由 caicai8478 On the logistics chessboard of international trade, air and sea freight are two major players. They not only determine the speed of goods transportation but also conceal drastically different pricing logics. Many foreign trade professionals are well aware of the existence of “volume weight,” but may not be aware of the crucial differences in its rules between air and sea—these are precisely the “hidden rules” affecting logistics costs.
Understanding these differences means you can no longer passively accept quotes but proactively choose the most “cost-effective” mode of transport for your goods.
I. Common Logic, Different Scales: Why Does “Volume Weight” Exist?
Whether it’s air or sea freight, carriers share a common core consideration: maximizing the use of limited transportation resources and fairly pricing resource consumption.
For air freight, the scarcest resource is the aircraft’s cargo hold volume. An aircraft may not have reached its load limit, but its cargo hold is already filled with bulky, lightweight cargo.
For maritime transport, the scarcest resource is also the cubic space of the ship’s hold, although a ship’s carrying capacity is also crucial.
Therefore, both air and sea freight introduce the concept of “volumetric weight” to balance the proportion of “weight” and “volume” in costs. The real difference lies in the “scale” used to measure volume.
II. Core Difference: The “Mysterious Number” That Determines Cost—Conversion Factor The general formula for calculating volumetric weight is:
Volumetric weight (kg) = Length (cm) × Width (cm) × Height (cm) ÷ Conversion Factor
This “conversion factor” is the most fundamental “rule of difference” between air and sea freight. It represents how many kilograms of weight one cubic meter (or one cubic centimeter) of space is equivalent to.
- Air Freight’s “Strict Scale”: Conversion Factor 5000
Rule: 1 cubic meter (1,000,000 cm³) of cargo has a volumetric weight calculated as 200 kg.
(Calculation: 1,000,000 ÷ 5000 = 200 KG)
The underlying logic: Airplane space is extremely expensive and limited. An airline considers 5000 cubic centimeters of space worth 1 kilogram of cargo weight. This makes volumetric weight easily exceed actual weight, which is very disadvantageous for lightweight, bulky cargo.
- The “relaxed scale” of ocean freight: Conversion factor 6000 (or higher)
Rule: 1 cubic meter (1,000,000 cm³) of cargo has a volumetric weight calculated as 167 kilograms.
(Calculation: 1,000,000 ÷ 6000 ≈ 167 KG)
The underlying logic: Ships have relatively ample cargo space, and the cost per unit of space is lower than air freight. A shipping company considers 6000 cubic centimeters of space worth 1 kilogram of cargo weight. For some very lenient sea or rail transport routes, the conversion factor can even reach 7000 or 8000, meaning that for the same volume, the calculated volumetric weight is smaller.
III. Practical Comparison: Same Cargo, Vastly Different Fates
Let’s look at a typical case to see the huge impact of this difference in rules.
【Cargo Information】
Product: A batch of fluffy lounge chairs
Outer Carton Dimensions: 100 cm × 80 cm × 60 cm
Actual Weight: 25 KG
【Claimable Weight Calculation】
Transportation Method Conversion Factor Volumetric Weight Calculation Volumetric Weight Result Claimable Weight (Larger Principle)
International Air Freight 5000 (100×80×60) ÷ 5000 96 KG 96 KG
International Sea Freight 6000 (100×80×60) ÷ 6000 80 KG 80 KG
Conclusion Analysis:
This batch of cargo is a typical example of “bulky cargo” because its volumetric weight is much greater than its actual weight.
If you choose air freight, you will be charged for 96 kg, while the shipment itself only weighs 25 kg.
If you choose sea freight, your chargeable weight is 80 kg, a direct reduction of 16 kg compared to air freight.
More importantly, the unit price per kilogram for sea freight is usually much lower than for air freight. Therefore, choosing sea freight results in a double reduction in both “chargeable weight” and “unit price,” leading to significant cost savings.
IV. Smart Decision-Making: How to Optimize Logistics Costs by Utilizing Rule Differences?
By understanding this “unwritten rule,” you can make the smartest choice for your goods:
For “Heavy Goods” (High-Density Goods):
Characteristics: Actual weight > Volumetric weight.
Strategy: Your chargeable weight is determined by the actual weight, and rule differences have little impact. In this case, it’s more important to compare the unit price per kilogram and delivery time requirements for air freight and sea freight.
For “Vacuum Goods” (Low-Density Goods):
Characteristics: Volumetric weight > Actual weight.
Strategy: Sea freight is your only economical choice. A larger conversion factor means a lower chargeable weight. Combined with the lower unit freight rate of ocean freight, this can save you significant costs. Ocean freight should be prioritized whenever time permits.
For “Critical Goods”:
Characteristics: Actual weight is close to volumetric weight.
Strategy: Precise calculations are essential. If the chargeable weight for air freight is only slightly higher than for ocean freight, but it gains you valuable market time, then air freight may be more cost-effective.
Conclusion: From “Passive Acceptance” to “Proactive Design”
In international logistics, knowledge is money. The difference in cubic weight rules between air and sea freight is not a complex technical issue, but a crucial business choice.
Next time you arrange a shipment, don’t just ask “How much per kilogram?” Instead, first measure the container dimensions, calculate the chargeable weight for air and sea freight respectively, and then make a comprehensive judgment based on the unit freight rate and time requirements. Only in this way can you break the “unwritten rules” and transform them into your “explicit advantages,” truly achieving lean management of logistics costs.