What is Cargo Density?
Cargo Density is a function of weight over volume.
Although conventionally speaking, weight and mass are used interchangeably, it is scientifically different. Nevertheless, understanding cargo density doesn’t require a logistician to be a scientist.
For this article, we will use weight or mass to indicate an object’s “heaviness”, measured in kilograms or pounds.
Volume, roughly described, determines how much space an object occupies. The larger the volume, the larger space the object occupies. Volume is internationally measured in cubic meters.
There are other permutations to measure volume, such as liters, ounce, gallon, quart, pint, cubic inch, barrels, etc… It all depends on the specific industry that imports/exports the cargo, and also the nature of the cargo transported.
To determine the cargo’s density, in layman terms, is to determine how much space is occupied for one-unit weight measured.
As we all know, in logistics and transportation, knowing the transported cargo’s weight and volume is very important. It helps logisticians decide what mode of transportation to use, and more crucially, ensures that the safety of the hands-on transporters does not endanger their lives by transporting cargos that exceed the capacity of the transportation mode.
What is a High Density Cargo?
Now we have understood what is cargo density, we need to be able to understand when cargo can be categorized as high density or as low density.
To do that we need a reference. Water has a density of 1 gram per cubic centimeter, or 1 kg per cubic meter, whichever the measurement is.
We can intepretate that density measurement as: – for every 1 gram of water, it occupies 1 cubic centimeter of space.
So, we can say that a high density cargo, relatively speaking, has a density of more than 1000 kg/m3.
An example of a high density cargo is any ferrous/non-ferrous cargo such as Iron, Steel, and Magnesium.
Perhaps we can learn from Archimedes if we want to conceptualize what cargo density is. Archimedes exclaimed “Eureka!” when he finally discovered that the volume of water displaced equals the volume of the object placed in water.
For a high density cargo, if placed in a water, it would sink as the object has a higher density than water.
What is a Low Density Cargo?
Low-density cargo, evidently, is the opposite of high-density cargo. The weight of the cargo is relatively low concerning the volume of the cargo.
Again, using H2O as a proxy, any cargo that has a density measurement that is lower than 1000 kg/m3, can be considered as low-density cargo.
Cotton is a low density cargo, it floats on water since water is more dense than cotton.
Low Density Cargos are, a supply chain’s nightmare. it requires a relatively larger space to transport a unit weight of cargo.
To compare the difference between high density and low-density cargo. Let’s use a case example of transporting 1 ton Cotton and 1 ton Steel.
Density of Cotton: 405 kg/m3
Density of Steel: 8050 kg/m3
To calculate exactly how much space is required to transport 1 ton worth of cotton, we divide 1,000kgs with 405 kg/m3 to derive the cubic meter space required, the same applies to steel cargos.
Cubic meter space required for Cotton: 1000kg / 405 kg/m3 = 2.47 m3
Cubic meter space required for Steel: 1000kg / 8050 kg/m3 = 0.124 m3
In theory, to transport the same weight value of the cargo, a transporter is required to allocate almost 20 times more space for cotton as compared to steel.
Practical application of cargo density on freight
Intermodal transportation may some alien to some, but it is a simple concept. As long as the cargo transported can be easily conveyed from one mode of transport to another, the arrangement is termed intermodal transportation.
The most common mode of intermodal transportation is the Land-Sea-Land mix, with the use of ISO containers.
We won’t be explaining in-depth on the contributions of the invention of intermodal containers, but we need to emphasize that the use of cargo containers has changed the way cargos are transported, with transportation now faster, more efficient, and more cost-effective.
Cargo density on intermodal transportation is not a significant factor that supply chain operators pay attention to. This is because transporters or logisticians using intermodal containers are bound by the limitations of the containers themselves.
Therefore, understanding the limitations of a cargo container is important for any transport arrangement.
|Container Type||20’ Container|
|Length x Width x Height (Internal Dimensions) in m||5.89m x 2.35m x 2.36m|
|Length x Width x Height (Internal Dimensions) in ft||19.3ft x 7.7ft x 7.74ft|
|Capacity (in kg or m3)||21,700 kgs or 33m3|
|Capacity (in lbs or ft3)||47,840 lbs or 1130 ft3|
|Container Type||40’ Container|
|Length x Width x Height (Internal Dimensions) in m||12.05m x 2.35m x 2.36m|
|Length x Width x Height (Internal Dimensions) in ft||39.5ft x 7.7ft x 7.74ft|
|Capacity (in kg or m3)||26,500 kgs or 66m3|
|Capacity (in lbs or ft3)||58,423 lbs or 2,354ft3|
It is worth pointing out that, although a 40’ container is twice the size in cubic capacity, its maximum weight capacity only increased by 22%.
In other words, cargos that are denser is more suitable for 20’ container loading, whereas cargos that are less dense is more suitable for 40’ container loading.
These are guidelines when performing transportation arrangement, it should be used as a general guide, not as a definite rule. A transporter must always refer to the cargo’s actual dimensions and its weight before deciding on which containers to use.
Of course, there is also an element of cost as well, a 40’ container generally has a higher ocean freight rate compared to a 20’ container, depending on the container’s loading point and delivery point.
Bulk Cargo Transportation
The stowage factor is essentially the same measurement as cargo density. It measures how much space occupies the space of one long ton.
Before moving forward, we believe it is pertinent to highlight that a ton can be measured in three distinct ways.
A long ton, or the UK ton is measured at 1,016 kgs per long ton.
A short ton, or the US ton is measured at 907 kgs per short ton.
A tonne, an international system of unit is measured at 1,000 kgs per tonne.
So, when we are referring to a ton in mass weight, I can range between 907 kgs to 1,016 kgs. So, we have to be careful to make sure to collect the right information. Especially when global trade involves multiple countries at a time. Conventionally, it is inferred that the international system of unit is the standard used.
You have recently chartered a Handymax bulk grain carrier capable of carrying 50,000 DWT.
Deadweight tonnage = how much absolute weight the vessel can carry including crew, ballasts, fuel and provision.
A DWT is to be distinguished from the Deadweight Cargo Capacity (DWCC), which measures the actual cargo weight the vessel is capable of carrying.
You concluded that the vessel’s net tonnage is actually 40,000 m3.
Net tonnage is the total space available to vessels to transport cargos.
Grain’s Stowage factor is calculated as 1.27 m3 per metric tonne. Meaning to say, the cargo has a lower overall density therefore require more space to store per metric tonne.
With that in mind, you concluded that the recently chartered vessel is capable of carrying
40,000m3 / 1.27 m3 per MT = 31,496 kgs of grain.
As this example suggests, although the net tonnage of the vessel is 40,000m3, you can realistically only carry 31,496kgs of grain as the stowage factor of grains are higher.
A cargo with higher stowage factor is equivalent to a cargo that is low in density. As an example, Iron ore has a stowage factor of 0.4 m3 per MT.
Physics is a universal law of nature that supply chain and logistics are bound by. Hopefully, this write up about cargo density can help you demystify what cargo density is.