Percentage Calculator
Pick the type of percentage problem you want to solve. Enter your values and click calculate.
1) What is X% of Y?
2) A is what percent of B?
3) Percentage increase or decrease
4) Find original value before percentage change
How do I calculate a percentage?
A percentage is just a way to express a number as “out of 100.” The word percent literally means “per hundred.” So when you see 25%, it means 25 out of 100, or 0.25 in decimal form.
Most percentage questions fall into four common types. If you learn these patterns, you can solve almost any everyday problem—tips at restaurants, discounts while shopping, grade calculations, tax rates, savings growth, and more.
The core percentage formulas
- Find X% of Y:
(X/100) × Y - Find what percent A is of B:
(A/B) × 100 - Find percentage change:
((New - Old)/Old) × 100 - Find original value after change: divide by the change factor (
1 ± p/100)
Example 1: What is 15% of 200?
Convert 15% to decimal: 0.15. Then multiply:
0.15 × 200 = 30.
So 15% of 200 is 30.
Example 2: 30 is what percent of 120?
Use (part ÷ whole) × 100:
(30 ÷ 120) × 100 = 25%.
So 30 is 25% of 120.
Example 3: Price goes from $80 to $100
Difference is 100 - 80 = 20.
Divide by old value: 20 ÷ 80 = 0.25.
Convert to percent: 0.25 × 100 = 25%.
This is a 25% increase.
Step-by-step method you can always use
- Identify what each number means (part, whole, old, new, etc.).
- Pick the right formula type.
- Convert percentages to decimals when multiplying (e.g., 8% = 0.08).
- Keep units in mind (dollars, people, points, etc.).
- Round at the end, not in the middle, for better accuracy.
Percentage increase vs. percentage points
People often mix these up:
- Percentage points: 10% to 15% is +5 percentage points.
- Percent increase: from 10% to 15% is a 50% increase, because
(15-10)/10 = 0.5.
This distinction matters in finance, statistics, and news reporting.
Quick mental math shortcuts
- 10% = move decimal one place left (10% of 250 = 25).
- 1% = move decimal two places left (1% of 250 = 2.5).
- 5% = half of 10% (5% of 250 = 12.5).
- 15% = 10% + 5% (37.5 on 250).
- 20% = 10% × 2.
Real-world uses
Shopping discounts
If a $120 jacket is 25% off, discount = 0.25 × 120 = 30.
Final price = 120 - 30 = 90.
Tips and tax
For a 18% tip on a $50 bill, tip = 0.18 × 50 = 9.
Total = 50 + 9 = 59 (before tax).
Grades
If you got 42 out of 50, percentage score is (42/50) × 100 = 84%.
Common mistakes to avoid
- Using the wrong “whole” value when finding a percentage.
- Forgetting to divide by 100 when converting a percent to decimal.
- Using the new value as denominator when calculating percentage change (it should be the old value).
- Rounding too early, which creates compounding error.
Final takeaway
If you remember one thing, remember this: percentages are relationships to 100. Once you know whether your problem is “part of whole,” “percent of number,” or “change over time,” the math becomes simple. Use the calculator above for speed, and use the formulas here when you want to understand the logic.