An overweight container is one of the fastest ways to turn a profitable shipment into a port rejection, rollover, or safety incident — and the bill rarely stops at demurrage. Since SOLAS mandated Verified Gross Mass (VGM) declarations in 2016, every packed container must have a signed gross mass before it loads aboard a vessel. Yet shippers still lose time and money because payload math was done on product weight alone, ignoring pallets, dunnage, moisture barriers, and the container's own tare.
This guide explains legal gross mass limits, how VGM must be calculated and filed, where shippers most often fail, and how digital load planning with Palletizr keeps weight and cube aligned before the truck reaches the terminal gate.
Gross mass vs. payload: know both numbers
Every ISO container carries two weight references on its CSC plate:
| Reference | Meaning |
|---|---|
| Max gross (MGW) | Maximum total weight of container + cargo — often 30,480 kg on standard units |
| Tare | Empty container weight — typically 2,200–4,000 kg depending on size and type |
| Payload | MGW minus tare — the actual cargo budget |
A common mistake is quoting "28 tonnes of product" without subtracting pallet weight (~20–25 kg each), dunnage, desiccant, and packaging overhang. A 40GP with 2,800 kg tare leaves roughly 27,680 kg payload at standard MGW — but road limits in the US, EU, and many Asian corridors cap lower, often ~26,000 kg all-in for domestic drayage.
Rule: Calculate verified gross mass (container + cargo + all packing materials), not net product weight.
VGM: two approved methods
SOLAS permits only two methods for establishing VGM:
Method 1 — Weigh the packed container
The entire packed container is weighed on a certified scale (weighbridge, terminal scale, or approved portable system). This is the gold standard when cargo density is uncertain or mixed-SKU loads make calculation unreliable.
Method 2 — Weigh all contents and add tare
Sum the mass of all cargo items, pallets, dunnage, and securing materials, then add the container tare weight from the CSC plate. This method requires documented accuracy — every SKU weight must be traceable, and estimations must be conservative.
| Method | Best for | Risk |
|---|---|---|
| Weigh packed container | Heavy machinery, mixed loads, first-time lanes | Scale availability at origin |
| Sum contents + tare | Uniform palletised FMCG, repeatable SKU mixes | Underestimation → overweight at gate |
The shipper (or a party acting on the shipper's behalf) is responsible for signing the VGM declaration. Carriers will not load a container without it.
Where shippers fail — and what it costs
1. Ignoring road weight before ocean weight. A container legal at 30,480 kg MGW may be illegal on US highways at 80,000 lb gross combination weight. Split bookings or transloading may be required.
2. Stacking without crush limits. Double-stacking cartons on a pallet adds height but also concentrates mass on lower tiers. VGM Method 2 fails when lower cartons compress in transit.
3. Last-minute dunnage. Airbags, timber chocks, and lash straps added at stuffing can add 200–800 kg — enough to breach payload on tight loads.
4. Wrong tare on SOC boxes. Shipper-owned containers have non-standard tare. Using a fleet average instead of the actual CSC plate invites rejection.
5. Filing VGM before final stuffing. Any change after declaration — even one pallet swap — requires re-weigh or recalculation.
Costs of failure: container rollover fees, re-stuff charges, missed vessel, terminal fines, and in worst cases cargo shift incidents at sea.
Practical compliance workflow
- Define payload budget upfront — MGW minus tare minus planned dunnage allowance.
- Model load plan digitally — Palletizr validates weight per tier, COG, and cube before warehouse execution.
- Weigh at stuffing — If Method 2 sum is within 2% of scale weight, file; if not, re-plan.
- Photo + document — Capture scale ticket or weight calculation sheet with the packing list.
- File VGM in carrier/terminal system before CY cutoff — not at the gate.
Bottom line
VGM is not paperwork — it is operational discipline. Teams that treat weight as a load-planning input alongside cube avoid the expensive loop of gate rejection, re-stuff, and missed sailings. Model gross mass before you seal the container, not after the terminal scale says no.