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.

Comment Feed 16 comments on this post

Bil Simser:

The simple math is fine for 10, 15, and 20% tips and yes, people should do this in their heads. If they can't, then probably need to go back to school.

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:

I know some people feel the need to assign everyone the exact price of the things that they ordered when there's a group involved. I prefer just to divide everything evenly and let the rule of large numbers (aka god) sort it out.

Wednesday, Dec 29, 2010, 9:33 PM

Chris Sells:

Also, this has nothing to do with tipping, but my step-father helped me get past the 9s in the multiplication tables by telling me "multiply the number by 10 and subtract the number," e.g. 9x7 = 10*7-7 = 70-7 = 63. I still do that to this day.

Wednesday, Dec 29, 2010, 9:55 PM

Bil Simser:

True that people really don't need to get down to the exact price. I prefer everyone just split the tip but then I usually roll in crowds where that's the norm, no matter who had what. Life needs to be simpler.

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:

While it has nothing to do with tipping, lately my favorite group dinner game to play (when the bill rolls around) is something we call "Card Roulette". Basically, everyone takes out a credit card, and places it on top of the check face-down. When the waiter or waitress comes back to collect, we ask them to pick a card from the cards presented - and whoever it is that gets their card picked pays the full bill.

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


Even easier: if the total is less than $50, it is a $10 tip. If the total is greater than $50, but less than $100, it is a $20 tip. If it is greater than $100, do the math.

Thursday, Dec 30, 2010, 11:30 AM

Judah Himango:

I usually give 15% tips.

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


I divide by 7... 1/7 = 15/105 ~ 15%
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:

I like to read the blog because it fits the wp7 screen so well. Great job!
Happy New Year!

Friday, Dec 31, 2010, 8:40 AM

Matt Katz:

I also play credit card roulette!

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


I do simple budgeting in my head. If I need say cash (yes, I still use cash) from a bank for several items or bills (yes, I still pay bills with cash) then I perform the mental exercise of adding it all up in my head and then checking it in my head.

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


Haha! This post really made my day. With all the gadgets in my phone, I hardly use mental computations as a basis for doing math work. But doing something like calculating for tips... I let my husband do that job for me. He's the tip calculator in the family. - Janet www.theleasingcentre.com.au

Thursday, Mar 17, 2011, 8:17 PM


I think the biggest factor in so many programs like this is it's a persons FIRST program for say the environment (e.g. Windows Phone) and they want to get an understanding. I know for one I would write one but would only want it for my own use as it let's me learn the basic concepts. If I could deploy only to my own phone it would be great but you do need to deploy to the market place and being you have to do that I feel people release it to everyone to get feedback, their names out there, etc.

Monday, Apr 4, 2011, 6:21 AM


I just don’t get the concept of way you would a tip for bad service. However I tend to think of a tip as a bribe for someone to what their job. I don’t understand why the USA that complains so match about bribes in the rest of the world, put up with a system when you will not get served next time if you don’t pay the retrospective bribe.

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


This is stupid. The quickest way to tip is to just tell them to keep the change.

Monday, Jan 9, 2012, 7:02 AM

Chris Sells:

That doesn't work if you're using a credit card, but thanks hello for the thoughtful feedback. : )

Monday, Jan 9, 2012, 9:53 AM

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