Category: Finance


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.  It 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.

The previous Advanced Tip Calculator is simply an extracted version from the Advanced Financial Calculator. It doesn’t have a good usability. So I redesigned the tip calculator over the weekend and release it as donationware. There are both free-ads-support version “Tip Calculator Donation Free” and paid version “Tip Calculator Donation”. The ads revenue and sales revenue from both  apps will be COMPLETELY donated to some non-profit/charity organizations to help people in need (such as fighting with AIDS or cancer, post-catastrophe reliefs, helping people in poor countries). And I will post receipts with the amount when it happens.

Advanced Financial Calculator is a collection of useful calculators to help you making important financial decisions. Talking about complex financial calculations, usability is on top of our mind. Instead of asking people to input a lot of numbers, we tried our best to keep the required inputs minimum and do the complex calculation for you automatically. There are settings that you input just once and shared by all the calculators when needed. You can also save any calculations and bring it up anytime later. Want to share ther result with other people? No problem. Email/SMS features are right there waiting for you.
Below are the list of calculators available while we are still actively adding more.
  1. Mortgage Payment
  2. Rental ROI
  3. Loan Affordability
  4. Refinance or not
  5. Compare Auto Loans
  6. Rebate or Dealer Special Financing
  7. Lease or Purchase
  8. Pay Off Loan
  9. Accelerated Loan Pay Off
  10. Compare Loans with Points
  11. House Affordability
  12. CD Analyzer
  13. Saving Analyzer
  14. APR to APY Converter
  15. Tip Calculator
  16. Historical Inflation
  17. Tax Bracket
  18. Wage to Salary Converter
  19. Salary to Wage Converter

See below for some screenshots.

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