Percentage Calculator: Three Questions, One Tool

A percentage calculator answers the three percentage questions that come up most often: what is X percent of Y, X is what percent of Y, and what is the percentage change from one value to another. Pick the mode, enter two numbers, and the tool returns the result with the formula and a plain-language explanation.

Most percentage mistakes are not arithmetic mistakes. They happen because the wrong question was asked, or because the base for the comparison was not the one the reader assumed. This tool makes the mode explicit, so you can see which values are being divided by which before you quote the answer.

#The three modes

#What is X% of Y?

This is the everyday discount-and-tax calculation. The tool multiplies X by Y and divides by 100.

Example: 20% of 150 is (20 ÷ 100) × 150 = 30.

Use it for a sale discount, a tip, a tax amount, a commission or a deposit. The answer is in the same unit as Y. If Y is money, the result is money.

#X is what percent of Y?

This reverses the question. It divides X by Y and multiplies by 100.

Example: 30 is what percent of 150? (30 ÷ 150) × 100 = 20%.

Use it when you know the part and the whole and want the share. Marking, test scores, market share, budget allocation and completion percentages all use this form.

#Percentage increase or decrease

This compares a starting value with a later value.

Example: from 100 to 125 is ((125 − 100) ÷ 100) × 100 = 25% increase.

The tool reports the direction separately from the size, so a fall from 100 to 75 comes back as a 25% decrease, not negative 25% in the headline. The start value cannot be zero, because percentage change is relative to the starting point and division by zero has no meaningful answer.

#How to use the percentage calculator

1. Open the percentage calculator.
2. Choose the question that matches what you are trying to find.
3. Enter the two values in the fields that appear.
4. Press Run.
5. Read the result, the formula and the explanation together. The formula is the check: if the formula does not describe your situation, you chose the wrong mode.

You can switch modes without reloading the page. Only the fields for the selected mode are submitted.

#Which mode solves which problem?

The fourth row is worth spelling out because it is where people get stuck. A 20% discount on 150 is 30 off, so the sale price is 120. The calculator gives you the 30; the subtraction is yours. Do not use the percentage-change mode for this, because 120 compared with 150 is a 20% decrease, which happens to match here but will not match after a price increase.

#Common problems and fixes

The answer is 100 times too big or too small. You probably divided in the wrong order or forgot to multiply by 100. Use the formula shown with the result and check the position of the decimal point.

A percentage increase and a percentage decrease do not cancel out. A 50% fall from 100 gives 50; a 50% rise from 50 gives 75. Percentages are relative to the value you start from, which changed. This is not a tool error.

The discount result is not the final price. The tool returns the amount of the percentage, not the amount left after subtracting it. Subtract the result from the original value for a sale price, or add it for tax.

Percentage points and percent are confused. Going from 10% to 15% is a 5 percentage-point rise but a 50% relative increase. Decide which one your reader needs. The change mode measures relative change; subtract the two percentages directly if you mean percentage points.

The base is zero. Percentage change from zero is undefined. If the starting value is genuinely zero, describe the change in absolute terms instead of a percentage.

Negative values are rejected in two modes. "What is X% of Y" and "X is what percent of Y" require non-negative inputs. The change mode accepts negative values because temperatures, balances and deltas can be negative, but the starting value still cannot be zero.

The result has more decimals than expected. Results are rounded to two decimal places. For money, round at the final step rather than at every intermediate step, or small differences accumulate.

#Tips for getting percentages right

• Name the base. Every percentage is a fraction of something. Write "20% of the pre-tax total" rather than "20%" so the reader cannot substitute a different base.
• Do the check in reverse. If 20% of 150 is 30, then 30 divided by 150 should be 20%. The second mode is a built-in verification of the first.
• Keep percentage points separate. When comparing two percentages, say "percentage points" if you mean the arithmetic difference and "percent" if you mean relative change.
• Round once, at the end. Rounding each step is the most common source of a total that does not add up.
• Be careful with percentage change on small bases. A move from 2 to 4 is a 100% increase, which sounds huge but is two units. Report the absolute numbers alongside the percentage.
• Do not use percentage change to compare two different things. It measures movement over time or between two states of the same measure, not the size difference between unrelated quantities.

#How percentages connect to text and token counts

Percentages turn up in writing tools more often than people expect. If you cut a draft from 1,400 words to 1,120, the percentage-change mode tells you the cut was 20%. Use the word counter to get the before and after lengths, then paste them into the percentage calculator.

The keyword density checker applies the same part-over-whole maths to words: a phrase used 12 times in 600 words has a 2% density. That is a readability figure, not a ranking target, but the arithmetic is the same. And if you are estimating API cost, the LLM token counter gives you the token counts; percentages help you compare one model's price with another.

#Frequently asked questions

#Bottom line

The hard part of percentages is choosing the right question, not pressing the buttons. This calculator makes the three common questions separate and shows the formula with every answer, so you can catch a wrong base before it reaches a report. Start with the percentage calculator, and use the word counter when the numbers you need come from a piece of text.