Slab weight, packing allowance and the 26-tonne limit: how to calculate the number of containers.
Because of the weight of natural stone the weight limit is reached before the container is full, so tonnes, not volume, decide. Buyers ask at the quote stage: 'How many m² fit in one container?' The answer depends on stone type, thickness and packing. You can do the calculation in three steps.
Step 1: Slab weight
Slab weight = area (m²) × thickness (m) × density (kg/m³). Example: a 280 × 160 cm marble slab is 4.48 m². At 2 cm thickness its volume is 0.0896 m³ and, at a density of 2,700 kg/m³, its weight is about 242 kg.
Step 2: Packing allowance
Wooden stands, crates and strapping make up about 4% of the total load. The slab above is counted as 242 × 1.04 = 252 kg when packed.
Step 3: Container capacity
The net load limit of a 20 ft container is in practice 26 tonnes, a value attainable in most countries given road-weight rules. 26,000 / 252 = 103 slabs, or about 461 m² (103 × 4.48). The same calculation gives 68 slabs and 305 m² at 3 cm, and 41 slabs and 184 m² at 5 cm.
With different stones
- Travertine (2,500 kg/m³), 2 cm: about 500 m²
- Granite (2,700 kg/m³), 2 cm: about 460 m²
- Basalt (2,950 kg/m³), 3 cm: about 280 m²
- Limestone (2,450 kg/m³), 3 cm: about 340 m²
These values are approximate and vary by ±5% with moisture, packing method and slab size. The calculator on our site does this calculation automatically when you enter size and thickness. The loading plan and exact container count are stated separately in the proforma invoice.
Points for the loading plan
- Weight must be spread evenly across the container floor; piling to one side creates a tipping risk
- Slabs travel upright on A-frames or wooden stands; horizontal loading can break weak slabs
- Wooden material must be treated to the ISPM-15 standard
- Rubber or foam separators are placed between bundles
If these rules are not followed, problems may arise in the insurance claim process. Loading should therefore be documented by camera recording and photographs. The Petrova team loads each container under its own responsibility and shares the loading plan with the buyer.
