5 Percentage Tricks That Make Everyday Math Easier
Mental-math shortcuts for tips, discounts, and comparing prices — plus the two percentage formulas everyone should know.
Percentages come up constantly — tips, discounts, tax, growth rates — and a few mental-math tricks make them painless.
Trick 1: Reverse the operands
X% of Y = Y% of X. So 8% of 25 = 25% of 8 = 2. Look for the easier arrangement.
Trick 2: 10% is a shift, 1% is two shifts
10% of any number is that number with the decimal moved one place left. 1% moves two places. Combine them: 15% of 60 = 10%(6) + 5%(3) = 9.
Trick 3: The tip shortcut
For a 20% tip, take 10% (move decimal) and double it. For 18%, take 20% and subtract a tenth.
Trick 4: Discount ladders
"30% off then extra 20% off" is not 50% off — it's 1 − 0.7 × 0.8 = 44% off. Multiplying, not adding, is the rule.
Trick 5: Percentage change formula
(new − old) / old × 100. If it's negative, you had a decrease. Try it in the Percentage Calculator.