Percentages come up everywhere — a tip, a discount, a test score, a raise, a statistic in the news — and yet the formula slips most of us at the worst moment. The good news is there are really just three percentage questions, and once you can spot which one you're facing, the math is easy.
Here are the three, with plain formulas and worked examples.
1. Finding a percentage of a number
This is the most common: 'What is 20% of 80?' Convert the percentage to a decimal (divide by 100) and multiply.
Formula: number × (percentage ÷ 100)
Example: 20% of 80 = 80 × 0.20 = 16. So a 20% tip on an $80 meal is $16.
2. Finding what percent one number is of another
'You got 45 out of 60 — what percent is that?' Divide the part by the whole and multiply by 100.
Formula: (part ÷ whole) × 100
Example: 45 ÷ 60 = 0.75, × 100 = 75%. That's your test score.
3. Percentage change (increase or decrease)
'A price went from $40 to $50 — what's the increase?' Subtract the old value from the new, divide by the *old* value, and multiply by 100.
Formula: ((new − old) ÷ old) × 100
Example: (50 − 40) ÷ 40 = 0.25, × 100 = 25% increase. A negative result means a decrease.
Always divide by the *original* value for percentage change — dividing by the new value is the most common mistake.
Skip the arithmetic
The free Percentage Calculator handles all three of these — of a number, what percent, and percentage change — so you can just type the numbers in.
Percentage CalculatorWork out percentages, increases and changes.Related everyday calculations
- Discount Calculator — work out a sale price and how much you save.
- Tip Calculator — split a bill and add a tip.
- Sales Tax Calculator — add tax to a price.
- Markup Calculator — set a selling price from cost.