Percentage Calculator
Calculate percentages, percentage change, increase/decrease, and reverse percentages.
What is X% of Y?
What is a Percentage Calculator?
A percentage calculator expresses a part of a whole as a fraction of one hundred. The word percent simply means "per hundred", so 15 percent is the same as 15 out of 100, or 0.15 as a decimal. This makes percentages a convenient, scale-free way to compare quantities.
This tool bundles four of the most common percentage tasks into a single page. The first finds a percentage of a number, the second finds what percent one number is of another, the third calculates the percentage change between an original and a new value, and the fourth reverses the operation to recover the original total.
Each mode shows its own formula directly beneath the result, so you can see exactly how the answer was produced. Percentage-of and reverse results are rounded to four decimals, while proportion and percentage change results are rounded to two decimals for readability.
Because every mode updates as you type and runs entirely in the browser, the calculator is quick to use for shopping, finance, statistics, and everyday arithmetic without any risk to your data.
Key Concepts
- Percent — a ratio expressed as a fraction of 100.
- Percent of — the value obtained by multiplying a number by a percentage.
- Proportion — what percentage one value is of a total.
- Percentage change — the increase or decrease from an original value to a new one.
- Reverse percentage — recovering the whole when a part and its percentage are known.
- Formula display — the exact arithmetic is shown beneath every result.
How to Use
- Choose a mode — click one of the four mode buttons to switch the calculator.
- Enter the values — fill in the two input fields for the selected mode.
- Read the result and formula — the answer appears with the formula that produced it.
- Copy the result — use the copy button beside the result field when you need it elsewhere.
- Compare modes — switch between modes to see the same numbers from a different angle.
Calculation Modes Reference
The four modes answer four different questions. The table below pairs each mode with the question it answers, the formula it applies, and the number of decimals it rounds to.
| Mode | Question | Formula | Rounding |
|---|---|---|---|
| What is X% of Y? | a part of a total | (X / 100) × Y | 4 decimals |
| X is what % of Y? | a proportion | (X / Y) × 100 | 2 decimals |
| % Change | increase or decrease | ((new − original) / original) × 100 | 2 decimals |
| Reverse % | find the whole | X / (Y / 100) | 4 decimals |
Worked Examples
- Percent of — 15% of 200 is (15 / 100) × 200 = 30.
- Proportion — 30 is (30 / 200) × 100 = 15% of 200.
- Increase — from 100 to 150 is ((150 − 100) / 100) × 100 = 50% increase.
- Decrease — from 200 to 150 is ((150 − 200) / 200) × 100 = 25% decrease.
- Reverse — if 30 is 15% of a number, the number is 30 / (15 / 100) = 200.
- Simple percent of — 10% of 250 is (10 / 100) × 250 = 25.
Tips and Common Mistakes
- Original cannot be zero — percentage change divides by the original value, so a zero original returns an "enter valid numbers" message.
- Total cannot be zero — the proportion mode divides by the total, so a zero total is rejected.
- Percentage cannot be zero — the reverse mode divides by the percentage, so a zero percentage is rejected.
- Watch the sign — percentage change reports an increase for a positive result and a decrease for a negative one.
- Rounding differs by mode — percent-of and reverse round to four decimals, while proportion and change round to two.
- Percentages can exceed 100 — a value larger than the whole, such as a 150% increase, is valid and displayed normally.
Common Use Cases
Percentages appear wherever quantities are compared. These are the everyday and professional situations where a percentage calculator is most useful.
- Discounts and sales tax — work out the saving on a sale price or the tax added to a subtotal.
- Tips and gratuities — calculate a 15% or 20% tip on a restaurant bill.
- Test and exam scores — convert a mark into a percentage grade.
- Profit margins — measure the change in revenue, cost, or price over time.
- Usage and battery — find how much data or battery remains from a total.
- Statistics and surveys — express responses as a share of the total sample.
Frequently Asked Questions
How do I calculate a percentage of a number?
Multiply the number by the percentage divided by 100. For example, 15% of 200 is 200 × 15/100 = 30. Use the "What is X% of Y?" mode for this.
How do I find what percent one number is of another?
Divide the part by the total and multiply by 100. For example, 30 is 30 / 200 × 100 = 15% of 200. Use the "X is what % of Y?" mode.
How is percentage change calculated?
Subtract the original value from the new value, divide by the original value, then multiply by 100. A positive result is shown as an increase and a negative result as a decrease.
What is reverse percentage?
Reverse percentage recovers the whole when you know a part and its percentage. For example, if 30 is 15% of a number, the number is 30 / (15/100) = 200.
Why can't the original value be zero in percentage change?
The change formula divides by the original value, so a zero original would be undefined. The tool shows "Enter valid numbers (original cannot be zero)" instead of a result.
How many decimals are shown?
Percent-of and reverse percentage results are rounded to four decimals, while "what percent" and percentage change results are rounded to two decimals.
Can a result be greater than 100%?
Yes. Proportions and changes can exceed 100%, for example when a value more than doubles or when a part is larger than the total it is compared against.
Is this tool free and private?
Yes. All four modes run in your browser and no data is sent to a server, so the calculator is free and private to use.