The store sign says "40% off already reduced prices." The original price was $120, marked down to $80, now 40% off that. You stand there, phone out, trying to figure out if this is actually a good deal or retail theater. The line is growing. You buy it anyway and do the math in the car. It was 56% off total. You could have negotiated better knowing that.
Percentage math is money math. Every time you can't calculate it quickly, you're making decisions blind.
We built the Percentage Calculator because we got tired of Googling "what is 15% of 87" every time we needed quick math. Whether you're calculating tips, discounts, tax, or splits, understanding percentages—or having a reliable tool—transforms shopping, budgeting, and negotiating from stressful to confident.
The Three Core Percentage Calculations
Every percentage problem falls into one of three categories. Master these, and you'll never be stuck again.
1. Finding a Percentage of a Number
Question: What is X% of Y?
Formula: Y × (X ÷ 100) = Answer
Example: What is 15% of $80?
- $80 × 0.15 = $12
Real-world uses:
- Calculating tips at restaurants
- Finding sale prices (20% off $50)
- Determining tax amounts
- Calculating down payments
2. Finding What Percentage One Number Is of Another
Question: X is what percent of Y?
Formula: (X ÷ Y) × 100 = Answer%
Example: 15 is what percent of 60?
- (15 ÷ 60) × 100 = 25%
Real-world uses:
- Test scores (42 out of 50 questions)
- Budget tracking (spent $300 of $1,000)
- Sales performance (12 sales out of 40 calls)
- Nutrition labels (8g protein out of 50g daily value)
3. Finding a Number When You Know the Percentage
Question: X is Y% of what number?
Formula: X ÷ (Y ÷ 100) = Answer
Example: 30 is 20% of what number?
- 30 ÷ 0.20 = 150
Real-world uses:
- Reverse-engineering original prices from sale prices
- Finding full amounts from partial payments
- Calculating totals from tax-inclusive prices
Using Our Percentage Calculator
Our Percentage Calculator handles all three calculation types instantly:
How to Use It
- Select the calculation type you need
- Enter your known values
- Get instant, accurate results
- Copy or use the answer
Why We Built It
Mental math is great until you're trying to calculate 17.5% of $243.89 while someone waits for an answer. Our calculator eliminates the friction between needing an answer and having it.
Mental Math Shortcuts
For quick estimates without a calculator, use these tricks:
The 10% Anchor
10% of any number: Move the decimal point one place left.
- 10% of $85 = $8.50
- 10% of $347 = $34.70
- 10% of $12.50 = $1.25
From 10%, derive everything:
- 5% = half of 10%
- 15% = 10% + 5%
- 20% = 10% × 2
- 25% = 20% + 5%
- 30% = 10% × 3
Quick Tip Math
For a $47 bill:
- 10% = $4.70
- 15% = $4.70 + $2.35 = $7.05
- 20% = $4.70 × 2 = $9.40
Round for sanity: $7 for 15%, $9.50 for 20%.
The Double-and-Halve Trick
To find 50%, halve the number. To find 25%, halve twice. To find 75%, find 50% + 25%.
$80 at 75% off:
- 50% = $40
- 25% = $20
- 75% = $40 + $20 = $60 off → Pay $20
Percentage Change Calculations
Percentage Increase
Formula: ((New - Old) ÷ Old) × 100
Example: Price went from $50 to $65
- (($65 - $50) ÷ $50) × 100 = 30% increase
Percentage Decrease
Formula: ((Old - New) ÷ Old) × 100
Example: Price dropped from $80 to $60
- (($80 - $60) ÷ $80) × 100 = 25% decrease
Common Mistakes to Avoid
Mistake: Thinking a 50% increase followed by a 50% decrease returns to the original.
Reality: $100 + 50% = $150. Then $150 - 50% = $75. You're down 25%.
Mistake: Adding percentages of different bases.
Reality: 20% off, then 10% off ≠ 30% off. It's actually 28% off total.
- $100 × 0.80 = $80
- $80 × 0.90 = $72
- ($100 - $72) ÷ $100 = 28%
Real-World Percentage Scenarios
Shopping Discounts
Scenario: "Buy one, get one 50% off" on $40 items.
Calculation:
- First item: $40
- Second item: $40 × 0.50 = $20
- Total: $60 for two items
- Per-item cost: $30 each
- Actual discount: 25% off each
Restaurant Bills
Scenario: $78.50 bill, want to leave 18% tip.
Quick method:
- 10% = $7.85
- 8% = $7.85 × 0.8 = $6.28
- 18% = $7.85 + $6.28 = $14.13
Or use our Tip Calculator for instant results including bill splitting.
Sales Tax
Scenario: Item costs $45, sales tax is 8.25%.
Calculation:
- Tax: $45 × 0.0825 = $3.71
- Total: $45 + $3.71 = $48.71
Reverse (finding pre-tax from total):
- If total is $48.71 with 8.25% tax
- Pre-tax: $48.71 ÷ 1.0825 = $45
Pay Raises
Scenario: You make $65,000 and get a 4% raise.
Calculation:
- Raise amount: $65,000 × 0.04 = $2,600
- New salary: $67,600
Loan Interest
Scenario: $10,000 loan at 7% annual interest (simple).
Calculation:
- Annual interest: $10,000 × 0.07 = $700
- Monthly interest: $700 ÷ 12 = $58.33
Percentage Calculator for Business
Profit Margins
Gross margin: (Revenue - Cost) ÷ Revenue × 100
Example: Sell for $50, costs $30 to make.
- ($50 - $30) ÷ $50 × 100 = 40% margin
Markup vs. Margin
Markup: (Price - Cost) ÷ Cost × 100 Margin: (Price - Cost) ÷ Price × 100
Same numbers, different percentages:
- Cost $30, sell $50
- Markup: 66.7%
- Margin: 40%
Know which one you're calculating—they're often confused.
Growth Rates
Year-over-year growth: ((This Year - Last Year) ÷ Last Year) × 100
Example: Revenue $120K this year, $100K last year.
- ($120K - $100K) ÷ $100K × 100 = 20% growth
When Mental Math Fails
Use our Percentage Calculator when:
- Numbers aren't round (17.3% of $847.92)
- You need precision (financial calculations)
- You're under pressure (negotiations, quick decisions)
- Multiple calculations stack (discounts on discounts)
- You want to verify your mental math
Common Percentage Reference
| Fraction | Decimal | Percentage |
|---|---|---|
| 1/2 | 0.50 | 50% |
| 1/3 | 0.333 | 33.3% |
| 1/4 | 0.25 | 25% |
| 1/5 | 0.20 | 20% |
| 1/8 | 0.125 | 12.5% |
| 1/10 | 0.10 | 10% |
| 2/3 | 0.667 | 66.7% |
| 3/4 | 0.75 | 75% |
Conclusion
Percentages are everywhere—prices, tips, taxes, raises, discounts, interest. Understanding them isn't optional if you want to make informed financial decisions.
You don't need to be a math genius. Know the three core formulas, memorize the 10% anchor trick, and keep our Percentage Calculator bookmarked for when precision matters or numbers get ugly.
The person who can quickly calculate that "40% off already reduced prices" is actually 56% off total is the person who negotiates better, shops smarter, and makes confident financial decisions. That can be you.
Keep Reading
- Everyday Calculations Quick Reference - More mental math shortcuts
- Personal Finance Calculations Guide - Master money math
- Tip Calculator Tipping Guide - Perfect restaurant math
Related Tools
- Percentage Calculator - Calculate any percentage instantly
- Tip Calculator - Tips and bill splitting made easy