How to Calculate Percentages: Simple Formulas With Everyday Examples

A practical guide · about 6 min read

Percentages are everywhere: sale signs, exam results, restaurant tips, interest rates, and news headlines about prices going up or down. Yet many people freeze the moment they have to work one out without a phone. The good news is that percentages rest on just a handful of simple rules. Once you know them, you can solve most everyday percentage problems in your head. This guide walks through each one with clear examples.

What a percentage really is

The word percent means "per hundred." A percentage is simply a way of describing a part out of one hundred. So 25% means 25 out of every 100, or one quarter. 50% means 50 out of 100, or one half. Holding on to this idea, "out of a hundred," makes every calculation below easier to picture.

1. Finding a percentage of a number

This is the most common task. The rule is: multiply the number by the percentage, then divide by 100.

A handy shortcut: to find 10% of anything, just move the decimal point one place left. 10% of 80 is 8. From there you can build other amounts. Need 20%? That is just 10% doubled, so 16. Need 5%? That is half of 10%, so 4. This trick makes tips and discounts easy to estimate without any device.

2. Working out what percent one number is of another

Sometimes you have two numbers and want to know the percentage relationship, for example, you scored 30 out of 150 on a test. The rule is: divide the part by the whole, then multiply by 100.

3. Percentage increase

This tells you how much something has grown, like a price rise or a pay increase. The rule: find the difference, divide by the original value, then multiply by 100.

4. Percentage decrease

This works exactly the same way, but the value has gone down. The rule: find the difference, divide by the original value, then multiply by 100.

One important warning: always divide by the original value, not the new one. Going from 50 to 75 is a 50% increase, but going from 75 back to 50 is a 33% decrease, because the starting points are different. Getting the original number right is the key to an accurate answer.

5. Increasing or decreasing a number by a percent

To add a percentage to a number quickly, multiply by 1 plus the percentage as a decimal. To subtract, multiply by 1 minus it.

This is the fastest way to apply sales tax, a discount, or a tip in one step.

Where you will use these

These five rules cover almost every percentage you meet in daily life: a 20% tip on a meal, a 30% off sale, a 5% pay rise, the percentage you scored on a test, or how much a bill has gone up since last year. With a little practice, most of them become quick mental maths.

Let the calculator do it for you

When the numbers are awkward or you just want to be sure, our free Percentage Calculator gives you the answer instantly. Enter the percentage and the value, and it does the multiplying and dividing for you, no mental arithmetic required. It is handy for tips, discounts, taxes, and grades.

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