How often have you left a tip that you later realized wasn't what you desired to give? Well, I guess from personal experiences, it does happen, as sometimes you miss the calculations a bit. That's why these days you would see a bill (or check which is the more common word for it here in USA) often with tip amounts suggested for our convenience. One could always use the calculator app on their phone to compute the right amount but I think I am yet to come across someone who has done that. And I don't know the reason why we consider using our smartphones to take pictures of food we eat is cool but it's not cool to use it to get the tip amount right. Although, it's the latter that makes a real difference in someone's day (or earnings).
Anyways, so, I googled if there are any apps for it, and surprise! You don't need an app when you got Google 🙂