Understanding how to calculate percentage is a fundamental math skill used in everyday life — from calculating discounts and tips to tracking statistics and test scores. This guide explains every type of percentage calculation with clear formulas and examples, plus a free calculator to do the math instantly.
What Is a Percentage?
A percentage is a number expressed as a fraction of 100. The word “percent” comes from the Latin “per centum,” meaning “by the hundred.” When you say 45%, you mean 45 out of every 100, or 45/100 = 0.45 in decimal form.
Basic Percentage Formula
The core formula for calculating a percentage is:
Percentage = (Part ÷ Whole) × 100
Example: What percentage is 25 out of 80?
(25 ÷ 80) × 100 = 31.25%
How to Calculate Percentage of a Number
Formula: Result = (Percentage ÷ 100) × Number
Example: What is 35% of 200?
(35 ÷ 100) × 200 = 0.35 × 200 = 70
Use the free ReadyGoTools Percentage Calculator to calculate this instantly without doing the math manually.
How to Calculate Percentage Increase
Formula: Percentage Increase = ((New Value – Old Value) ÷ Old Value) × 100
Example: A product price went from $80 to $100. What is the percentage increase?
((100 – 80) ÷ 80) × 100 = (20 ÷ 80) × 100 = 25% increase
How to Calculate Percentage Decrease
Formula: Percentage Decrease = ((Old Value – New Value) ÷ Old Value) × 100
Example: A price dropped from $120 to $90. What is the percentage decrease?
((120 – 90) ÷ 120) × 100 = (30 ÷ 120) × 100 = 25% decrease
How to Calculate What Percent One Number Is of Another
Formula: Percentage = (A ÷ B) × 100
Example: 45 is what percent of 180?
(45 ÷ 180) × 100 = 0.25 × 100 = 25%
Percentage Calculation Examples for Common Use Cases
| Scenario | Calculation | Answer |
|---|---|---|
| 20% off $85 | $85 × 0.20 = $17 discount | Pay $68 |
| 15% tip on $47 | $47 × 0.15 | $7.05 tip |
| Score: 42 out of 50 | (42 ÷ 50) × 100 | 84% |
| Sales tax 8.5% on $200 | $200 × 0.085 | $17 tax |
| Growth from 500 to 650 | ((650-500) ÷ 500) × 100 | 30% increase |
How to Calculate Percentage on a Calculator
On a basic calculator, to find X% of Y: enter Y, press ×, enter X, press %, and press = . For example, to find 15% of 200: press 200 × 15 % = and the answer is 30.
For more complex calculations — percentage change, reverse percentage, or compound percentage — use the free ReadyGoTools Percentage Calculator which handles all these scenarios instantly.
How to Calculate Reverse Percentage (Find the Original Value)
Formula: Original = Final ÷ (1 – Percentage/100) for percentage decrease.
Formula: Original = Final ÷ (1 + Percentage/100) for percentage increase.
Example: A price after a 20% discount is $80. What was the original price?
$80 ÷ (1 – 0.20) = $80 ÷ 0.80 = $100
Frequently Asked Questions
How do you calculate a percentage quickly in your head?
For 10%: move the decimal point one place left (10% of 250 = 25). For 5%: halve the 10% value (5% of 250 = 12.5). For 25%: divide by 4 (25% of 200 = 50). For 50%: divide by 2.
What is 20% of 150?
20% of 150 = (20 ÷ 100) × 150 = 0.20 × 150 = 30. Use the Percentage Calculator for instant results.
How do I calculate a percentage increase between two numbers?
Subtract the old value from the new value, divide by the old value, then multiply by 100. The Percentage Calculator automates this with a dedicated “percentage change” mode.
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