How to Find the Percentage of a Number: Math Made Simple

The Basic Formula

To find what X% of a number is: Result = (Percentage ÷ 100) × Number

Or simply: move the decimal point of the percentage two places left, then multiply.

Example: What is 20% of 150?
20 ÷ 100 = 0.20
0.20 × 150 = 30

Mental Math Tricks for Percentages

• 10%: Move the decimal one place left. 10% of 250 = 25.
• 5%: Take 10% then halve it. 5% of 250 = 12.5.
• 20%: Take 10% and double it. 20% of 250 = 50.
• 25%: Divide by 4. 25% of 250 = 62.5.
• 50%: Divide by 2. 50% of 250 = 125.
• 1%: Divide by 100. 1% of 250 = 2.5. Then multiply for other percentages.

Percentage Increase and Decrease

Percentage change formula:

% Change = ((New Value − Old Value) ÷ Old Value) × 100

Example (increase): A price rises from $80 to $100.
% Change = ((100 − 80) ÷ 80) × 100 = (20 ÷ 80) × 100 = 25% increase

Example (decrease): A salary drops from $60,000 to $54,000.
% Change = ((54,000 − 60,000) ÷ 60,000) × 100 = (−6,000 ÷ 60,000) × 100 = −10% (a 10% decrease)

Finding What Percentage One Number Is of Another

Formula: % = (Part ÷ Whole) × 100

Example: You scored 42 out of 56 on a test. What percentage did you get?
(42 ÷ 56) × 100 = 0.75 × 100 = 75%

Reverse Percentage: Finding the Original Number

If a number after a percentage increase or decrease is known, you can find the original:

Formula: Original = Current Value ÷ (1 ± percentage/100)

Example: A product costs $130 after a 30% markup. What was the original price?
Original = 130 ÷ 1.30 = $100

Practical Percentage Examples

Scenario	Calculation	Result
15% tip on $65 meal	$65 × 0.15	$9.75
20% off a $120 jacket	$120 × 0.80	$96
7% sales tax on $200	$200 × 0.07	$14
Score of 34/40	(34 ÷ 40) × 100	85%
5% interest on $1,000	$1,000 × 0.05	$50