Percentage Calculation Tricks: Mental Math Shortcuts
Percentages are the math I do most often. After years of practice, here are the shortcuts that make common percentage calculations fast and reliable. Percentages are the math I do most often, and after years of practice, I have a collection of tricks that make common percentage calculations fast and reliable. These are not party tricks. They are the shortcuts I use for tips, discounts, tax, and quick comparisons throughout the week. Here are the ones I find most useful, with the math underneath each one. The 10 Percent Anchor Everything starts with 10 percent. To find 10 percent of any number, move the decimal one place to the left. Ten percent of 80 is 8. Ten percent of 245 is 24.5. Ten percent of 1,200 is 120. Once you have 10 percent, every other common percentage is a simple multiple or fraction of it. 10% of 80 = 8 5% = half of 10% = 4 15% = 10% + 5% = 12 20% = double 10% = 16 1% = one-tenth of 10% = 0.8 This is how I calculate tips. For an 87 dollar bill, 10 percent is 8.70. For 15 percent, add half of that, about 4.35, to get about 13. For 20 percent, double 8.70 to get 17.40. The mental math takes about three seconds and is more reliable than fumbling with a phone calculator in a noisy restaurant.