How to Calculate Percentages Without Guesswork

Percentages appear everywhere: sale prices, pay rises, tips, bills, exam marks and business reports. Yet they can feel confusing because the word “percentage” is used for several different questions.

You do not need to memorise dozens of formulas. Start by deciding what the question is asking, then use the matching calculation below.

1. How much is a percentage of a number?

This is the calculation to use when you see a question such as “What is 20% of £80?” Convert the percentage to a decimal by dividing it by 100, then multiply by the original number.

The answer is £16. Try different values with our percentage of a number calculator.

2. What percentage is one number of another?

Use this when you know the part and the total. Divide the part by the total, then multiply by 100.

The important detail is choosing the correct total. In this example, 45 is the whole amount, so it belongs underneath the division.

3. What is the percentage increase or decrease?

Percentage change compares the difference with the original value—not the new value. First subtract the original number from the new number. Divide that change by the original number and multiply by 100.

It is a 20% increase. If the result is negative, the value has decreased. Our percentage change calculator handles both directions.

4. How do you reverse a percentage?

A reverse percentage finds the original amount before a percentage was added or removed. This is where simply subtracting the percentage often gives the wrong answer.

Suppose a price is £120 after a 20% increase. The £120 represents 120% of the original price. Divide it by 1.20:

For an amount after a 20% reduction, the remaining value represents 80% of the original, so divide by 0.80. You can check either type with the reverse percentage calculator.

A quick way to choose the right calculation

• Finding a portion? Multiply the number by the percentage as a decimal.
• Comparing a part with a total? Divide the part by the total and multiply by 100.
• Comparing an old and new value? Divide the change by the original.
• Working backwards? Divide by the percentage multiplier.

Common percentage mistakes

Using 20 instead of 0.20

Before multiplying, divide a percentage by 100. Ten per cent becomes 0.10, 5% becomes 0.05 and 125% becomes 1.25.

Dividing by the wrong number

For percentage change, always compare the difference with the original value. Swapping the original and new values changes the answer.

Adding and removing the same percentage

A 20% increase followed by a 20% decrease does not return to the starting value. Starting at £100 gives £120 after the increase, then a 20% decrease takes £24 away and leaves £96.

Rounding too early

Keep the full number while calculating and round only the final result. This matters most when working with money or chaining several calculations together.