Percentages in Everyday Life: Discounts, Tips & Tax
Percentages are everywhere: sale signs, restaurant bills, interest rates, exam scores. A few mental shortcuts and a clear understanding of the common traps will save you money and confusion.
What a percentage really is
βPercentβ means βper hundredβ. So 15% is 15 out of every 100, or 0.15. To find a percentage of a number, multiply by the decimal: 15% of 80 is 80 Γ 0.15 = 12. The percentage calculator handles the four most common cases.
Quick mental tricks
- 10%: move the decimal point one place left. 10% of 64 is 6.4.
- 5%: half of 10%. 1%: move two places.
- 15% tip: take 10%, add half of that. On 64: 6.4 + 3.2 = 9.6.
- 20%: double 10%.
- 25%: divide by four.
Discounts do not simply add up
A 30% discount followed by an extra 10% off is not 40% off. The second percentage applies to the already reduced price: 100 β 70 β 63, a total of 37% off. Try combinations in the discount calculator.
Increase then decrease is not a round trip
If a price rises 50% and later falls 50%, you do not return to the start: 100 β 150 β 75. To undo a 50% increase you need a 33.3% decrease. The base changes each time.
Removing tax correctly
If a price including 20% tax is 120, the pre-tax price is 120 Γ· 1.2 = 100, not 120 minus 20%, which would be 96. The sales tax, GST and VAT calculator does it in both directions.
Percent versus percentage points
When an interest rate moves from 4% to 5%, it has risen by one percentage point, but by 25% in relative terms. News headlines often blur the two, so check which one is meant.
Tipping and splitting
Tipping customs differ by country, and some bills already include service. Use the tip calculator to split a bill fairly among friends.
Margin versus markup
Sellers should know the difference: markup is profit divided by cost, while margin is profit divided by the selling price. A 50% markup is only a 33% margin. Our profit margin calculator converts between them.