100. Going from 80 to 100 is (100 − 80) ÷ 80 × 100 = a 25% increase. The direction (increase or decrease) is shown automatically.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100. If you scored 42 out of 60, that is 42 ÷ 60 × 100 = 70%.
Why does percent change divide by the old value?
Because percentage change is always relative to where you started. A rise from 50 to 100 is a 100% increase, but falling back from 100 to 50 is only a 50% decrease — the base value changed.
Is a 50% discount the same as taking 50% twice?
No. Two successive 50% discounts leave 25% of the original price, not zero. Successive percentage changes multiply, they don't add — try the change mode twice to see it.