The invention provides methods and apparatus, including computer program products, for of determining a possible lot size of units with respect to a fixed date for a chain of at least two process steps, each process step requiring a respective assigned resource, and consuming a respective time per unit for being performed by the respective assigned resource, where the process steps are sequentially dependent on each other. This is achieved by the following: (a) determining, for each resource, a respective time interval of availability during which the respective resource is continuously available, each time interval being limited by the fixed date, (b) calculating, for each determined time interval of availability, a respective largest possible number of times the respective process step can be performed by the respective resource, and (c) selecting the lot size of units to the minimum of the largest possible numbers.