Work Out Percentage

May 5, 2026

A restaurant bill is £120, and you want to leave a 15% tip. A shop offers 25% off an £80 jacket. Knowing how to work out percentage quickly saves time and avoids mistakes. The methods below cover everyday situations with straightforward formulas and examples.

The calculator covers three common tasks: finding a percentage of a number, working out what percent one number is of another, and calculating the original amount before a percentage change. Enter the values you know, and it gives the answer instantly.

How Do You Work Out a Percentage of a Number?

To work out a percentage of a number, convert the percentage to a decimal and multiply.

Formula:
Result = (Percentage ÷ 100) × Total

Example: What is 15% of 120?
15 ÷ 100 = 0.15 → 0.15 × 120 = 18

You can also multiply directly: 120 × 15% = 120 × 0.15 = 18. This works for any percentage and total amount.

For quick mental calculations, use a base of 10%. 10% of 120 is 12, so 15% is 12 + half of 12 (6) = 18.

How to Work Out What Percentage One Number Is of Another

When you need to express one quantity as a percentage of another, divide the part by the whole, then multiply by 100.

Formula:
Percentage = (Part ÷ Whole) × 100

Example: You answered 45 questions correctly out of 60.
45 ÷ 60 = 0.75 → 0.75 × 100 = 75%

The same logic applies to financial ratios, test scores, or any comparison: the “part” goes on top, the “whole” on the bottom.

How to Work Out Percentage Increase or Decrease

To find the percentage change between an old and a new value:

Formula:
Percentage Change = ((New – Old) ÷ Old) × 100

Example (increase): Your salary rose from £2,500 to £2,750.
(2,750 – 2,500) ÷ 2,500 = 250 ÷ 2,500 = 0.10 → × 100 = +10%

Example (decrease): A product drops from £200 to £160.
(160 – 200) ÷ 200 = (–40) ÷ 200 = –0.20 → × 100 = –20%

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

How to Work Out the Original Number Before a Percentage Change (Reverse Percentage)

When you know the final amount after a percentage increase or decrease and need the original figure:

Formula for a decrease:
Original = New Number ÷ (1 – Percentage ÷ 100)

Formula for an increase:
Original = New Number ÷ (1 + Percentage ÷ 100)

Example: A jacket costs £64 after a 20% discount.
Original = 64 ÷ (1 – 0.20) = 64 ÷ 0.80 = £80

Example: An investment of £1,130 after a 13% return.
Original = 1,130 ÷ 1.13 = £1,000

Always divide by the decimal equivalent of the remaining or total percentage.

Three Mental Shortcuts for Working Out Percentages

10% and its multiples: Move the decimal point one place left for 10%, then multiply that result for 20%, 30%, etc.
5%: Half of the 10% value. For 5% of 240, 10% is 24, so 5% is 12.
1%: Move the decimal two places left. 1% of 450 is 4.5. Build any percentage by combining: 3% is 3 × 1%.

These tricks help with tips, discounts, and quick estimates without reaching for a calculator.

This guide offers general mathematical methods; always verify critical financial or tax figures with official sources or a professional.

Frequently Asked Questions

What is the easiest way to work out 10% of a number?

To find 10% of any number, simply move the decimal point one place to the left. For example, 10% of 340 is 34. For other percentages, build on that: 20% is double the 10% value, and 5% is half of it. This mental shortcut works instantly without a calculator.

How do I work out percentage change in Excel?

Use the formula = (new_value - old_value) / ABS(old_value) * 100. Format the cell as a percentage. For example, if old value is in A1 and new value in B1, enter =(B1-A1)/ABS(A1)*100. The result shows the increase as a positive or decrease as a negative percentage.

What is the formula to work out what percentage one number is of another?

Divide the part by the whole and multiply by 100: (part ÷ whole) × 100. If you scored 18 out of 25, 18 ÷ 25 = 0.72, then × 100 = 72%. This works for any two numbers where you need to express one as a percentage of the other.

Can I use an online calculator to work out percentages?

Yes, the calculator above lets you work out percentage of a number, calculate what percent one number is of another, or find the original value before a percentage increase or decrease. It removes the need for manual formulas and is free to use.

How do I work out reverse percentage without a calculator?

Divide the current number by (1 ± percentage as a decimal). If a price after a 15% discount is £85, divide 85 by 0.85 to get the original £100. For a markup, divide by 1 plus the percentage. This works for any percentage scenario when the final amount is known.

What is the difference between percentage and percentage points?

A percentage point is the absolute difference between two percentages. For example, if interest rates rise from 4% to 5%, they increase by 1 percentage point, not 1%. The relative increase is 25%. Always clarify the difference when discussing changes in rates or statistics.