 Nearly everyone has a calculator in their pockets these days thanks to nearly ubiquitous smartphone ownership, but it’s easy to calculate the tip after your next meal without pulling it out.

Simple math tricks that make life easier are the best math tricks, and in terms of frequency of use, this one ranks near the top of our list. Adding a tip onto the cost of a meal is a common practice in American culture, and one most of us will do thousands of times in our lives. Rather than wing it with a guess or pull out a calculator you can use the following trick instead. For the sake of example, we’ll use a meal cost of \$44.37.

1. Take the cost of the meal and round it up to the nearest whole number. Our example meal rounded to the nearest whole number it would is \$45.00.
2. Calculate 10% of that by simply moving the decimal place one place to the left—\$45.00 becomes \$4.50.
3. If you want to tip 15%, you then take half the amount you just arrived at and add it on: half of \$4.50 is \$2.25, so added together our tip is \$6.75. If you want to tip 20%, you’ll double it—\$4.50 * 2 is \$9. Great service? Triple it for a 30% tip—\$4.50 * 3 is \$13.50.

Of course, if you want to be super precise and prefer cold hard calculations over clever math parlor tricks, you can always bust out your calculator and use a simple multiplier to calculate the tip. For those of you a little rusty on your grade school math, the equation for calculating a percentage is Original Value * 0.X where X is the percent. If we figured the exact 20% tip on our hypothetical meal, it would be \$44.37 * 0.20 = \$8.95.

It looks like our little parlor trick worked out pretty well. We spent a nickel more on the tip but didn’t have to fuss with our phone and got a little mental stimulation out of the experience.