Calc Plus! 1.8
the author of this app says:
This calculator is designed for mobile professionals to make all types of basic and advanced calculations. One of its main characteristics is the availability of four independent design layouts for different types of calculations: standard, unit conversions, base conversion and statistics.
You can enter the whole expression, including brackets, functions and operators. You can always return to an expression you calculated before, change it and recalculate it.
It also converts more than 100 different units in 11 categories, and users can define new ones.
If you buy this program, you get unlimited technical support plus all future versions free of charge.