Percentage Calculator

Calculate percentages three ways: find a percentage of a number, find what percent one number is of another, or calculate percentage change.

Percentage

Of Number

Part (X)

Whole (Y)

From (Old Value)

To (New Value)

Result

—

Formula Used

—

Finding a Percentage of a Number

Formula: Result = Number × Percentage / 100

20% of 150 = 150 × 20 / 100 = 30
15% tip on $80 = $80 × 15 / 100 = $12
8.5% tax on $50 = $50 × 8.5 / 100 = $4.25

Finding What Percent X is of Y

Formula: Percentage = (Part / Whole) × 100

18 out of 90 = (18 / 90) × 100 = 20%
Test score: 42 out of 50 = 84%

Percentage Change

Formula: Change = ((New - Old) / Old) × 100

$50 to $60 = ((60 - 50) / 50) × 100 = +20%
$100 to $75 = ((75 - 100) / 100) × 100 = -25%

Mental Math Shortcuts

10%: Move the decimal point one place left
5%: Find 10% and divide by 2
25%: Divide by 4
50%: Divide by 2
1%: Move the decimal point two places left

When to Use This Calculator

Discounts and sales: Find how much a 30% discount saves on a $129 item — instantly.
Year-over-year growth: Measure revenue, traffic, or salary changes between two time periods.
Grade calculation: Find what percentage of points you earned on a test or assignment.

Real-World Examples

Example 1 — Sale price: Item costs $299, on sale at 35% off. Discount = $299 × 35 / 100 = $104.65. Sale price = $299 − $104.65 = $194.35.

Example 2 — Investment return: Portfolio grew from $12,500 to $15,750. Change = ((15,750 − 12,500) / 12,500) × 100 = +26% return.

Limitations & Assumptions

Percentage change requires the original value to be non-zero — division by zero is undefined.
Percentages above 100% are valid (e.g., 150% increase means value 2.5x the original).
Percentage points and percentages are different: going from 20% to 25% is a 5 percentage point increase but a 25% relative increase.

Data Sources

Formulas follow standard arithmetic definitions used by the US Bureau of Labor Statistics for CPI calculations and the SEC for investment return reporting. No external data source required.

Frequently Asked Questions

How do I calculate a percentage of a number?

To find X% of Y, multiply Y by X and divide by 100. For example, 25% of 200 is 200 × 25 / 100 = 50. This works for discounts, tips, taxes, and any other percentage calculation.

What is percentage change and how is it calculated?

Percentage change measures how much a value has increased or decreased relative to its original value. The formula is ((New Value - Old Value) / Old Value) × 100. A positive result means an increase, and a negative result means a decrease.

What is the difference between percentage and percentage points?

A percentage is a relative measure (e.g., "prices rose 10%"), while percentage points describe the arithmetic difference between two percentages (e.g., "interest rates went from 3% to 5%, a change of 2 percentage points"). Going from 5% to 10% is a 5 percentage point increase, but a 100% increase.

How do I calculate what percentage one number is of another?

Divide the part by the whole and multiply by 100. For example, to find what percentage 30 is of 120: (30 / 120) × 100 = 25%. This is useful for calculating test scores, completion rates, and proportions.