We have developed two XML Schemas to support the implementation of calculations within XML-based order sets. The models support the representation of variable-based algorithms and include data elements designed to support ancillary functions such as input range checking, rounding, and minimum/maximum value constraints. Two clinicians successfully authored 57 unique calculated orders derived from a set of 11 calculations using the models within our authoring environment. The resultant knowledge base content was subsequently tested and found to produce the desired results within the electronic physician order entry environment.