"Loading more" is not the same as "loading correctly". Behind every physical limit in container loading sits engineering experience — and the real cost of getting it wrong is a refused insurance claim, a container that rolls over, or a shipment returned at destination. This page sets the limits out: why they exist, what to check before loading, and what happens when you exceed them.
Loading a container is not a matter of filling the space and stopping. Weight stacking, the position of the center of gravity, carton compression strength and the clearance you leave all decide whether a load arrives safely — miss any one of them and you risk the container being held, the load being re-stuffed, or the whole shipment becoming a write-off.
Heavy-on-light is the most basic physical rule in container loading: a heavy carton must not be placed on top of a light one. Do it the other way round and the lower cartons are crushed under long-term static pressure, and the whole container of cargo is written off.
LoadExpert turns heavy-on-light into a built-in constraint and checks every upper-to-lower stack against the packing list: the total weight of the upper cartons must not exceed what the layer below can carry. It applies in both rule-based layout and full optimization — the two differ only in how cartons are arranged within a layer, not in this criterion.
In the app the tightness of this rule is set with a tolerance percentage: 0% = strict (a light carton may never be under a heavier one), 100% = no limit, and an intermediate value is a weight allowance. The loose-carton page defaults to 30% and the pallet-loading page to 50% (the latter only appears with "pallets required" plus multiple carton sizes). Light, bulky cargo (garments, foam) can be relaxed a little; cartons of one size and one weight can be tightened to close to strict; on a mixed-SKU container, keep the heavy-cargo zone strict so the lower layer is not crushed.
A container rolls and vibrates on a vessel and on a train. If the center of gravity sits off the container's geometric center, vessel stability drops and the container can be refused at the terminal. In the worst case a whole stack of containers topples in a chain.
LoadExpert checks the container's center of gravity automatically, against three criteria: the offset from the container floor center is no more than ±10% lengthwise and no more than ±10% widthwise, and the center-of-gravity height is no more than 60% of the internal height. A plan that goes out of range is flagged as a warning, prompting you to redistribute the cargo rather than ship a container at risk of one-sided-load rollover.
The industry guidelines above are not applied identically by every carrier; LoadExpert uses the three thresholds set out here rather than matching the list item by item — when in doubt, your own contract or the carrier's requirement governs.
CoG too far forward → move the heavy cargo toward the rear of the container. CoG too high → heavy cargo on the floor, light bulky cargo on top. CoG off to one side → place identical carton types symmetrically.
An upper carton has to land on the supporting surface of the cartons or pallet below. Support ratio = the area where the base of the upper carton overlaps the surface below, divided by the base area of the upper carton.
LoadExpert checks the support ratio of every upper carton — only enough of the base resting on the layer below passes, which is what stops a small carton sitting in mid-air and collapsing. The threshold is built in and there is no switch in the interface: loose carton loading uses 80% and pallet mixed loading uses 90% (a palletized stack is more prone to collapse, so the test is stricter). The threshold cannot be adjusted per shipment; if you need something stricter, the only way is to lay the carton dimensions out more conservatively yourself.
Below 70% support, upper cartons under heavy pressure slide off the collapsing edge of the layer below — the classic topple. The usual protection in a warehouse is to fill the unsupported gap with foam board or airbags (inflatable dunnage).
BCT (Box Compression Test) is the compression strength of a carton, in kg. From the BCT you can work out how many layers of the same goods one carton can carry at most.
| Carton type | Typical BCT | Where it suits |
|---|---|---|
| 5-ply corrugated large carton | 200–400 kg 441–882 lb |
Home appliances, machinery outer cartons |
| 3-ply corrugated medium carton | 60–150 kg 132–331 lb |
Electronics, general merchandise |
| Single-wall small carton | 20–50 kg 44–110 lb |
Food, light manufactured goods |
| Honeycomb paperboard | 100–300 kg 220–661 lb |
Furniture, glass outer cartons |
BCT is measured in a temperature- and humidity-controlled laboratory, and the high humidity of an ocean voyage takes 30–50% off a carton's compression strength. So a layer count worked out from BCT is a ceiling only, and the real stack comes down a notch — be especially conservative on tropical and refrigerated routes; cartons collapsing from damp cost far more than the few cartons you left out.
