Tag Archive: tip calculator

Admit it, as a developer, we all assume tip calculator is simple. There are probably hundreds of tip calculator apps on Android and iPhone. We made our tip calculator app simply because it is a good app to start. We have a new design of this app a couple of months ago. ┬áIt looks cool and very functional. I thought we are done with it, until one day, a guy named Sean left a comment on our app, saying “Can’t add 9.95+1.00(=10.94?) 10% tip”. I didn’t really pay much attention until I grab my tipcal app and did exactly what Sean said. I entered 9.95 as the bill, adjusted tip percentage to 10%, the app shows $1 as tip but the total is 10.94 which is wrong.

My first thought is it is a rounding error. After debugging, I realized it is not the case. 10% of 9.95 is 0.995, so total should be 9.95 + 0.995 = 10.945. If it rounds, it should still be 10.95, not 10.94. It turned out the root reason is when doing 9.95 + 0.995 in computer word, the result is 10.944999999999999. After rounding it becomes 10.94, not 10.95 as expected.

I downloaded a few other popular tip cal apps, on Android and on iPhone. To my surprise, all of them have the same error. See below. It is very common for developers to do it this way. We were trained to keep the precision at the maximum during the intermediate steps and only round at the final step. That’s why all apps have the same error. However from user point of view, 9.95 + 1 = 10.94 is wrong. Even it is just one cent difference, developers have no excuse for it.

And here is our tip cal app after fixing the bug. Thanks Sean.


We changed the graphics for the Tip Calculator and post a new update for it a while ago. Here is the new design. Hope you like it better than the old one.

We also decided to remove the donation tag from the tip calculator. The reason is it doesn’t sell very well as there are way too many tip calculators out there on the market. This one cannot get enough attention. We made probably less than $50 from this tip calculator so there is really no point keeping it as donateware. But as promised, we will donate in good cause when needed.

Looks like there are tons of tip calculators on the android market. Nevertheless since we already did it as part of Advanced Financial Calculator, we extracted it out and made it a free app so that it is available to everyone. What we like about it is that in addition to the tip splitting and rounding features that other tip apps have, it also can save the tips into history and bring it back any time. All previous inputs are remembered so next time to find out the tip, all you need to do is to input the bill total, simple as that. You can even email it to your own email address to keep a record or to your friend to remind he/she owes you some money since you pay the bill this time. It kind of changes from a simple tipping app to a prefect application to keep track of all the dinning expenses. You may want keep an eye on it as you might know already, dinning expense could be huge sometimes.

Here are some screenshots.