Wall Street Steward Blog

The Mathematics of Tipping

Ok peeps…fair disclosure: this will be a short blog entry. However, I was asked to write about this and I thought the idea had some legs.

Have you ever been at a restaurant, and when you received the bill, you stare at the subtotal and struggle to determine what tip to give? Not because you are thinking about how good/bad the service was, but because you cannot do the simple math in your head to determine what is 15% of X.

Really? Nobody? L-I-A-R-S!

Recently, I was eating lunch with a group of people and I shared my method with them. Someone shared it with me many years ago, and I assumed that everyone knew it. After sharing it, I was expecting someone to say “hey Copernicus, we know how to do that!”

That didn’t happen.

I told my friend, as she struggled…“just move the decimal.”


“The bill is $12.50, so if you move the decimal once to the left, it becomes $1.25. That is 10% of $12.50. If the service was great and you want to leave 20%, then just double it to $2.50.”

“Oh. Wow. That is a pretty cool trick. Thanks. BUT what if I want to leave 15% smart guy?”

“Well, then move the decimal (to get to $1.25) and then add half of that number to the total. Half of $1.25 is 62.5 cents….so you can either leave $1.88 or round it to a two spot.”

The truth is that after 15 years of working with numbers, I can calculate the figures in my head. If you ask me what 11% of 645 is, I can spew off 71 pretty easily. I am not trying to brag…but there are so few things that I do well that I have to toot my own horn every now and then, right? Don’t answer that!

Bill: $130
Move the decimal: $13
Double it for 20% tip: $26
Add back half of the amount for 15%: $13 + $6.50 = $19.50
if the service was bad, just leave the 10%

I DO NOT expect to get any feedback on this blog entry. I am sure some of you are reading this saying “this is useless…who doesn’t know how to do that.” However, judging my how many people I watch stare at their bills and get out the pocket tipping guide, someone will surely learn something useful from this entry.

AND I don’t expect those people to be honest enough to admit it. Oh well, I know you are out there!

Told you it would be short. Till next time.