20 Mental Math Tricks

I used to freeze up whenever I had to do math in my head.

That feeling of standing in a grocery aisle or looking at a restaurant bill and my brain just going completely blank. It was embarrassing. For a long time, I just figured I wasn't a "math person" and reached for my phone calculator for even the simplest sums. But as I worked on rebuilding my life, getting healthy and building better habits, I realized something important. My brain wasn't broken. It was just untrained.

Sharpening my mind with simple mental math became a small but powerful part of my journey. It was a way to build confidence, one small calculation at a time. It's a skill, like any other, and you can learn it. Here are 20 simple tricks that helped me turn mental math from a source of anxiety into a daily confidence boost.

Making Addition and Subtraction Easier

Let's start with the basics. These tricks make adding and subtracting in your head feel less like a chore and more like a simple puzzle.

1. Work from Left to Right. We’re all taught to add from right to left on paper. In your head, it’s often easier to do the opposite. For 54 + 28, first add the big numbers: 50 + 20 = 70. Then add the small ones: 4 + 8 = 12. Finally, add your two results: 70 + 12 = 82.

2. Round Up and Adjust. For numbers that are close to a round number, this is a lifesaver. To calculate 497 + 380, just think of it as 500 + 380 = 880. Then, since you added 3 to 497 to make it 500, you just subtract that 3 from the end. 880 – 3 = 877.

3. Find Pairs that Make 10. When you have a long string of numbers to add, look for friends. In a list like 8 + 3 + 5 + 2 + 7, spot the pairs that make 10. You have 8 + 2 = 10 and 3 + 7 = 10. Now your problem is 10 + 10 + 5. Much easier.

4. Subtract by Adding Up. Big subtraction problems can be intimidating. So flip it. Instead of 1000 – 730, ask yourself, "What do I need to add to 730 to get to 1000?" You can add 70 to get to 800, then 200 to get to 1000. You added 70 + 200, so the answer is 270.

Mastering Multiplication

Multiplication feels like a big leap, but most of it is just breaking things down into simpler steps.

5. Multiply by 10, then Halve (for x5). To multiply any number by 5, just multiply it by 10 and then cut it in half. For 64 x 5, do 64 x 10 = 640. Half of 640 is 320. Done.

6. Double, then Double Again (for x4). To multiply by 4, just double the number twice. For 35 x 4, double 35 to get 70. Double 70 to get 140.

7. Multiply by 10, then Subtract (for x9). To multiply a number by 9, multiply it by 10 and then subtract the original number. For 45 x 9, do 45 x 10 = 450. Then subtract 45. 450 – 40 is 410, and 410 – 5 is 405.

8. The Easy 11s Trick. To multiply a two-digit number by 11, add the two digits together and place the sum in the middle. For 34 x 11, add 3 + 4 = 7. Put that 7 between the 3 and 4. You get 374. If the sum is a two-digit number (like for 85 x 11, where 8 + 5 = 13), you carry the 1. Put the 3 in the middle and add the 1 to the 8. You get 935.

9. Double and Halve. This is great for bigger problems. If you have to multiply 16 x 35, you can halve the even number and double the other. It becomes 8 x 70, which is 560. Much simpler.

10. Break It Down. For something like 14 x 12, just break one number down. Do 14 x 10 (140) and 14 x 2 (28). Add them together: 140 + 28 = 168.

11. Squaring Numbers Ending in 5. This one feels like a real party trick. To square a number like 45, take the first digit (4) and multiply it by the next number up (5). 4 x 5 = 20. Then just tack on a 25 at the end. The answer is 2025. It works every time.

Tackling Percentages and Tipping

This is where mental math becomes a true everyday tool. No more fumbling with your phone to calculate a tip or a sale discount.

12. The 10% Rule. This is the foundation for almost all percentage calculations. To find 10% of any number, just move the decimal point one place to the left. 10% of $85 is $8.50. 10% of $120 is $12.00.

13. Find 5%. Once you know 10%, finding 5% is easy. It’s just half of the 10% amount. So, 5% of $85 is half of $8.50, which is $4.25.

14. Calculate a 15% Tip. Find 10% and then add half of that amount back on. For a $50 bill, 10% is $5. Half of that is $2.50. Your tip is $5 + $2.50 = $7.50.

15. Calculate a 20% Tip. Find 10% and just double it. For a $50 bill, 10% is $5. Double it for a $10 tip.

16. The Reverse Percentage Trick. This one is amazing. Sometimes, "8% of 50" seems hard. But it’s the exact same as "50% of 8." That’s easy. It’s 4. This works for any percentage. 4% of 75 is the same as 75% of 4.

17. Figure Out Discounts. If an item is 30% off, don’t calculate the 30%. Calculate what you will pay, which is 70%. For a $200 jacket that’s 30% off, you pay 70%. Find 10% of $200 ($20) and multiply by 7. You’ll pay $140.

Building Your Mental Muscle

Knowing the tricks is one thing. Making them a habit is another.

18. Practice Little and Often. You don’t need to drill for hours. Just use these tricks in your daily life. At the grocery store. At a restaurant. When you’re driving past a gas station. Small, consistent practice is key. This is the same principle that helped me lose over 110 pounds. It wasn't about one giant workout. It was about small, daily choices that added up over time.

19. Estimate First. Don't worry about getting the exact answer right away. Just try to get close. If the bill is $78.50 and you want to leave a 20% tip, just round it to $80. 10% is $8, so 20% is $16. Your estimate is close enough to be confident.

20. Celebrate the Small Wins. This is so important. When I was on my health journey, I learned to celebrate losing one pound just as much as losing ten. The same goes for this. When you successfully calculate a tip in your head, give yourself a mental pat on the back. That small win builds momentum. It's proof that you're getting sharper and more capable.

Math isn't a measure of your worth. It's just a tool. And like any tool, the more you use it, the better you get with it. Training my brain with these small tricks gave me a sense of control and confidence that spilled over into so many other areas of my life.

So here’s my challenge to you: The next time you see a percentage sign or need to split a bill, don’t pull out your phone. Take a deep breath and try one of these tricks.

What’s the first one you’re going to master?