How to Calculate a Percentage: The Complete Step-by-Step Guide (UK)

Knowing how to calculate from percentage is one of the most useful maths skills for everyday UK life — from working out a discount in a sale, to checking VAT on a receipt, to understanding exam results or a pay rise. This guide walks through the exact formula, step-by-step examples, and the fastest way to get an accurate answer every time, with or without a calculator.

What Is a Percentage?

A percentage is simply a way of expressing a number as a fraction of 100. The word itself comes from the Latin per centum, meaning "per hundred". So 25% means 25 out of every 100 — or 0.25 as a decimal, or ¼ as a fraction.

Percentages let you compare quantities on a consistent scale, which is why they're used everywhere from payslips and bank statements to school reports and shop discounts.

The Basic Formula: How Calculate a Percentage of a Number

If you're asking how do you calculate a percentage, the core formula is:

Percentage of a number = (Percentage ÷ 100) × Number

Worked Example

To find 15% of £240:

1. Divide the percentage by 100: 15 ÷ 100 = 0.15

2. Multiply by the number: 0.15 × 240 = £36

So 15% of £240 is £36.

This same three-step method works for any figure — wages, exam marks, recipe quantities, or discounts. Once you're comfortable with it, you can apply it almost instantly to any everyday calculation.

A Quicker Mental Method

If you want to know how calculate percentage without writing anything down, break the percentage into easy chunks:

• 10% of a number = divide by 10

• 5% = half of 10%

• 1% = divide by 100

• Combine these to build any percentage. For example, 15% = 10% + 5%.

Using the £240 example: 10% of £240 = £24, and 5% = £12, so 15% = £24 + £12 = £36 — the same answer, worked out in your head.

How to Calculate the Percentage of 2 Numbers

A different but equally common question is how to calculate percentage of 2 numbers — that is, expressing one number as a percentage of another. This answers questions like "what percentage is 35 out of 140?"

Formula: (Part ÷ Whole) × 100

Worked Example

If 35 students out of 140 passed an exam:

1. Divide the part by the whole: 35 ÷ 140 = 0.25

2. Multiply by 100: 0.25 × 100 = 25%

25% of students passed.

This formula applies whenever you're comparing two related figures — test scores, sales conversion rates, or comparing salaries between two roles.

How to Calculate Percentage for Money

Money-related percentages are among the most frequent real-world uses, and how to calculate percentage for money usually falls into one of three categories.

Discounts

Discount % = ((Original Price − Sale Price) ÷ Original Price) × 100

Example: a jacket reduced from £120 to £84. (£120 − £84) ÷ £120 × 100 = 30% off

VAT (UK)

UK VAT is charged at 20% on most goods and services. To add VAT to a net price, multiply by 1.20. To find the net price from a VAT-inclusive (gross) price, divide by 1.20.

Example: a gross price of £60 ÷ 1.20 = £50 net, meaning £10 was VAT.

Percentage Increase or Decrease

Percentage change = ((New Value − Old Value) ÷ Old Value) × 100

Example: a price rising from £80 to £100. (£100 − £80) ÷ £80 × 100 = 25% increase

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

Percentage Formulas at a Glance

What you want to findFormulaExampleX% of a number(X ÷ 100) × Number15% of £240 = £36One number as a % of another(Part ÷ Whole) × 10035 out of 140 = 25%Percentage increase/decrease((New − Old) ÷ Old) × 100£80 → £100 = 25% increaseDiscount percentage((Original − Sale) ÷ Original) × 100£120 → £84 = 30% offAdding UK VAT (20%)Net × 1.20£50 × 1.20 = £60

Common Mistakes to Avoid

• Dividing by the wrong number. For percentage change, always divide by the original (old) value — not the new one.

• Confusing percentage points with percentages. A rise from 20% to 25% is a 5 percentage point increase, but a 25% relative increase — these aren't the same thing, and mixing them up is a common error in reports and news coverage.

• Rounding too early. Round only your final answer, not the numbers used mid-calculation, to avoid compounding small errors.

• Forgetting the decimal conversion. Percentages must be divided by 100 before multiplying — skipping this step is the most frequent source of wrong answers.

Manual Calculation vs. Using a Calculator

Both approaches are valid, and which one suits you depends on the situation:

• Manual calculation is useful for building number sense, exam settings where calculators aren't allowed, or quick mental estimates while shopping.

• A calculator is faster and removes the risk of arithmetic slips, particularly for numbers with several decimal places or when working out VAT, discounts, or increases where precision matters.

For everyday accuracy without doing the maths by hand, our free percentage calculator handles all three formulas above instantly — just enter your two numbers and choose the calculation you need.

Frequently Asked Questions

How do you calculate a percentage?

To calculate a percentage, divide the percentage figure by 100, then multiply by the number in question. For example, 20% of 50 is (20 ÷ 100) × 50 = 10.

How do you calculate the percentage of 2 numbers?

Divide the first number (the part) by the second number (the whole), then multiply by 100. For example, 45 out of 90 is (45 ÷ 90) × 100 = 50%.

How do I calculate percentage for money, like VAT or a discount?

For VAT at the UK rate of 20%, multiply the net price by 1.20 to get the gross price, or divide the gross price by 1.20 to find the net. For a discount, subtract the sale price from the original price, divide by the original price, then multiply by 100.

What's the fastest way to calculate a percentage without a calculator?

Break the percentage into 10% (divide by 10), 5% (half of 10%), and 1% (divide by 100), then add these building blocks together to reach the percentage you need.

How do you calculate a percentage increase or decrease?

Subtract the old value from the new value, divide the result by the old value, then multiply by 100. A positive answer is an increase; a negative answer is a decrease.

Conclusion

Whether you're working out a discount, checking VAT, comparing exam results, or reviewing a pay rise, the same handful of percentage formulas cover almost every situation. The key steps are always the same: convert the percentage to a decimal, apply the right formula for what you're comparing, and double-check you're dividing by the correct original value.

For instant, error-free results every time, try our free percentage calculator — it covers all three core calculations in this guide, with no sign-up required. Explore more free tools at CalcMax.