Calculating your actual body fat via an app wouldn't be too accurate I would think.
The app would be simply taking a guess based on the numbers you feed it.
If you want your actual body fat % then the best way is to have your readings taken correctly with a skin fold test. This is done with specifically designed calipers in certain places on your body.
I took a lot of peoples body fat measurements with calipers over a decade at the gym where I worked and I can tell you that every ones body (and body fat) is different to some degree.
Thats why I think an app where you feed numbers into it to give you a percentage wouldn't work so well. Every now and then someone comes out with a new gimmick to test body fat percentage but they cant beat the skin fold test.
Of course if it's BMI that you mean then thats entirely possible.