7 Percentage Tricks That Make You a Mental Math Genius
Calculating 15% tip, 30% discount, or your raise in your head is easier than you think.
Most people reach for their phone to calculate a tip or figure out a sale price. With these seven tricks, you will do it faster in your head.
1. The 10% Anchor
10% of anything: move the decimal point left. 10% of $67.50 = $6.75. Now you can build any percentage: 20% = double it ($13.50). 5% = half of 10% ($3.38). 15% = 10% + 5% ($6.75 + $3.38 = $10.13).
2. The Flip Trick
8% of 50 = 50% of 8 = 4. Pick whichever is easier to calculate. 4% of 75 = 75% of 4 = 3. This works because multiplication is commutative (a percent of b = b percent of a).
3. Discounts: Subtract From 100
30% off means you pay 70%. $80 item at 30% off: 70% of $80 = 7 times 8 = $56. Faster than calculating the discount and subtracting.
4. Double Discounts Are Not What You Think
40% off plus additional 20% off is NOT 60% off. It is: pay 60%, then pay 80% of that = 0.6 times 0.8 = 0.48 = 52% off. Always less than the sum of discounts.
5. Tipping: The Double Tax Method
In the US, sales tax is roughly 7-10%. Double the tax amount for a 15-20% tip. Tax on $50 meal is about $4 — double it for an $8 tip (16%). Easy.
6. Salary Raise Quick Math
Quick annual to monthly: divide by 12. But even quicker: divide by 10 (close enough for estimation). $5,000 raise / 10 = $500/month (actual: $417 but close enough for a quick mental check).
7. The Rule of 72
Years to double your money = 72 / interest rate. At 8% returns: 72/8 = 9 years to double. At 12%: 6 years. At 4%: 18 years. Works for any compound growth.
Practice with our percentage calculator and percent increase calculator.