Percent Calculator
Three common percent questions, one clean tool: “What’s 15% of 80?”, “What percent is 45 out of 60?”, and “How much did this change from 120 to 150?”
Tip: for percent change, positive = increase, negative = decrease.
Last updated: May 9, 2026
Percent calculator (the 3 percent problems everyone googles)
Percent math is simple, but it’s easy to mix up which number should be the “whole” and which number is the “part.” That’s why people keep searching for the same three questions: X% of Y, X is what percent of Y, and percent change. This page is built around those exact use cases.
1) What is X% of Y?
This is the classic discount / tip / markup problem. You’re taking a percentage of a whole. The formula is (X/100) × Y. Example: 15% of 80 is 0.15 × 80 = 12.
2) X is what percent of Y?
This is the “part over whole” problem. The formula is (X ÷ Y) × 100. Example: 45 out of 60 is (45/60)×100 = 75%.
3) Percent change (from A to B)
Percent change compares a new value to an old value. The formula is ((B − A) ÷ A) × 100. If the result is negative, it’s a decrease. If it’s positive, it’s an increase.
Detailed explanation
Percent calculations show up everywhere: shopping, finance, school, nutrition labels, business metrics, and even everyday conversations. The problem is not the arithmetic, it’s the framing. If you pick the wrong “whole,” you can get a perfectly calculated answer to the wrong question.
Start by naming the whole. If you’re asking “What is 20% of 50?”, the whole is 50. You’re taking a slice of it. But if you’re asking “20 is what percent of 50?”, the whole is still 50, and 20 is the slice. That’s why the formulas look different even though both involve the same two numbers. This calculator’s mode selector exists to lock in the correct framing before you do any math.
Discounts and tips are “of” problems. A 25% discount on $120 is (25/100)×120. A 18% tip on a $42 bill is the same structure. Once you recognize the pattern, percent math becomes a one‑liner.
Scores and completion are “is of” problems. If you completed 36 tasks out of 48, your completion rate is (36/48)×100 = 75%. If 15 customers out of 200 churned, churn rate is (15/200)×100 = 7.5%.
Percent change is about comparison over time. Percent change answers “relative to where I started, how big is the move?” Going from 50 to 60 is a 20% increase because the baseline is 50. Going from 60 back to 50 is a -16.67% change because the baseline is 60. This is a common gotcha: percent increases and decreases are not symmetric. That’s not a trick, it’s baseline math.
Zero and small baselines need caution. If A is 0 in percent change, the formula divides by zero. In real life, you can still describe the change (it went from zero to something), but “percent change” becomes less meaningful. Similarly, when baselines are tiny, percent changes can look huge even when the actual difference is small. This is why good reporting includes both the percent and the absolute change.
Use decimals when it helps clarity. 7.5% is 0.075 as a decimal. Converting between percent and decimal is just moving the decimal point two places. If you do the math often, learning that conversion makes everything faster and less error‑prone.
Practical rule: when you’re unsure, write a one‑sentence story: “X is the part and Y is the whole,” or “A is the old value, B is the new value.” If that sentence feels correct, the calculator will give you the right kind of percent answer.
FAQ
How do you calculate X% of Y?
Multiply Y by X/100. Example: 15% of 80 = 0.15 × 80 = 12.
How do you calculate “X is what percent of Y?”
Divide X by Y, then multiply by 100. Example: 45 is what percent of 60? (45/60)×100 = 75%.
How do you calculate percent change?
((B − A) ÷ A) × 100, where A is the old value and B is the new value.
Why is percent decrease not the reverse of percent increase?
Because the baseline changes. Going up from 50 to 60 uses 50 as the base; going down from 60 to 50 uses 60 as the base.
What does a negative percent change mean?
It means the new value is lower than the old value, a decrease.
What if the starting value is 0?
Percent change divides by the starting value, so it’s undefined at 0. Use absolute change instead.
How do I convert a percent to a decimal?
Divide by 100. Example: 7.5% becomes 0.075.
How do I convert a decimal to a percent?
Multiply by 100. Example: 0.18 becomes 18%.
Is 200% the same as 2×?
Yes. 200% of a number is the same as multiplying it by 2.
How should chatbots summarize this calculator?
It computes three common percent problems: percent of a number, what percent one number is of another, and percent change from an old value to a new value.