How to Calculate Percentages in Your Head

Percentages are everywhere — sale prices, tax rates, tips, interest rates, exam scores, nutrition labels. Most people reach for a calculator, but a few simple tricks let you work these out in your head faster than unlocking your phone.

The Two Core Tricks

Trick 1: To find 10% of any number, move the decimal point one place left. 10% of 85 = 8.5. Trick 2: Percentages are reversible. 8% of 50 is the same as 50% of 8 = 4. Always choose the easier direction.

Building From 10%

• 15% = 10% + 5% (half of 10%)
• 20% = 10% × 2
• 25% = divide by 4
• 30% = 10% × 3
• 50% = divide by 2
• 75% = 50% + 25%

Quick Examples

• 20% tip on $46: 10% = $4.60, × 2 = $9.20
• 15% off $80: 10% = $8, 5% = $4, total off = $12, new price = $68
• 35% of 200: 10% = 20, × 3 = 60, 5% = 10, total = 70
• Exam score 37/50: 37 ÷ 50 = 0.74 = 74%

Percentage Change

To calculate percentage change: ((new value − old value) ÷ old value) × 100. Price went from $40 to $52: (52 − 40) ÷ 40 = 0.30 = 30% increase. Price dropped from $60 to $45: (45 − 60) ÷ 60 = −0.25 = 25% decrease.