The centre of gravity (CoG) is the point where the whole weight of the cargo can be considered to act. In a container, the cargo CoG should sit close to the middle lengthwise and across, and low. Widely used guidance keeps the lengthwise CoG within about ±5% of the inside length from the midpoint. This guide explains why, how to calculate it and what it means for the truck.
Why centre of gravity and weight distribution matter
Cargo bunched at one end of a container causes several problems:
- Unbalanced lifts. Crane spreaders and reach stackers lift from the four corners. An end-heavy container tilts in the air and loads the equipment unevenly.
- Uneven truck axle loads. One trailer axle can be overloaded even when the total weight is within limits; see truck axle load and GVW.
- Structural stress. Floors and frames are designed for loads that are spread out.
- Reduced vehicle stability. A high CoG that is also off-centre raises the risk of rollover in bends.
On the vessel, planners use the VGM and the stowage position to calculate stability. They generally assume the container's CoG is central, so a badly end-heavy box makes those numbers less reliable. See also what is a stowage plan.
Three directions of weight distribution
Lengthwise (front to back). The common guideline keeps the lengthwise CoG within about ±5% of the inside length from the midpoint. It is not a single official limit; refer to the CTU Code and each carrier's packing guidelines.
| Container | Inside length | Midpoint | ±5% | CoG window from front wall |
|---|---|---|---|---|
| 20GP | 5,898 mm | 2,949 mm | ±295 mm | 2,654–3,244 mm |
| 40GP / 40HC | 12,032 mm | 6,016 mm | ±602 mm | 5,414–6,618 mm |
| 45HC | 13,556 mm | 6,778 mm | ±678 mm | 6,100–7,456 mm |
Across (left to right). Keep the CoG on the centreline. Balance a heavy item on one side with cargo on the other, or place it centrally with blocking to both walls.
Vertical. Heavy at the bottom, light on top. A low CoG reduces the rollover risk for the truck and the forces on lashings. Strong packages support weaker ones, never the other way round.
How to calculate the combined centre of gravity
The method is a sum of moments:
- Choose a reference, for example the front wall of the container (the end away from the doors) as x = 0.
- For each cargo block, note its weight (kg) and the distance of its own CoG from the reference (m). For a block with evenly spread weight, that is the middle of its length.
- Multiply weight × distance to get the moment (kg·m).
- Add up all moments and divide by the total weight.
CoG = Σ(weight × distance) / Σ weight. The same method works across the width and vertically.
Worked example: four cargo blocks in a 40GP
An exporter in Surabaya loads four groups of goods into a 40GP. Distances are from the front wall.
| Block | Position | Weight | Block CoG | Moment |
|---|---|---|---|---|
| A (crated machine) | 0.0–3.0 m | 6,000 kg | 1.5 m | 9,000 kg·m |
| B (spare-part cartons) | 3.0–6.0 m | 4,000 kg | 4.5 m | 18,000 kg·m |
| C (cable drums) | 6.0–9.6 m | 5,000 kg | 7.8 m | 39,000 kg·m |
| D (light cartons) | 9.6–12.0 m | 2,000 kg | 10.8 m | 21,600 kg·m |
| Total | 17,000 kg | 87,600 kg·m |
CoG = 87,600 / 17,000 = 5.15 m from the front wall. The 40GP midpoint is at 6.016 m, so the CoG sits about 0.86 m forward, outside the ±0.6 m guideline window (5.414–6.618 m).
Fix: swap blocks A and B (both are 3.0 m long). Block B now centres at 1.5 m and A at 4.5 m:
- New moment sum: 4,000 × 1.5 + 6,000 × 4.5 + 39,000 + 21,600 = 93,600 kg·m.
- CoG = 93,600 / 17,000 = 5.51 m, about 0.51 m from the midpoint. Within the guideline.
At 17,000 kg the container is well below its 26,730 kg payload, yet an end-heavy stow could still load the trailer axles unevenly. These results are a simulation: the official weight still comes from the VGM, and vessel stability remains the job of the ship's planners and loading computer.
Centre of gravity on a truck
On a truck, the position of the cargo CoG relative to the axles decides how much weight each axle carries. Too far forward overloads the steer axle and makes steering heavy; too far back unloads the front axle and the vehicle becomes hard to control. The split follows the same moment principle, taken about the axle positions.
A simple example with assumed numbers: a two-axle truck with a 4.0 m wheelbase carries 5,000 kg of cargo. With the cargo CoG 2.6 m behind the front axle, the rear axle takes 5,000 × 2.6 / 4.0 = 3,250 kg and the front axle 1,750 kg (cargo only, before vehicle weight). If the cargo shifts so its CoG is 3.4 m behind the front axle, the rear axle takes 4,250 kg and the front only 750 kg. Same total, but an extra 1,000 kg on the rear axle. On a truck already near its limit, that decides whether it passes the weighbridge.
Check the result with the Axle Load Calculator and compare it with the permitted axle load for the road class on your route. To pick a truck that suits the cargo weight and dimensions, use the Truck Finder, and replace catalogue figures with the actual vehicle's KIR test book or registration data.
Practical tips for stuffing
- Follow a stuffing sequence from the plan, not the order in which goods reach the warehouse.
- Mark the container's midpoint on the floor with chalk or tape as a quick reference.
- Place the heaviest items around the middle, then fill both ends with lighter cargo.
- Record the actual weight of each pallet or crate; packing-list weights are often estimates.
Common mistakes
- Loading heavy goods first at the front end just because of stuffing order, then filling the rest with light cargo.
- Checking only the total weight, without checking where the CoG is.
- Stacking heavy items on top of light ones, raising the CoG and crushing the lower packages.
- Heavy cargo on one side with no counterweight or blocking.
- Ignoring concentrated loads; heavy machinery needs spreader beams, see the flat rack container guide.
- Not securing the cargo, so the CoG moves during the trip; read cargo lashing and securing.
Check the centre of gravity in the planner
The 3D Load Planner shows the combined CoG and flags it when it falls outside the guideline, so you can swap blocks before stuffing. The planner is a simulation and does not replace official weighing, the VGM, the vessel's loading computer or operator approval.