The procedures recommended by API  for the interconversion of ASTM D 86 and TBP (True Boiling Point) distillation curves are transposed into Mathcad computation platform. Two Mathcad versions are presented for each of direct, ASTM to TBP, and the backward, TBP to ASTM, curves conversions according to the API procedure. The first version, simpler, has straightforward calculations, and the second one contains the relations in the Mathcad worksheet written as functions, in a sequence which finally allows its call as a single variable function. The Mathcad worksheets comprising the API procedures are thus providing a convenient tool for fast prediction of a distillation curve starting from the other one. Several examples of distillation data conversion, which in the case of herein presented version are included in the Mathcad worksheets with the procedures, illustrate the use of Mathcad written API methods. Beside a brief description of the API procedures, Part 1 gives the simpler version of their Mathcad implementation, while function containing version will be given in the second part.