How to Calculate Percentages: Every Formula You Need
September 3, 2026 · 5 min read
Percentages show up everywhere — shopping discounts, tax rates, tips, test scores, business growth, investment returns. Yet many people struggle with percentage calculations because there are several different types, and each requires a slightly different approach. This guide covers every common percentage formula with clear examples.
The 6 Most Common Percentage Calculations
1. What is X% of Y? Formula: Y × (X / 100). Example: What is 15% of $80? → $80 × 0.15 = $12. Use this for tips, taxes, and discounts.
2. X is what percent of Y? Formula: (X / Y) × 100. Example: 35 is what percent of 200? → (35 / 200) × 100 = 17.5%. Use this to find your score on a test, your savings rate, or what portion of your budget goes to rent.
3. Percentage increase Formula: ((New - Old) / Old) × 100. Example: Revenue went from $4,000 to $5,200 → ((5,200 - 4,000) / 4,000) × 100 = 30% increase.
4. Percentage decrease Same formula, result is negative. Example: Price dropped from $50 to $35 → ((35 - 50) / 50) × 100 = -30% (a 30% decrease).
5. Find the original price before a discount Formula: Sale Price / (1 - Discount%/100). Example: You paid $68 after a 15% discount. Original price: $68 / (1 - 0.15) = $80.
6. Add or remove tax To add tax: Price × (1 + Tax%/100). To find the pre-tax price: Total / (1 + Tax%/100). Example: $50 item with 8.5% tax → $50 × 1.085 = $54.25.
Quick Percentage Shortcuts
| To find... | Shortcut |
|---|---|
| 10% of any number | Move the decimal one place left |
| 5% of any number | Find 10%, then halve it |
| 1% of any number | Move the decimal two places left |
| 20% of any number | Find 10%, then double it |
| 25% of any number | Divide by 4 |
| 33% of any number | Divide by 3 |
| 50% of any number | Divide by 2 |
| 15% tip | Find 10% + half of that |
Common Percentage Mistakes
A 50% increase followed by a 50% decrease does not return to the original. If $100 increases by 50%, it becomes $150. A 50% decrease from $150 gives $75 — not $100. This is why understanding the base matters.
Percentage points vs. percentages. If interest goes from 5% to 7%, that is a 2 percentage point increase — but a 40% increase in the rate itself. These are very different things.
Calculate Any Percentage Instantly
Use our free Percentage Calculator for all six types of calculations. Enter your numbers and get instant results with step-by-step breakdowns.
Open Percentage Calculator →Need to calculate a discount? Our Discount Calculator handles sale pricing, and the Tip Calculator splits tips for any group size.