Board Feet Calculator

Why is this so difficult to use? I made this thing for my phone, and if you are reading this you probably aren't on a phone. It doesn't use actual HTML input tags, because I didn't want the device keyboard to pop up. Why not? I made my own keypad with exactly what I need to calculate board feet.

board feet
1 2 3 ft
4 5 6 in
7 8 9 .
0 spc del /