Performing even relatively simple conversion calculations such as from pounds to kilograms from memory can be fairly difficult, let alone calculating more esoteric conversions such as from square meters to acres. To automate this conversion process, I used the Zend Framework to create a calculator capable of easily migrating data among the most commonly used formats.
Zend_Measure lets you define the type you want to convert from and convert to based on some constants included with the component. A simple call to the "convertTo" method and you'll get your answer. Based on this he creates a simple calculator and shows you can can integrate roman numeral support in as well.