Percentages quietly run a huge amount of daily life — sales, raises, grades, interest — yet most of us never learned a fast, reliable way to work them out on the spot. Here's the practical version.
Working Out a Discount
To find a sale price, multiply the original price by (100% minus the discount). A $60 item at 25% off: 100% − 25% = 75%, so $60 × 0.75 = $45. The "what's 25% off" question is really just "what's 75% left," which is often the easier way to think about it.
Figuring Out a Raise
A raise is the original salary multiplied by (100% plus the increase). A $50,000 salary with a 4% raise: $50,000 × 1.04 = $52,000. The same shortcut works for any compounding increase — investment growth, population growth, or price inflation.
Grades and Test Scores
A score of 42 out of 50 is 42 ÷ 50 = 0.84, or 84%. The part-over-whole pattern is the core of almost every percentage question: whatever you're measuring, divide it by the total, then multiply by 100.
"What Percent Did It Change?"
This is the one that trips people up. If a price moves from $80 to $100, the increase is $20, and the percentage change is the increase divided by the original number: $20 ÷ $80 = 25%. It's easy to mistakenly divide by the new number instead, which gives a different (wrong) answer — a common source of disagreements over reported statistics.
A Quick Mental Shortcut
For fast estimates, 10% of any number just moves the decimal point one place left ($84 → $8.40). From there, 20% is double that, 5% is half that, and 15% is 10% plus 5%. This combination covers most everyday tipping and discount math without a calculator.
For Anything More Precise
Mental shortcuts are great for estimates, but for an exact number — taxes, payroll, grading — our Percentage Calculator handles discounts, increases, and part-to-whole calculations instantly.