Many customers specify a maximum stacking layer count in the carton specification (for example "this carton stacks no more than 5 high"), usually an engineering conservative figure from the carton factory based on BCT testing.
The "stack no more than N high" figure from a carton factory is an engineering conservative value based on BCT. A high-utilization plan can stack 7–8 cartons in a single position, so if that exceeds the layer count promised for the carton, confirm the ceiling before the plan goes out — do not discover it after the container is loaded.
Every container type has a maximum payload limit: 20GP 28,200 kg (62,170 lb) / 40GP 26,600 kg (58,640 lb) / 40HQ 26,500 kg (58,420 lb) / 45HQ 27,600 kg (60,850 lb). LoadExpert totals the weight for you and warns on an overload with the actual number of kilos over.
The clear opening of the door frame is lower than the internal height, so a carton taller than the door height will not pass through the door (turning it diagonally depends on how it is actually handled — do not gamble on it): a 40GP door is 228 cm (89.8 in) high and a 40HQ door 258 cm (101.6 in). In loose carton loading LoadExpert compares the height of a single carton against the door height once, and flags it clearly as "a risk at the door" if it is over. Reefer doors are lower still (about 215 cm / 84.6 in) — because of the insulation and temperature-control structure, reefer loading has to be worked out as a dedicated refrigerated plan and cannot simply reuse the result for a dry container.
The total dimensions of all cartons (length × width × height) have to fit inside the container's internal dimensions, and you have to keep a little tolerance back yourself, so that there is room to reach in during handling and the rubber door seal is not crushed. That clearance is yours to reserve when you enter the carton dimensions — LoadExpert lays the cartons out against the sizes you enter and the standard internal dimensions of the container type, and does not deduct anything on top.
Carton dimensions expand with changes in humidity and temperature. An ocean voyage crosses several climate zones — from the humid heat of Ningbo to the dry cold of Hamburg — and cartons can expand by 2–4%.
Between the container wall and the cargo you normally leave about 2–4 cm (0.8–1.6 in) of reach / pallet-pull clearance: if the cargo is pressed hard against the wall, a forklift may not be able to pull the pallet out at unloading. This gap is yours to leave as well — either enter slightly larger carton dimensions, or leave one carton out of the innermost row as experience suggests. The pallet side has its own handling allowance (10 cm / 3.9 in by default), which keeps the total pallet height clear of the door.
LoadExpert works out the loading plan only, it does not lash the load. Securing a real shipment is done by the warehouse operators, and there are four common methods:
| Method | Material | Where it suits |
|---|---|---|
| Stretch film | PE stretch film, 2–3 layers | Cartons on pallets in general — the most common method |
| PET strapping | Plastic strap plus metal buckle | Heavy palletized cargo, paper reels |
| Inflatable dunnage | Airbags wedged between cartons | Filling the space left over, stopping side-to-side movement |
| Wood bracing | Timber plus wooden wedges | Heavy or bulky cargo, open top containers |
At least 2 horizontal and 2 longitudinal straps; stretch wrap goes on thick at the bottom and thin at the top — 3–4 layers at the base, decreasing upward; for heavy cargo fill the four corners with airbags to stop it sliding. LoadExpert does not work out the lashing, but it does use space-conflict detection to show where airbag filling is needed.
With the constraints clear, try LoadExpert free — every loading constraint ships with an industry default, so a workable plan comes straight out of the box. Or request a full account to unlock mixed carton sizes, pallet loading and multi-container allocation.
The free plan covers single-SKU container capacity. A full account unlocks all five carton-loading modes, pallet loading (per-pallet capacity, pallets required, four container modes), rule-based and optimized layouts, custom pallet sizes, custom payload limits, calculation history, the 3D loading view and shareable results. Leave a contact and we will set up your account within one business day.