Most cargo fills a container on volume. Stone fills it on the scale, and every planning error in stone logistics comes from forgetting that. This guide is the arithmetic, and the limits that arithmetic runs into.
The densities
- Granite — 2,600–2,750 kg/m³
- Marble — 2,600–2,750 kg/m³
- Limestone — 2,400–2,700 kg/m³
- Sandstone — 2,200–2,500 kg/m³
Use the actual figure for your stone where you have it. The ranges above are for planning.
Weight per square metre
Taking granite at 2,700 kg/m³ as the working figure:
- 10 mm — 27 kg/m²
- 20 mm — 54 kg/m²
- 22 mm — 59 kg/m²
- 30 mm — 81 kg/m²
- 40 mm — 108 kg/m²
- 50 mm — 135 kg/m²
For sandstone at 2,300 kg/m³: 22 mm is about 51 kg/m², 30 mm about 69 kg/m², and 50 mm about 115 kg/m².
Then add 8–12% for packing — A-frames, crates, dunnage, protection. This is not optional in the calculation; it is real weight that goes on the same scale.
The container limits
A standard 20ft general purpose container has a maximum gross weight of typically 30,480 kg, and a tare weight of typically 2,200–2,300 kg, leaving a maximum payload of roughly 28,000–28,300 kg.
But that is the shipping line’s limit, not your limit.
The limit that actually binds: road weight
The number that stops you is almost never the container’s rated payload. It is the legal road weight limit at destination, and it is lower — often considerably.
Road weight limits vary by country, by vehicle configuration, by axle arrangement and sometimes by route. Depending on the jurisdiction and the truck, the practical cargo limit for a 20ft container moving on public roads is frequently in the range of 20–26 tonnes, well below the container’s rated payload.
A container loaded to 28 tonnes may be legal at sea and illegal on the road to your yard. What that produces is a haulier refusing the load, a container that has to be stripped at the port at your cost, demurrage running the whole time, and in some cases a fine.
Ask your haulier for your actual limit before you agree the quantity. Then work backwards to what goes in the container.
The other destination limits
Your unloading equipment. A single A-frame of 30 mm slabs can weigh several tonnes. Your forklift or crane has a rated capacity, and it needs to reach into the container at that capacity. Confirm before the vessel sails.
Your floor. A yard slab or warehouse floor has a loading limit, and stacked stone concentrates weight severely.
Your storage. Twenty-five tonnes of stone needs somewhere to go immediately.
VGM — the weight you must declare
Under the SOLAS convention, the Verified Gross Mass of a packed container must be declared before it is loaded onto a vessel. The shipper is responsible for the declaration, and there are two permitted methods: weigh the packed container, or weigh the contents and add the tare.
For you as a buyer, ask your supplier what their VGM procedure is and ask for the declared figure. It is the most accurate weight number you will get, it comes before the vessel sails, and it lets you brief your haulier correctly.
Worked examples
A container of 30 mm granite slabs. Cargo limit set by your haulier: 24,000 kg. Less a 10% packing allowance: 21,800 kg of stone. At 81 kg/m² that is roughly 265 m², or about 2,850 sq ft.
A container of 22 mm calibrated sandstone paving. Cargo limit 24,000 kg, less 12% for pallets and crating: 21,100 kg. At 51 kg/m² that is roughly 410 m².
A container of 100 mm granite setts. At 2,700 kg/m³, 100 mm depth is 270 kg/m². Cargo limit 24,000 kg less 10%: 21,600 kg. That is roughly 80 m² of setts — which sounds small until you remember you are shipping a tenth of a metre of solid rock. Setts are the densest cargo in this business: a container of them is mostly air by volume and entirely full by weight.
Why stone ships in 20ft, not 40ft
Because a 40ft container of stone is either dangerously overweight or half empty. A 40ft has roughly twice the cubic capacity and a maximum gross weight not much higher than a 20ft. Fill it with stone and you exceed the limit long before you fill the space; load it to the limit and you have paid for a 40ft to move a 20ft quantity. A supplier who offers you a 40ft of granite has not done the arithmetic.
Loading distribution
Weight has to be spread along the container floor, not concentrated. A container with all its weight in the front third cannot be safely lifted by a spreader, the truck’s axle loading will be illegal even if the total weight is not, and the container floor has a point-loading limit of its own.
What to specify: load distributed evenly along the length, secured against fore-aft movement, and photographed. The loading photograph set — yard, frames, part load, full load, sealed doors with the seal number visible — is the evidence, and the full standard is in our guide to sea packing for natural stone.
The demurrage arithmetic
Free time at the terminal and on the container is limited, and stone is unusually exposed. It needs a specific truck and sometimes a permit; it often goes to a yard with limited unloading equipment; customs may select it for examination, and a stone container is slow to examine; and the importer frequently has nowhere to put 25 tonnes at short notice.
A week of demurrage on one container can exceed the entire duty saving from a trade agreement. Confirm your haulier, your unloading equipment and your customs entry before the vessel sails, and know your free-time allowance. The related decision — whether to ship a full container at all — is covered in FCL vs LCL for stone shipments.
The planning checklist
- Get the actual density for your stone
- Calculate weight per square metre at your thickness
- Add 8–12% for packing
- Ask your haulier for your legal road limit, not the container’s rated payload
- Work backwards to the quantity that fits
- Ask for the VGM figure before the vessel sails
- Confirm your unloading equipment capacity
- Specify even weight distribution, and get the loading photographs
- Know your free time at the terminal and on the container
How Gemarix plans a load
We calculate the load against your road limit, not the container’s rated payload, and we tell you the quantity that actually fits rather than filling the box and letting you find out at the port. We supply the VGM figure and the loading photograph set before documents are released, so your haulier is briefed with a real number rather than an estimate. Buying terms are covered in Incoterms for stone importers.
Frequently Asked Questions
How much does a container of granite weigh?
A 20ft container of stone is limited by weight, not space. Maximum gross weight is typically 30,480 kg and the empty container weighs about 2,200 to 2,300 kg, leaving roughly 28,000 kg of payload. In practice the binding limit is your destination road weight limit, which is often between 20 and 26 tonnes.
What does 20mm granite weigh per square metre?
Granite runs around 2,700 kg per cubic metre, so 20mm weighs about 54 kg per square metre and 30mm about 81 kg per square metre, before packing. Add 8 to 12 per cent for A-frames, crates and protection, because that weight goes on the same scale.
Why does stone ship in 20ft containers rather than 40ft?
Because a 40ft container of stone is either dangerously overweight or half empty. A 40ft has roughly twice the cubic capacity but a maximum gross weight not much higher than a 20ft, so stone reaches the weight limit long before it fills the space.
What is VGM and who is responsible for it?
Verified Gross Mass is the declared weight of a packed container, required under the SOLAS convention before the box is loaded onto a vessel. The shipper is responsible for the declaration. Ask your supplier for the declared figure, because it is the most accurate weight number you will get before sailing.
How many square metres of paving fit in a container?
It depends on thickness and the road limit at destination. As a worked example, at a 24,000 kg cargo limit and 12 per cent packing allowance, 22mm calibrated sandstone at about 51 kg per square metre gives roughly 410 square metres. Always calculate against your own haulier's limit.