How do I calculate a 15 percent tip in my head fast?

Question

I always struggle to calculate tips at restaurants. Whats the fastest mental math trick?

For 15% tip:

Calculate 10% (move decimal left one place)
Take half of that for 5%
Add them together

Example: Bill is $\$ 47.50$

10% = $\$ 4.75$
5% = $\$ 2.38$ (half of 4.75, roughly)
15% = $\$ 4.75 + \ $2.38 = \$ 7.13$

For 20%: just double the 10% amount. For 18%: 10% + 5% + 3%(rounded).

But is there a faster way? I saw someone calculate 15% of $\$ 82$ instantly:
10% = 8.2, half = 4.1, total = 12.3. But they did it in like 2 seconds.

What about sales tax? If tax is 8.25%, can you quickly estimate the total? I usually just do 10% - 2% (rough estimate).

Any other useful everyday mental math tricks?

Robert Fischer · Accepted Answer

A quick way to calculate a 15 percent tip is: 15 % = 10 % + 5 % . Find 10 percent by moving the decimal one place left. Then take half of that for 5 percent. For a bill of $48: 10 % = 4.80. Half of 4.80 is: 2.40. So: 15 % = 4.80 + 2.40 = 7.20. The tip is about $7.20. If you want to be faster in a restaurant, round the bill first. For 47.80 , r o u n d t o48 and use the same calculation. The small difference is usually only a few cents.

How do I calculate a 15 percent tip in my head fast?

1 Answer

How do I calculate a 15 percent tip in my head fast?

1 Answer