It turns out, your school teacher was right. You need math on a regular basis. While most people won't typically have need of orthogonal trajectories, here are some of the most vital math shortcuts for your everyday life.


Figure Out Your Yearly Salary From Your Hourly (or Vice Versa)


When negotiating your salary or trying to figure out raises, comparing annual salaries with hourly ones are a needless complication. Fortunately, there's a quick and dirty way to transition between the two:

$1/hour = $2000/year

This formula assumes a 40-hour workweek and two weeks vacation every year. While your mileage may vary, this can help you get into a rough ballpark. If you're part time, you can adapt the formula. Just multiply $2000 by your hourly rate to get your yearly rate. Alternatively, divide your annual salary to get your hourly pay. For example:

$7.25/hour * 2000 = $14,500/year

$30,000/year / 2000 = $15/hour

You can also adapt this formula if you're only working part-time. For example, if you work a job 32 hours a week, that equals $1600/year. Every 8 hours per week amounts to $400/year for every $1 per hour.

Use the Rule of 72 to Estimate Interest or Returns on Investment


When you're considering where to invest your money and what kind of return you want to get, the Rule of 72 can put you in the ballpark. Here's how it works:

72/Interest Rate = Years To Double Investment

While the Rule of 72 isn't perfectly accurate, it can give you a frame of reference. So, say you have $5,000 and you want to place it in a mutual fund that generally has a return of 10%. That $5,000 will double to $10,000 in about 7.2 years. You can also use this to estimate the effect of inflation. While US inflations rates vary widely from year to year, we could reasonably estimate at least a 2% increase in inflation. Using the Rule of 72, we can see that your $10,000 today will be worth about $5,000 in about 36 years.


Calculate a Tip In Your Head Quickly


When it comes time for the tip, you don't want to waste time or pull out the calculator on your phone. To simplify how much you need to give for a tip, move the decimal in your total one place to the left to reach 10%. If your total is $20, you get $2 for a 10% tip. From there you can easily calculate a higher percentage. If you want to do a 15% tip, multiply that number by 1.5 ($3 in our example). For 20%, multiple by two, and so on.

Of course, you do have a calculator in your smartphone, so if you don't mind pulling it out briefly, you can figure out the tip with a single calculation. Multiply your total time 1.[tip%]. So, for a 20% tip, multiply your total by 1.2. This will give you your new total. The difference between your new total and the subtotal will give you your tip amount.


Convert Between Fahrenheit and Celsius


Until Earth establishes a universal Federation and unites all nations and planets under a single banner, we'll probably keep wrestling with converting between different standards of measurement. Fortunately, one of the most common—converting between Fahrenheit and Celsius—is also pretty easy to calculate.


Celsius to Fahrenheit: Multiply the temperature by 2, then add 30.

Fahrenheit to Celsius: Subtract 30, then divide by 2.

This is more of a ballpark estimate, but for most things, it will work alright. If you do need to get more specific, however, replace 2 with 1.8 and replace 30 with 32 in the above equations.


Figure Out the Annual Cost of Daily Habits


We all have our habits that cost small amounts of money that eventually add up. If you want to find out just how much you'll spend on them every year, there's a quick way to find out:


Estimate the weekly cost of each purchase. If it costs $4 and you do it four times per week, then the weekly cost is $16. Add two zeros to the end. In this example, we get $1600. Divide that number by half. Here, we get $800. This will be a rough approximation of the yearly cost.


Strictly speaking, this will actually give you the cost of 50 weeks of the habit, though it's reasonable to assume there would be some variation throughout the year, anyway. It's also a great way to get a frame of reference for how much you might be able to save by changing certain habits.

Of course, the great thing about math is that there's no limit to the number of quick, time-saving shortcuts that can be useful in daily life. We're sure you've got plenty more, so share them in the comments below.