RPEngine is an RPN (Postfix) based calculator app for your iPad with built-in exponential, trigonometric, hyperbolic and statistical operations. RPN calculators use a stack to hold numbers. Operations are performed on the numbers stored in the stack in a last-in first-out order allowing you to keep intermediate steps stored while performing multi-step calculations. This allows arbitrarily complex calculations to be easily and quickly entered and performed. RPEngine has a built-in unit converter for quickly performing unit conversions.