In shipping, the stowage factor indicates how many cubic metres of space one metric tonne (or cubic feet of space one long ton) of a particular type of cargo occupies in a hold of a cargo ship. It is calculated as the ratio of the stowage space required under normal conditions, including the stowage losses caused by the means of transportation and packaging, to the weight of the cargo. The stowage factor can be used in ship design and as a reference to evaluate the efficiency of use of the cargo space on a ship.
The stowage factor varies from one type of commodity to another — for example iron ore has a stowage factor of 0.40 (m3/mt), meaning that the space needed by one tonne of ore is only one sixth of that required to stow one tonne of woodchips that have a stowage factor of 2.5. This means that if a ship designed to carry woodchips is loaded with iron ore, only a small part of the hold capacity can be utilized, and a bulk carrier designed to carry iron ore cannot be loaded to the maximum draft with woodchips, leaving much of its deadweight tonnage unutilized. Thus the stowage factor is taken into account in ship design when determining the size of cargo holds, and specialized ships such as ore carriers and car carriers are built for cargoes with a stowage factor that departs significantly from the average.