This research is the development of a forecast model according to the needs of
PT. Wijaya Karya (WIKA) to support the improvement of the steel price
forecasting and analysis process. The purpose of the forecast model is to create
efficiency and effectiveness as well as accountability in the inventory system
through electronic media. The development of the forecast model uses the Fourier
series method to predict steel prices by including the influence of billet prices and
the Mid BI rate. To measure the performance of forecasting, the Mean Absolute
Percentage Error (MAPE) is used and the MAPE value is 0.48%. Based on these
results, it is proven that statistically the model developed in this study is suitable
to describe the pattern of fluctuations in steel prices.