Calculator? 
Seriously, what more do you want in the app specifically? If it was just for what you said then a calculator would be more than enough but I am guessing you want a lot more. Personally I can’t beat Excel as I can modify my budget targets and targets as I go as life changes and interlink it to my shares and investments etc.
Might not be an app but I don’t need it on my phone as I don’t see why I would need to constantly check it.