The Three Most Common Percentage Calculations

  1. 1

    Finding a percentage of a number

    Formula: (Percentage ÷ 100) × Number

    Example: What is 15% of 200?
    15 ÷ 100 = 0.15
    0.15 × 200 = 30

    Quick mental maths trick: Find 10% first (move decimal one place left), then adjust. 10% of 200 = 20. Half of that = 5% = 10. Add them: 15% = 30.

  2. 2

    What percentage is X of Y?

    Formula: (X ÷ Y) × 100

    Example: 45 is what percentage of 180?
    45 ÷ 180 = 0.25
    0.25 × 100 = 25%

    Use this when you want to express one number as a portion of another — test scores, discounts, completion rates.

  3. 3

    Percentage change (increase or decrease)

    Formula: ((New − Old) ÷ Old) × 100

    Example: Price went from $80 to $100. What is the percentage increase?
    (100 − 80) ÷ 80 = 0.25
    0.25 × 100 = 25% increase

    If the result is negative, it is a percentage decrease. Example: $100 to $80 = (80−100)÷100 × 100 = −20% (a 20% decrease).

Reverse Percentage — Finding the Original Number

Formula: Amount ÷ (Percentage ÷ 100)

Example: A price after a 20% discount is $64. What was the original price?
After 20% discount means you paid 80% of the original.
$64 ÷ 0.80 = $80

Quick percentage shortcuts10% — move the decimal point one place left (350 → 35). 5% — find 10% then halve it. 25% — divide by 4. 50% — divide by 2. 1% — move the decimal two places left (350 → 3.5). Build any percentage from these building blocks.

Percentage on a Calculator

Most calculators have a % button. To find 15% of 200: press 200 × 15 % = and the answer appears. On a phone calculator, type 200 × 15% and it calculates automatically.

Frequently Asked Questions

In Excel: =(New-Old)/Old and format the cell as a percentage. For example if old value is in A1 and new in B1: =(B1-A1)/A1 then format as percentage. Excel handles the ×100 conversion automatically when you use percentage formatting.
Percentage points measure the arithmetic difference between two percentages. If interest rates go from 2% to 5%, that is a 3 percentage point increase, but a 150% percentage increase (3÷2×100). Politicians and media sometimes confuse these deliberately or accidentally — always check which one is being used.
Multiply the price by (1 + percentage as decimal). To add 10% GST to $50: $50 × 1.10 = $55. To add 20% VAT to $80: $80 × 1.20 = $96. To remove tax from a GST-inclusive price, divide by 1.10.