Production planning is an important activity in the company, production planning at PT. WDY experienced several problems, namely the company tended to use intuition or experience in determining lot sizes and sometimes ordered only as needed. This has resulted in several problems such as increased costs, shortage of raw materials, etc. Therefore, it is necessary to improve the lot determination process. The improvement process starts from forecasting, where the forecasting method chosen based on the smallest error rate of 19.46% is the winter method. Then proceed with the manufacture of MPS, checking capacity using the gross capacity method, calculating the number of lots and calculating the capacity of each machine. The results of the calculation of factory capacity show that the factory capacity is larger so that MPS can be implemented. The method of calculating the number of lots selected is the Wagner within method with a cost reduction of 14.24%. In the calculation of machine capacity, it is found that each machine has a capacity greater than what is needed, which means that each machine is able to meet consumer demand