How to Calculate a Percentage of a Number

Percentages express a quantity as parts per hundred, making proportions and changes easier to compare. This guide focuses on a practical situation: finding a percentage of a number is useful for discounts, budgets and proportions.

The short answer

Twenty percent of £80 is £16, so an £80 item with 20% off costs £64 before other charges. The useful result is not only the final number or code; it is understanding what was measured, what assumptions were made and how the answer will be used.

A reliable step-by-step method

1. 01Decide whether you need a share, a change or a new value.
2. 02Identify the base value that represents 100 percent.
3. 03Calculate before rounding, then label the result clearly.
4. 04Sense-check the answer against the original values.

A practical example

Twenty percent of £80 is £16, so an £80 item with 20% off costs £64 before other charges. Start with the information you actually know, keep the original unit or context beside every figure, and avoid rounding until the end. If the result affects money, health, safety or a formal requirement, confirm it with an authoritative source or qualified professional.

Common mistakes to avoid

• Using the new value as the base when the old value is required
• Confusing percentage points with percent change
• Treating a percentage as a complete explanation without context

A quick plausibility check is valuable. Ask whether the result is in the expected direction, whether the scale makes sense, and whether another person could understand the assumptions from your notes.

Use the free tool

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Authoritative further reading

• HMRC: VAT rates