Three Types of Percentage Problems
Every percentage question falls into one of three patterns:
1. "What is X% of Y?" Multiply: Y × (X/100). Example: 15% of 230 = 230 × 0.15 = 34.5
2. "X is what percent of Y?" Divide: (X/Y) × 100. Example: 45 is what % of 180? = (45/180) × 100 = 25%
3. "What is Y after X% increase/decrease?" Add or subtract: Y × (1 + X/100) for increase, Y × (1 - X/100) for decrease. Example: $89 after 20% discount = 89 × 0.80 = $71.20
The confusion people have: "percentage" vs "percentage points." If a tax rate goes from 10% to 12%, that's a 2 percentage point increase but a 20% relative increase (2/10 = 0.20). Politicians and media often mix these up. "Taxes increased by 2%" could mean either thing. This calculator works with relative percentages (the mathematical kind).
How to Use
- 1Enter the base value (the number you're calculating a percentage of).
- 2Enter the percentage (positive for increase, negative for decrease).
- 3See the percentage amount and the final total instantly.
- 4Use negative percentages for discounts and decreases.
When You'll Use This
Calculating discounts while shopping
Item is $89 with a 30% off sale. What's the final price? 89 × 0.70 = $62.30. Or: what's the actual savings? 89 × 0.30 = $26.70 off. Quick math before deciding if the "deal" is worth it.
Figuring out tips at restaurants
Bill is $67. 15% tip = $10.05. 18% = $12.06. 20% = $13.40. Quick trick: 10% of $67 is $6.70, double it for 20% ($13.40), or add half for 15% ($10.05).
Understanding salary increases or tax rates
You got a 4% raise on a $75,000 salary. That's $3,000 more per year, or $250/month before tax. Is it worth celebrating? Depends on inflation. If inflation is 3.5%, your real raise is only 0.5%.
Calculating margins and markups in business
You buy a product for $40 and sell for $60. Markup is 50% (20/40). But margin is 33% (20/60). These are different calculations that confuse many business owners. Markup is based on cost, margin is based on selling price.
Common Mistakes
Percentage increase then decrease doesn't return to original
A 20% increase followed by a 20% decrease does NOT give you the original number. $100 + 20% = $120. $120 - 20% = $96 (not $100). The second 20% is calculated on the larger number. This is why stock markets need a 25% gain to recover from a 20% loss.
Percentage points ≠ percentage
Interest rate going from 3% to 4% is a 1 percentage point increase but a 33% relative increase. "Sales grew by 5%" and "market share grew by 5 percentage points" mean very different things. Always clarify which is meant.
Margin vs markup: know which you're calculating
Markup = (profit / cost) × 100. Margin = (profit / revenue) × 100. A 50% markup ($40 cost → $60 price) is only a 33% margin. If someone says "we need 40% margin," don't apply a 40% markup. You'll underprice.
Compounding percentages multiply, they don't add
Three consecutive 10% increases is not 30%. It's 1.10 × 1.10 × 1.10 = 1.331, or 33.1%. The difference grows with larger percentages and more periods. This is why compound interest is more powerful than it seems.
Examples
25% discount on $89
A common shopping calculation. What do you actually pay?
Input
Base: $89 | Percentage: -25%Output
Discount: $22.25 | Final price: $66.75What percentage is 45 of 180?
Finding what fraction one number is of another.
Input
45 ÷ 180 × 100Output
25%Limitations
- Handles standard percentage operations only (percent of, percent change, what percent). Does not support compound percentage chains.
- Results use JavaScript floating-point arithmetic. Very large numbers or many decimal places may show minor rounding artifacts.
- Cannot process percentages from text descriptions (e.g., "20% off $50"). Input must be numeric values.
- Does not support percentage-based financial calculations like markup vs margin — use dedicated tools for those.
Features
- Calculates percentage of any number instantly
- Shows both the percentage amount and the total after applying
- Supports negative percentages for discounts/decreases
- Handles decimal percentages (7.5%, 2.25%, etc.)
- Visual indicators for increases vs decreases
- No data sent anywhere. Runs 100% in your browser
Frequently Asked Questions
How do I calculate X% of a number?
Multiply the number by X/100. 15% of 230 = 230 × 0.15 = 34.5. Quick mental math: 10% is just moving the decimal (230 → 23), then adjust. 15% = 10% + 5% = 23 + 11.5 = 34.5.
How do I find what percentage one number is of another?
Divide the part by the whole, multiply by 100. "45 is what % of 180?" = (45 ÷ 180) × 100 = 25%. Think of it as: what fraction is 45 of 180? It's 1/4, which is 25%.
Why doesn't a 20% increase then 20% decrease give me the original?
Because the decrease is calculated on the increased amount. $100 + 20% = $120. Then $120 - 20% = $120 × 0.80 = $96. The 20% decrease is 20% of $120 ($24), not 20% of the original $100 ($20). To get back to $100 from $120, you need a 16.67% decrease (20/120).
What's the difference between markup and margin?
Markup is profit as a percentage of COST: (60-40)/40 = 50% markup. Margin is profit as a percentage of REVENUE: (60-40)/60 = 33% margin. Same transaction, different percentages. Retail typically talks margin; wholesale talks markup. A 100% markup = 50% margin.
How do I calculate percentage change between two numbers?
Formula: ((new - old) / old) × 100. Revenue went from $80K to $95K: ((95-80)/80) × 100 = 18.75% increase. If it went from $95K to $80K: ((80-95)/95) × 100 = -15.8% decrease. Note: the percentage is different depending on direction.
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