Calculators are awesome, but they’re not always handy. More to the point, no one wants to be seen reaching for the calculator on their mobile phone when it’s time to figure out a 15 percent gratuity. Here are ten tips to help you crunch numbers in your head.

Mental math isn’t as difficult as it might sound, and you may be surprised at how easy it is to make seemingly impossible calculations using nothing but your beautiful brain. You just need to remember a few simple rules.

**Add and Subtract From Left to Right**

Remember how you were taught in school to add and subtract numbers from right to left (don’t forget to carry the one!)? That’s all fine and well when doing math with pencil and paper, but when performing mental math it’s better to do it moving from left to right. Switching the order so that you start with the largest values makes it a bit more intuitive and easier to figure out. So when adding 58 to 26, start with the first column and calculate 50+20=70, then 8+6=14, which added together is 84. Easy, peasy.

**Make It Easy on Yourself**

When confronted with a difficult calculation, try to find a way of simplifying the problem by temporarily shifting the values around. When calculating 593+680, for example, add 7 to 593 to get 600 (more manageable). Calculate 600+680, which is 1280, and then take away that additional 7 to get the correct answer, 1273.

You can do a similar thing with multiplication. For 89x6, calculate 90x6 instead, and then subtract that additional 6, so 540-6=534.

### Memorize Building Blocks

Memorizing multiplication tables is an important aspect of mental math, and it shouldn’t be discounted.

Spencer Greenberg, a mathematician and founder of ClearerThinking.org, says that by memorizing these basic “building blocks” of math, we can instantly get answers to simple problems that are embedded within more difficult ones. So if you’ve forgotten these tables, it would do you well to quickly brush up. While you’re at it, memorize your 1/n tables so you can quickly recall that 1/6 is 0.166, 1/3 is 0.333, and 3/4 is 0.75.

**Remember Cool Multiplication Tricks**

To help you do simple multiplication, it’s important to remember some nifty tricks. One of the most obvious rules is that any number that’s multiplied by 10 just needs to have a zero placed at the end When multiplying by 5, your answer will always end in either a 0 or 5.

Also, when multiplying a number by 12, it’s always 10 times plus two times that number. For example, when calculating 12x4, do 4x10=40, and 4x2=8, and then 40+8=48. One of my favorites is multiplying by 15: just multiply your number by 10, and then add half to the answer (e.g. 4x15 = 4x10=40, plus half that answer, 20, giving you 60).

There’s also a neat trick for multiplying by 16. First, multiply the number in question by 10, and then multiply half the number by 10. Then add those two results together with the number itself to get your final answer. So to calculate 16 x 24, first calculate 10 x 24 = 240, then figure out half of 24, which is 12, and multiply by 10, giving you 120. Simple math finishes it up: 240+120+24=384.

Similar tricks exist for other numbers, which you can read about here.

### Squares Are Your Friends

These simple tricks are all fine and well, but large numbers present a different challenge. For that, a physicist from askamathematician.com says it’s a good idea is use the difference of squares (a square being a number multiplied by itself).

“Take the two numbers you’re multiplying and think of them as their average, x, plus and minus the difference between each and their average, ±y,” he says. “These two numbers are squared, so rather than memorizing entire multiplication tables you only memorize squares.”

It may seem like a daunting task, but memorizing all the squares from 1 to 20 isn’t as bad as it sounds. It’s just 20 numbers, after all. Armed with this prior knowledge, you can perform some pretty incredible calculations.

Here’s how it works, starting with a simple example. Let’s assume for a moment that we don’t know the answer to 10x4. The first step is to figure out the average number between these two numbers, which is 7 (i.e. 10-3=7, and 4+3=7). Next, determine the square of 7, which is 49. We now have a number that’s close, but not close enough. To get the correct answer, we have to square the difference between the average (in this case 3) providing us with 9. The last step is to do some simple subtraction, 49-9=40, and wouldn’t you know it you have the correct answer.

That might seem like a roundabout way to calculate 10x4 (it is), but this same technique works for bigger numbers. Take 15x11 for example. Once again, we have to find the average number between these two, which is 13. The square of 13 is 169. The square of the difference in the average (2) is 4. Finally, 169-4=165, the correct answer.

### It’s Okay to Approximate

When doing mental math, particularly for large numbers, it’s often a good idea to make an informed estimate, and not worry about getting a perfect answer. Back during the Manhattan Project, for example, physicist Enrico Fermi wanted a rough estimate of the atomic blast’s power before the diagnostic data came in. To that end, he dropped pieces of paper when the blast wave hit him (from a safe distance, of course). By measuring the distance the paper traveled, he estimated the blast strength to be about 10 kilotons of TNT. This estimate was fairly accurate, as the true answer was 20 kilotons of TNT.

## DISCUSSION

Granted, I have a background in finance and accounting, but I can’t believe how many people have their mind’s blown when I clue them in on the tipping tip. 10% plus half again. I remember sitting at lunch with some highly intelligent classmates (all top 10 students), they had to get calculators to determine tips.