My brain has been gifted with a pretty good spell-checker but not so good calculator. Instead of doing straight out calculations, I sometimes resort to some math magic. Let’s take going out to eat at a restaurant as an example. When going with a group, it’s more complicated if people want to pay according to what they ordered. I used to resort to taking out my mobile phone or PDA to use the calculator. I haven’t had to do that since I figured the trick to calculating tax and tip without a calculator.

To start off, I don’t try to figure out tax and then tip. I calculate them together. Where I live, the tax rate is 8.25%. Tip is 15%. The sum percent of tax and tip is 23.25%. To simplify matters in my simple brain, I round up to 25%. The tip will be more than 15% but that’s ok with me.

Twenty-five is a good number because it makes it easy to deal with subtotals that are factors of 4. For example, if a meal costs $12, tax & tip (t&t) will be $12/4=$3 . The total will be $15. A $100 meal will total $125.

How about when the meal cost is not a factor of 4? No problemo. What if it is $42? This is the thought process in my head to arrive at t&t:

Double 42: 84 (200% of 42)

Divide 84 by 10, or move the decimal to the left by one: **8.4** (20% of 42)*

–

Divide 42 by 10, or move the decimal to the left: 4.2 (10% of 42)

Divide 4.2 in half: **2.1** (5% of 42)*

With those pairs, you can do the decimal change before or after the doubling or halving, depending on which one makes it easier to do the math.

* Add the values: 8.4+2.1 =10.5 (20%+5% = 25%)

Cost + T&T = 42 + 10.5 = $52.50. This number could be rounded off to $52 or $53

It might seem complicated but it’s not that hard. At least it’s not as difficult as it would be to try to come up with exact numbers:

Subtotal: 42

Tax: 3.47 (8.25%)

Tip: 6.3 (15%)

Total: $51.77

It’s lower than the estimated cost, but it took more effort to go through, if one isn’t blessed with a built-in calculator.

I’ll try another example but with more of a stream-of-consciousness method to the walkthrough: $27

27… 54… **5.4**… (20% of 27)

27… ~13… **1.3**… (5% of 27)

5.4+1.3= 6.7

27+ ~7= 34 total including t&t

exact sum: 33.2775

Well, I hope that helps, and doesn’t end up causing confusion.

Uh, why go through all these mental acrobatics? Just take 10% of the bill, take it down by one decimal place and add half of that to it. Simple and accurate.

Great article on how to calculate tax easily. This is a scary task I generally try to avoid. But reading this made it less intimidating.

All the tricks are Nice and very useful. Great work.