Wednesday, Dec 29, 2010, 9:13 PM in The Spout

# Enabling the Tip Calculator in Your Brain

I can’t imagine anyone reading this blog needs to read this, but I can’t help myself.

When I was just a wee lad, probably the most valuable thing I learned was how to perform mathematical estimation, the importance of which and several techniques you can get by reading Jon Bentley’s The Back of the Envelope (this essay along with several others, are collected in his most excellent books Programming Pearls and More Programming Pearls, both of which are still relevant a decade later). Not only is estimation generally quicker than running a calculator, but even when you do run a calculator, it helps you figure out when you did it wrong, the latter of which has saved my bacon time and again.

For example, as much as I love the Windows Phone 7 marketplace and it’s quality and quantity of applications, the ones that puzzle me are the “tip calculator” apps (several!). I don’t understand why it’s worth the trouble of pulling out your phone and punching buttons when you can know the tip instantly.

For example, let’s assume the dinner bill is $37.42. If the service was bad, that’s a 10% tip (you have to tip them something ‘cuz the IRS assumes you will and taxes them accordingly – bastards). So, with a 10% tip, take the bill and move it right one decimal point: $3.74. Now, round up or down depending on how bad the service was, e.g. $3.50 or $4. Quick and easy.

Assuming the service was great, that’s a 20% tip, so double the bill and move it right one decimal point, making the math easier for yourself, e.g. $37.42 is close to $35, doubling is $70, so a $7 tip. Boom: 20% tip.

If you want to get fancy and provide a 15% tip for good but not great, then average the two numbers: ($4 + $7)/2 = $5.50. Zim zam zoom.

Honestly, as great as the apps are on your phone, tablet or BlueTooth headset (seriously), think about using the apps in your head first. Now only are they quicker and cheaper, but using them staves off dementia (which is a good thing!).

Oh, and if the tip is added as a mandatory minimum, then the additional tip is easy: $0.00. I don’t deal well with authority.

## 16 comments on this post

### Bil Simser:

I think where the tip calculators shine is when you have a group and not everyone is even on their bills. Imagine you have 9 friends (I know it's hard) and 3 of them have wine at dinner, 2 drink heavily on Crown Royale and Coke, 1 has water and is the designated driver but had a lobster dinner and 2 others including yourself had the buffet. That's not an easy thing to calculate in your head, especially if you have to add up the bill for each person (and the bill is all together). Separate bills help this and each person can perform the simple math again if that's the case and you could argue that once you had the bill amount for you or your party, simple math kicks in again. I think by that time you probably could have been able to knock off an answer from one of those apps. Maybe not.

Full disclosure. I have not written a tip calculator and don't plan on doing so. I have bigger fish to fry.

Wednesday, Dec 29, 2010, 9:30 PM

### Chris Sells:

Wednesday, Dec 29, 2010, 9:33 PM

### Chris Sells:

Wednesday, Dec 29, 2010, 9:55 PM

### Bil Simser:

Ha. Funny about the nines (yes, we're now using your blog post as a twitter thread). I watch watching my daughter on the iPad the other day as she was playing a multiplication game and it showed that trick with the nines. Didn't realize it. Pretty slick. I'm just old school and I keep the mutliplication tables in my head.

Thursday, Dec 30, 2010, 5:10 AM

### Bryan Porter:

The "placed upside down part" came into being when a friend of mine, who had a credit card with a distinctive picture on the front, had his card picked 4 times in a row - the waiters and waitresses would immediately reach for his card to get a better look at the picture on the front. Hence, the cards are placed upside down.

Thursday, Dec 30, 2010, 7:41 AM

### Consultutah:

Thursday, Dec 30, 2010, 11:30 AM

### Judah Himango:

Calculation is easy: Find 10% by moving the decimal left, e.g. 40 = 4

Then, find half of that number, and add it, e.g. half of 4 is 2, so 4 + 2 = 6.

Voila! There's your 15%.

Thursday, Dec 30, 2010, 3:26 PM

### Martin:

It's only one step and i've gotten really efficient at dividing by 7 over the years.

Friday, Dec 31, 2010, 2:37 AM

### big fan of your blog:

Happy New Year!

Friday, Dec 31, 2010, 8:40 AM

### Matt Katz:

It works best when you've got a good group of friends that go out regularly together so the pain averages out. It's even simpler than dividing the bill evenly!

The less you earn, the less attractive this game is and the more important it is to pay what you actually owe.

People who earn more than I do regularly vie to pick up the check rather than playing roulette. Their game is to see who can sneak a credit card to the waiter first.

Thursday, Jan 6, 2011, 2:15 PM

### MAC:

At first this was hard but as time has progressed; I've gotten faster and faster and faster.

I can take multiple digit decimal number and add them in my head with incredible speed! The trick is to not really think about it; let the super computer inside your skull do all the work! I know it sounds crazy and at first it is very hard to let go of adding, carrying wondering if it's right but after a lot of practice I can add up 10 multi-digit decimal number in a couple of seconds! It's probably far faster than that it's just it takes a couple of seconds to become cognizant of it. Maybe not as fast as silicone but I don't require any electricity.

And the kicker is, I’m 58 years old and, supposedly, way beyond my prime for doing this kind of thing.

Tuesday, Feb 1, 2011, 8:14 AM

### Janet:

Thursday, Mar 17, 2011, 8:17 PM

### John:

Monday, Apr 4, 2011, 6:21 AM

### Ian:

I will consider tipping if and only if the service is a better than I wold expect relative to the cost of the meal, e.g. someone has done more than is expected by their job.

Wednesday, Aug 24, 2011, 1:50 AM

### hello:

Monday, Jan 9, 2012, 7:02 AM

### Chris Sells:

Monday, Jan 9, 2012, 9:53 AM