How to Calculate Percentages
Percentages are one of the most frequently used calculations in everyday life. Whether you are working out a discount at the shops, calculating interest on a loan, interpreting a change in your website traffic, or understanding a pay rise, percentages appear everywhere. This guide covers the four most common percentage calculations in plain English.
1. Finding X% of a Number
The most basic percentage calculation: "What is 15% of 200?"
- Formula: Result = (Percentage ÷ 100) × Number
- Example: 15% of 200 = (15 ÷ 100) × 200 = 0.15 × 200 = 30
This calculation is used constantly: calculating tips, finding discounts, working out tax amounts, determining commission on a sale, or figuring out how much of your budget goes to a particular category.
2. What Percentage Is X of Y?
This is the reverse of the above — "30 is what percentage of 200?"
- Formula: Percentage = (Part ÷ Whole) × 100
- Example: 30 ÷ 200 × 100 = 15%
Use this to find market share, conversion rates, test scores, or survey response proportions. If 340 of your 2,000 email subscribers opened a campaign, your open rate is (340 ÷ 2000) × 100 = 17%.
3. Percentage Change
Percentage change tells you how much something has grown or shrunk relative to its original value.
- Formula: % Change = ((New − Old) ÷ |Old|) × 100
- Positive result = increase; negative result = decrease
- Example (increase): Revenue went from $80,000 to $100,000. Change = ((100,000 − 80,000) ÷ 80,000) × 100 = +25%
- Example (decrease): Traffic dropped from 5,000 to 3,500 visits. Change = ((3,500 − 5,000) ÷ 5,000) × 100 = −30%
Always use the original value as the denominator. A common mistake is to use the wrong base — if something increases 50% then decreases 50%, you do not end up where you started (you end up at 75% of the original).
4. Percentage Difference
Unlike percentage change, percentage difference is symmetric. It answers: "How different are these two values relative to their average?"
- Formula: % Difference = (|A − B| ÷ ((A + B) ÷ 2)) × 100
- Example: Comparing $90 and $110. |90 − 110| = 20. Average = 100. % Difference = 20%
Common Percentage Mistakes
Reversing an increase incorrectly: If a price increases 20%, you cannot subtract 20% from the new price to get back to the original. If the original was $100, 20% increase = $120. To reverse: $120 ÷ 1.20 = $100. Subtracting 20% from $120 gives $96 — wrong.
Confusing percentage points with percentages: If interest rates rise from 2% to 3%, that is an increase of 1 percentage point, but a 50% increase in the rate itself.
Adding percentages of different bases: A 10% increase followed by a 10% decrease is not flat — it results in a 1% net loss. Each percentage is applied to a different base.
Percentages in Business
| Metric | Formula | What it tells you |
|---|---|---|
| Profit margin | (Profit ÷ Revenue) × 100 | Profitability per dollar of revenue |
| Conversion rate | (Conversions ÷ Visitors) × 100 | % of visitors who take action |
| Email open rate | (Opens ÷ Delivered) × 100 | How engaging your subject lines are |
| Churn rate | (Lost customers ÷ Starting customers) × 100 | Customer retention over a period |
| YoY growth | ((This year − Last year) ÷ Last year) × 100 | Annual growth rate |