How to Calculate a Percentage (With Simple Examples)

Percentages show up everywhere: discounts, tips, test scores, interest rates, and price changes. The good news is that once you understand one simple idea, every percentage problem becomes easy. This guide walks you through it step by step, with plain-English explanations and worked examples you can follow along with.

What “Percent” Actually Means

The word percent literally means “per hundred.” A percentage is just a fraction with 100 on the bottom. So when you see a percent, picture splitting something into 100 equal pieces and counting how many you have.

For example:

• 25% means 25 out of 100 = 25/100 = 0.25
• 50% means 50 out of 100 = 50/100 = 0.50
• 7% means 7 out of 100 = 7/100 = 0.07

The key trick to remember: to turn a percent into a decimal, divide by 100 (or just move the decimal point two places to the left). To go the other way, multiply by 100. This single idea powers every calculation below.

How to Find X% of a Number

This is the most common percentage task. The rule in words: convert the percent to a decimal, then multiply by the number.

Example 1: What is 20% of 80?

1. Turn 20% into a decimal: 20 ÷ 100 = 0.20
2. Multiply: 0.20 × 80 = 16

So 20% of 80 is 16.

Example 2: What is 15% of 200?

1. Turn 15% into a decimal: 15 ÷ 100 = 0.15
2. Multiply: 0.15 × 200 = 30

So 15% of 200 is 30. That’s exactly how you’d work out a 15% tip on a $200 bill.

What Percentage Is A of B?

Sometimes you have two numbers and want to know what percent one is of the other, for example, a test score. The rule in words: divide the part by the whole, then multiply by 100.

The formula is: (A ÷ B) × 100

Example: You got 30 questions right out of 120. What percentage is that?

1. Divide: 30 ÷ 120 = 0.25
2. Multiply by 100: 0.25 × 100 = 25%

So 30 out of 120 is 25%. You can sanity-check this: 25% is one quarter, and 30 really is one quarter of 120 (because 120 ÷ 4 = 30).

Percentage Increase and Decrease (Percentage Change)

When a value goes up or down, “percentage change” tells you how big the jump was relative to where you started. The rule in words: subtract the old value from the new value, divide by the old value, then multiply by 100.

The formula is: ((new − old) ÷ old) × 100

A positive answer means an increase; a negative answer means a decrease.

Increase example: A price rises from 50 to 65.

1. Difference: 65 − 50 = 15
2. Divide by the old value: 15 ÷ 50 = 0.30
3. Multiply by 100: 0.30 × 100 = 30% increase

Decrease example: A price drops from 80 to 60.

1. Difference: 60 − 80 = −20
2. Divide by the old value: −20 ÷ 80 = −0.25
3. Multiply by 100: −0.25 × 100 = −25% (a 25% decrease)

Notice that you always divide by the original (old) number, not the new one. That’s the most common mistake people make.

Adding or Subtracting a Percentage (Tips and Discounts)

When you add a tip or take a discount, there’s a handy shortcut: instead of calculating the percentage and then adding or subtracting it, you can multiply in one step.

To add a percentage, multiply by (1 + the decimal). A 15% tip means multiplying by 1.15.

Example: Add a 15% tip to a $40 bill.

• 40 × 1.15 = $46

Why does this work? 1.15 is “100% of the bill plus 15% more.” You could also do it the long way: 15% of 40 is 6, and 40 + 6 = 46. Same answer, fewer steps.

To subtract a percentage, multiply by (1 − the decimal). A 20% discount means multiplying by 0.80.

Example: Take 20% off a $50 item.

• 50 × 0.80 = $40

The long way: 20% of 50 is 10, and 50 − 10 = 40. The shortcut gets you there instantly.

Reverse Percentage: Finding the Original Amount

This one trips a lot of people up. Suppose you know the price after a discount and want to find the original price. You can’t just add the percentage back, you have to divide.

The rule in words: divide the final amount by (1 minus the discount decimal) for a discount, or by (1 plus the decimal) for an increase.

Example: An item costs $90 after a 10% discount. What was the original price?

• A 10% discount means you paid 90% of the original, so the multiplier was 0.90.
• Original = 90 ÷ 0.90 = $100

Let’s verify: 10% of $100 is $10, and $100 − $10 = $90. It checks out perfectly.

The reason you divide instead of just adding $9 (10% of $90): the discount was 10% of the original $100, not 10% of the final $90. Dividing reverses the multiplication correctly.

Quick Mental-Math Tricks

You don’t always need a calculator. These shortcuts let you estimate percentages in your head:

Percentage	Quick trick	Example
10%	Move the decimal one place left	10% of 250 = 25
1%	Move the decimal two places left	1% of 250 = 2.5
50%	Take half	50% of 250 = 125
25%	Take half, then half again	25% of 250 = 62.5
5%	Find 10%, then halve it	5% of 250 = 12.5
20%	Find 10%, then double it	20% of 250 = 50
15%	10% + 5% (half of the 10%)	15% of 250 = 25 + 12.5 = 37.5

One more clever trick: X% of Y always equals Y% of X. That sounds odd, but it can make hard problems easy. For instance, 8% of 50 looks awkward, but flip it: 50% of 8 is just 4. Same answer, far less effort.

Let the Calculator Do the Work

Once you understand the formulas, the arithmetic is the only thing left, and that’s where a tool helps. Our free percentage calculator handles every type of problem covered here: finding a percentage of a number, working out what percent one number is of another, calculating percentage increases and decreases, applying tips and discounts, and even reverse percentages. Just type in your numbers and get the answer instantly, with no signup and no math anxiety required.

Understanding the “why” behind each formula means you’ll always know which calculation to reach for, and you’ll be able to double-check the results yourself. From there, the calculator simply saves you time.