A two-level algorithm to compute blend plans that have much smaller number of different recipes, much shorter execution times, and the same cost as the corresponding multiperiod mixed-integer nonlinear programming is introduced. These plans become a starting point for computation of approximate schedules, which minimize total number of switches in blenders and swing tanks. The algorithm uses inventory pinch points to delineate time periods where optimal blend recipes are likely constant. At the first level, nonlinear blend models are optimized via nonlinear programming. The second level uses fixed recipes (from the first level) in a multiperiod mixed-integer linear programming to determine optimal production plan followed by an approximate schedule. Approximate schedules computed by the multiperiod inventory pinch algorithm in most of the case studies are slightly better than those computed by global optimizers (ANTIGONE, GloMIQO) while requiring significantly shorter execution times. Such schedules provide constraints for subsequent detailed scheduling in Part II. (c) 2014 American Institute of Chemical Engineers AIChE J, 60: 2158-2178, 2014