Container Weight Limits & SOLAS VGM Explained

Volume fills a container; weight closes it. Every ISO container has a maximum gross mass, and international law requires you to declare a verified weight before your box is loaded on a vessel. Getting this wrong doesn’t mean a fee — it can mean your container simply does not sail.

Payload limits by container

Maximum gross mass (container + cargo) is 30,480 kg for both 20ft and 40ft standard boxes. Because the 40ft box weighs more empty (~3,750 kg tare vs ~2,300 kg), the 20ft actually carries more cargo: roughly 28,180 kg payload vs 26,680 kg. Dense cargo — tiles, stone, metal parts, liquids — regularly hits the weight ceiling with the container half empty by volume. That is why the calculator computes containers needed by volume and by weight separately and takes the worse of the two.

What SOLAS VGM requires

Since July 2016, the SOLAS convention requires shippers to provide a Verified Gross Mass for every packed export container before vessel loading. Two methods are permitted:

• Method 1: weigh the entire packed, sealed container on calibrated equipment.
• Method 2: weigh all cargo, dunnage and packing material and add the container’s tare weight (printed on its door), using a certified process.

No VGM, no loading — terminals will hold the container, and storage/demurrage accrues while you fix the paperwork. Your forwarder usually files the VGM, but the legal responsibility sits with the shipper named on the bill of lading.

Don’t forget the road leg

A container that is legal on the water can be illegal on a truck. Road weight limits in many countries cap the practical cargo weight below the container’s theoretical payload (rules differ by country and axle configuration). If your calculation shows you close to the payload ceiling, confirm the trucking limit on both the origin and destination legs before you book — overweight fines and forced repacking at the port are expensive ways to learn this.

What SOLAS VGM actually requires of you

Since July 2016, the international SOLAS convention has made it a legal requirement that a Verified Gross Mass is submitted for every packed export container before it may be loaded onto a vessel. “Verified” is the operative word: an estimate is not acceptable. The rule was introduced after a series of accidents caused by mis-declared container weights — collapsed stacks, damaged vessels, and in the worst cases lost ships. The responsibility sits legally with the shipper named on the bill of lading, even when a freight forwarder files the declaration on your behalf.

The two permitted methods

Method 1: weigh the entire packed, sealed container on calibrated weighbridge equipment. Simple, accurate, and the default at most modern terminals.

Method 2: weigh all the individual items — cargo, pallets, dunnage, packing materials — and add the container’s tare weight, which is stencilled on the container door. This method must follow a certified procedure approved by the relevant national authority. It suits shippers who pack their own containers and know their cargo weights precisely.

Whichever method you use, the VGM must reach the carrier and terminal by the cut-off time stated on your booking. Miss it and the container is not loaded — it sits at the terminal accruing storage and demurrage while you scramble to fix paperwork, and your cargo rolls to the next sailing.

Payload limits are not the same as gross mass

Two numbers get confused constantly. Maximum gross mass is the total the container may weigh, cargo plus the container itself — 30,480 kg for most standard boxes. Payload is what is left for cargo after subtracting the container’s tare weight: roughly 28,180 kg in a 20ft and 26,680 kg in a 40ft. When you plan a load, you care about payload. The calculator checks your total cargo weight against payload for each container type and flags an overweight configuration before it becomes a problem at the port.

The road leg can be stricter than the sea leg

A container that is perfectly legal on the water can be illegal on a truck. Road weight limits vary by country and by axle configuration, and in many jurisdictions they cap the practical cargo weight well below the container’s theoretical maritime payload. A 40ft container loaded to 26 tonnes may be fine at sea but require a special permit — or be flatly illegal — on the road at origin or destination. If your calculation puts you near the payload ceiling, confirm the trucking limits on both ends before you book. Overweight fines and forced repacking at the port are expensive ways to learn this rule.

Practical takeaways

• Always know your VGM before the booking cut-off — treat it as a hard deadline, not a formality.
• For dense cargo, expect the weight limit to bind before the volume limit; plan container count on weight.
• Keep the container tare weight from the door for your records if you use Method 2.
• Check road weight limits at both ends when you are within a couple of tonnes of payload.