This thesis is centered around the computional program PASGO. PASGO is used for calculations for making performance monitoring of the steam generators in Ringhals reactor 3 and 4, especially concerning fouling of the steam generator tubes. The main goal of this thesis project was to convert the program code from the programming language Turbo Pascal 6.0 into a more modern language that works with the current Windows operative systems. Apart from a straight conversion of the code, a function to make automatic input from an Excel-file to the program was also requested. The thesis also includes analyzing the correlations used for heat transfer in PASGO, to determine if these can be replaced with more exact correlations. The Dittus-Boelter correlation was used in the original program, and while it was not a bad correlation, studies show that the more modern Gnielinski correlation has a smaller margin of error. The program code was converted successfully into C++ in Visual Studio, which was the programming language of choice. A function to enable automatic input of data from Excel-files was successfully implemented as well. This function allows the user to run performance calculations regarding tube fouling and pressure correction values for hundreds of performance tests at the same time. Versions of PASGO using the Gnielinski correlation were also created. These versions are recommended for future tests, while the version using the Dittus Boelter correlation is recommended for tests which will be compared to studies made white the previous version of PASGO.