Ask any dry bulk operator what decides how much cargo actually fits on a ship, and the answer is rarely just deadweight. It is stowage factor: the relationship between how much a cargo weighs and how much space it takes up. Understanding this single concept explains why two cargoes of identical tonnage can fill a vessel's holds completely differently, and why the right ship for one commodity is the wrong ship for another.
What Stowage Factor Actually Measures
Stowage factor expresses the volume a given weight of cargo occupies once loaded in a ship's hold. It is a density measure, but a practical one, since it accounts for the way a commodity actually settles, trims and packs in a cargo space rather than its theoretical density on paper.
Dense cargoes such as iron ore or other minerals pack tightly and occupy relatively little space for their weight. Lighter, bulkier commodities such as grain, wood pellets or certain fertilisers take up considerably more volume for the same tonnage. Between these extremes sit cargoes like coal, which vary depending on grade and handling, and bagged or palletised goods, which stow less efficiently than free-flowing bulk due to the air gaps between packages.
Cubing Out vs Weighing Out
Every bulk carrier has two ceilings: a weight ceiling, set by its deadweight and permissible draft, and a volume ceiling, set by the cubic capacity of its holds. A cargo 'weighs out' a vessel when the holds still have spare space but the ship has reached its maximum permitted weight, typically the case with dense cargoes such as ores and minerals.
A cargo 'cubes out' a vessel when the holds are physically full while the ship is still well short of its deadweight limit. This is the typical outcome for light, voluminous cargoes. In that situation, the vessel sails below its maximum draft, carrying less tonnage than it is theoretically capable of, simply because there is no more room left in the holds.
Why This Drives Vessel Selection
Owners and charterers use stowage factor to match cargo to ship, not just by deadweight but by the ratio of deadweight to cubic capacity, often described as the vessel's grain or bale capacity relative to its tonnage. A ship with generous cubic capacity relative to its deadweight is well suited to light, bulky cargoes, while a ship with relatively modest hold volume but strong deadweight is better suited to dense cargoes.
This is one of the reasons vessel types are not interchangeable across all trades. A Panamax built with ore and coal in mind may have a tighter cubic-to-deadweight ratio than a geared vessel designed with flexibility across commodities in mind, a distinction covered in more detail in our comparison of Handysize and Supramax geared bulk carriers. Choosing the wrong type for a light cargo means sailing with unused deadweight and a weaker freight return per voyage.
Volume vs Deadweight: Two Different Ceilings
It is tempting to think of a bulk carrier's capacity purely in terms of deadweight, since that figure appears on every vessel description. But deadweight only tells half the story. The other half, cubic capacity, determines whether that deadweight can ever actually be used on a given cargo.
This is why charterers evaluating a fixture for a light commodity will ask for grain and bale capacity figures alongside deadweight, and why brokers routinely check stowage factor before proposing tonnage. A vessel that looks generously sized on deadweight alone can still turn out to be the wrong fit once the cubic arithmetic is done.
Practical Implications for Charterers and Owners
For charterers, stowage factor affects laycan planning, cargo quantity declarations and, indirectly, freight economics, since a cargo that cubes out a vessel effectively pays for deadweight the ship never carries. For owners, it affects which trades a vessel is competitive in, and whether a ship's hold geometry, including hatch size and box-shaped holds, suits cargoes prone to cubing out.
Bagged and palletised cargoes add a further layer, since packaging introduces additional void space beyond the commodity's own density, a topic explored further in our piece on the bagged vs bulk chartering decision for sugar and cement. In every case, getting the stowage factor right at the fixture stage avoids surprises once loading begins.
Stowage factor is one of those details that rarely makes headlines but quietly decides whether a fixture works commercially. Our desks at ER Drybulk weigh it alongside deadweight, draft and hold configuration on every enquiry we handle, so that the tonnage proposed genuinely fits the cargo, not just on paper but in the hold. If you are unsure whether a cargo will cube out or weigh out a given vessel, we are glad to talk it through.