The economic order quantity (EOQ formula) is the quantity that minimizes total annual cost by balancing ordering cost and holding cost. It equals the square root of 2 x D x S / H, with D the annual demand, S the cost of an order and H the cost of holding one unit per year. It assumes stable, known demand.
Example
Q = sqrt(2 * D * S / H) with D = 1200, S = 50, H = 6: Q = sqrt(20000), or about 141 units.