Percentage Calculations You Use Every Day (and How to Do Them Fast)

Percentages show up constantly — discounts at checkout, tip calculations, tracking a pay raise, or figuring out how much a stock moved. Almost every situation boils down to one of three calculations.

1. Finding a percentage of a number

Used for discounts, tips, and tax. The formula is straightforward: divide the percentage by 100, then multiply by the number. A 20 percent discount on a 1500 rupee item is (20 ÷ 100) × 1500.

2. Finding what percentage one number is of another

Used for figuring out things like test scores or budget allocation. Divide the part by the whole, then multiply by 100. If you spent 3000 out of a 12000 budget, that is (3000 ÷ 12000) × 100.

3. Finding percentage change

Used for tracking growth, price changes, or performance over time. Subtract the original value from the new value, divide by the original value, then multiply by 100. A salary that went from 40000 to 46000 increased by ((46000 − 40000) ÷ 40000) × 100.

The mistake people make most often

Percentage change calculations should always divide by the original value, not the new one. Mixing this up is the single most common error and can make an increase look like a decrease or badly distort the size of a change.