What this calculator does
It answers the five everyday percentage questions: what is X% of a number, what percentage one number is of another (like marks), the percentage increase or decrease between two values, a number after adding or subtracting a percentage, and the total when you know a part and its percentage.
How to use it
- Tap the question you want to answer, for example X is what % of Y.
- Type the two numbers. The labels change to tell you what goes in each box.
- For Add or subtract %, also choose Increase or Decrease.
- Read the answer and the one-line explanation below it. It updates as you type.
You can type numbers with commas, like 12,000 or 1,00,000.
Examples
These examples and answers are from the NCERT Class 8 Mathematics textbook (Ganita Prakash, Part II, Chapter 1):
| Question | Answer |
|---|---|
| 25% of 120 g | 30 g |
| 35% of 95 g | 33.25 g |
| 42 marks out of 50 | 84% |
| Price ₹30 → ₹42 | 40% increase |
| Footfall 160 → 100 | 37.5% decrease |
| ₹2,650 sold at 18% loss | ₹2,173 |
| 40% of a journey is 92 km; full journey? | 230 km |
| 650 kg compared with 260 kg | 250% |
Formula
Percent means “per hundred”, so X% = X ÷ 100.
- X% of Y = X ÷ 100 × Y
- X is what % of Y = X ÷ Y × 100
- % change from old to new = (new − old) ÷ old × 100. A positive answer is an increase, a negative one a decrease.
- Y increased or decreased by X% = Y × (100 ± X) ÷ 100
- Y is X% of what? = Y ÷ X × 100
Answers are shown with up to 2 decimal places, without extra zeros: 441, 86.4, 33.33. The calculation itself uses full precision.
Rules and limits
- Any number from −10,00,00,00,00,000 to 10,00,00,00,00,000 (1 lakh crore) is accepted, including decimals and negative numbers. The answer must also stay within this range.
- You cannot divide by zero: “5 is what % of 0” and “20 is 0% of what” have no answer.
- Percentage change from 0 is undefined, because there is no base to compare with.
- If the old value is negative, the change is measured against its size (without the minus sign), so a rise still shows as an increase. From −50 to −25 is a 50% increase.
These limits are set by RozTools for this calculator.
Common mistakes
- Using the wrong base for a change. A rise from 100 to 125 is 25%, but a fall from 125 to 100 is 20%, not 25%. The base is always the old value.
- Adding percentages one after another. A 10% rise followed by a 10% fall does not bring you back to the start: 100 → 110 → 99.
- Mixing up “% of” and “% more than”. 250% of 40 is 100, but 250% more than 40 is 140.
- Rounding too early. Round only the final answer. Rounding each step can change the result.
Disclaimer
Results are for general use and study. For exam results, admissions or official records, follow the rules published by your board, university or employer.