We quantify the normally unobservable forces that determine the boundary of the firm; that is, which transactions are mediated by ownership control as opposed to contracts or markets. To do so, we examine the shipment decisions of tens of thousands of establishments that produce and distribute a variety of products throughout the goods-producing sector. We examine how a firm’s willingness to ship over distance varies with whether the recipient is owned by the firm. Because shipping costs increase with distance for many reasons, a greater volume of internal transactions at any given distance reveals the size of the firm’s perceived net cost advantage of internal transactions. We find that the firm boundary is notably wide. Having one more vertically integrated downstream establishment in a location has the same effect on transaction volumes to that location as does a 40 percent reduction in distance between sender and destination. We further characterize how this “internal advantage” varies with observable attributes of the transaction or product being shipped. Finally, we conduct a calibration of a multi-sector general equilibrium trade model and find that costs associated with transacting across firm boundaries also have discernible economy-wide implications.