LPT: How to Convert °C to °F in Your Head

These are quick math tricks to make certain day-to-day calculations much easier.  Sometimes you need to convert temperatures in Celsius (centigrade) into Fahrenheit.

The formula I learned in elementary school was °C * 9/5 + 32, which is not particularly amenable to calculating in your head.  Here's how to figure that in your head without multiplying.

Let's say the temperature is 7°C.

• Calculate 1/10th (10%) of that.  Ten percent of a number is simply that number with the decimal place shifted left one spot.  So ten percent of 7 is 0.7
• Subtract 0.7 (1/10th) from 7 = 6.3
• Double 6.3 = 12.6
• Add 32 = 44.6
• 7°C is 44.6°F

How does this work?

In the original formula 9/5 = 1.8.  Half of 1.8 is 0.9

If you subtract 1/10th of the original number from itself, you get 0.9 * °C.  Then when you double that, you get 1.8 * °C.

After that, you merely need to add 32 to get the Fahrenheit equivalent.  Going the other way (°F to °C) is a little more complicated, and I will get to that in a subsequent post.

LPT: How to Calculate a Tip in Your Head

These are quick math tricks to make certain day-to-day calculations much easier.  When you eat at a restaurant, you'll typically tip your server.  Here's how to figure that in your head without multiplying.

Let's say the bill is $12.34.

Calculating an Exact Tip

• Calculate 10% of the bill.  Ten percent of a number is simply that number with the decimal place shifted left one spot.  So ten percent of 12.34 is just 1.234 (round to 1.23).

10% tip:

• Tip is $1.23

15% tip:

• Start with the 10% of $12.34: 1.23.
• Now cut that number in half to make 0.615 (rounded to 0.62).  This is 5% of the bill.
• Add 1.23 and 0.62 together to make 15%:
• Tip is $1.85

20% tip:

• Double the ten percent: 1.23 * 2
• Tip is $2.46

Easier Approximate Tip

That said, I typically approximate using only the dollar value, because it's easier.

• Round up to $13.  Calculate 10% of 13.  Ten percent of a number is simply that number with the decimal place shifted left one spot.  10% of 13 is 1.30

10% tip:

• Tip is $1.30

15% tip:

• Cut 1.30 in half to make 0.65.  This makes 5% of the bill.
• Add 1.30 and 0.65 together to make 15%: (1.30 + 0.65)
• Tip is $1.95

20% tip:

• Double the ten percent: 1.30 * 2
• Tip is $2.60